Chapter 5: Cell Cycle and Cell Division

In the late 1870s, a German anatomist named Walther Flemming sat at his microscope in Kiel with a problem that had defeated everyone before him. The inside of a living cell looked like a featureless grey soup. Whatever happened when a cell split in two was happening invisibly. Flemming’s advantage was chemical, not optical: the dye industry of the time had begun producing brilliant new aniline stains, and he found that some of them clung tightly to a particular material inside the nucleus. He called that material chromatin, from the Greek word for colour. Working with the transparent tissues of salamander larvae, he stained cell after cell and drew what he saw. Slowly a sequence emerged from the still pictures. The chromatin gathered itself into short, thick threads. The threads lined up neatly across the middle of the cell. Then they split, and the two halves were dragged apart to opposite ends before the cell pinched in two. Flemming published these drawings in 1882 and named the whole performance mitosis, after the Greek word for thread. He had shown that a dividing cell is not a bag of soup at all, but a machine running a precise, ordered programme.

Diagram

Fig. 5.1 – Opening illustration: a nineteenth-century microscope beside a row of four stained salamander cells showing chromatin gathering into threads, lining up at the equator, separating, and the cell pinching in two

Curiosity Corner

Q1. Why can chromosomes be seen clearly only during a short part of a cell’s life, and not at all for most of it? Q2. What stops a cell from starting to divide before its DNA has been copied correctly? Q3. Mitosis makes two daughter cells that are identical, but meiosis makes four that are all different. Where does that difference come from? Q4. By looking at a stained slide of tissue, how can you tell whether its cells are dividing rapidly or hardly at all? Q5. How do we know that a human body cell carries exactly 46 chromosomes?

Each of these questions is answered somewhere in this chapter — watch for them as you read.

5.1 The Cell Cycle

Every cell in your body came from an existing cell that divided. But a cell cannot simply split the moment it is formed. Before it can hand a complete set of instructions to two daughter cells, it must first grow to a workable size, build the machinery it will need, and make an exact copy of all its DNA. Only then can it divide. This orderly series of events — grow, copy the DNA, prepare, divide — is called the cell cycle.

Definition

The cell cycle is the life period of a cell, extending from its formation to the completion of its own division, during which it grows, replicates its DNA, and divides into two daughter cells.

Notice that two different things are happening at two different rhythms here. Growth of the cytoplasm is continuous — the cell is adding protein, membrane and organelles almost all the time. DNA synthesis is not continuous. It is squeezed into one specific window of the cycle and does not happen at any other time. This separation is deliberate: it lets the cell finish copying its genome completely before it begins the risky business of pulling that genome apart.

How Long Does One Cycle Take?

The duration of the cell cycle is not fixed. It differs from organism to organism and, within one organism, from tissue to tissue. A typical eukaryotic cell such as a human cell in culture completes one cycle in about 24 hours. A yeast cell races through the same cycle in about 90 minutes. Cells lining the human gut divide every few days, while a mature nerve cell may never divide again in a lifetime. The length of the cycle is a property of the cell type and its physiological condition, not a universal constant.

The Two Main Halves

A newly formed cell is never ready to divide immediately. It first passes through a long stretch of growth and preparation called interphase, sometimes described as the resting phase of the cell. The word “resting” is misleading, and it is worth correcting straight away — interphase is the busiest part of a cell’s life in chemical terms. What is resting is the dividing machinery, not the cell.

Interphase is immediately followed by the phase of division, the M-phase. Together, interphase and M-phase constitute the cell cycle. Interphase itself is not a single block; it is made of three successive phases — G₁-phase, S-phase and G₂-phase — as shown in Fig. 5.2.

Diagram

Fig. 5.2 – The cell cycle drawn as a wheel divided into G₁ (initial growth or pre-DNA synthesis phase, up to ten hours), S (DNA synthesis phase, up to eight hours), G₂ (post-DNA synthesis or second growth phase, up to five hours) and M-phase (one hour, with prophase, metaphase, anaphase and telophase marked along it); arrows showing the direction of progress

Compare Table

The Four Successive Phases

Working with radioactively labelled cells in the 1950s, Howard and Pelc showed that DNA synthesis occupies only a defined window of interphase, and divided the entire cell cycle into four successive phases.

1. G₁-phase. This phase immediately follows the previous division, whether that division was mitosis or meiosis. It is also called the pre-DNA synthesis phase. During G₁ the cell synthesises the substrates and the enzymes that DNA replication will shortly need — so G₁ is marked by heavy transcription of various types of RNA and by the synthesis of many different proteins. In effect, the cell is stocking its workshop before the work begins.

G₁ is also the phase in which the length of the whole cycle is chiefly decided, because regulation acts mainly by arresting the cell at a specific point in G₁. Of all the phases, G₁ is the most variable in length. Depending on the physiological condition of the cell, it may last days, months or years. In a cell cycle of 24 hours’ duration, G₁ takes up roughly the first ten hours.

2. S-phase. This is the phase of DNA synthesis, and by the end of it every chromosome consists of two identical sister chromatids. Histones — the proteins around which DNA is packaged — are also synthesised during this phase and become associated with the newly replicated DNA, so that the new DNA is packed as soon as it is made.

Replication does not begin everywhere at once. Euchromatic regions of the genome, which are loosely packed, replicate earlier than heterochromatic regions, which are tightly packed. In some cells the G–C rich regions replicate earlier than the A–T rich ones, because G–C rich regions are usually part of active, open, gene-rich chromatin, whereas A–T rich regions are often repressive, compact and gene-poor. The timing of replication therefore reflects not only base composition but also chromatin structure and gene function — active parts of the genome are copied first.

3. G₂-phase. Also called the post-DNA synthesis phase, G₂ is the period between the end of DNA synthesis and the start of cell division. All the metabolic activity concerned with the growth of the cytoplasm and of its constituent organelles and macromolecules is carried out here, and the factors needed for chromosome condensation during mitosis are synthesised in this phase. Because replication is complete, a cell in G₂ contains twice the amount of DNA present in the original diploid cell.

Side Note

The DNA content of a cell is often expressed as its C-value — the amount of DNA, in picograms, present in a haploid set of chromosomes. A normal diploid cell in G₁ is therefore 2C, and after S-phase it becomes 4C, even though the chromosome number has not changed.

4. M-phase, or the phase of division. During this phase the cell actually divides. The nucleus divides first — this is called karyokinesis — and it is followed by the division of the cytoplasm, called cytokinesis. The two together produce two separate daughter cells, and each of them re-enters G₁. The cycle is now closed.

Flowchart

FC 5.1 – The cell cycle: G₁ (growth, enzyme and RNA synthesis) → S (DNA replication) → G₂ (preparation for division) → M-phase (karyokinesis → cytokinesis) → two daughter cells → back to G₁; branch at G₁ → G₀ (quiescent stage) → re-entry into G₁ when growth conditions return

Curiosity Corner

Q1. Why can chromosomes be seen clearly only during a short part of a cell’s life, and not at all for most of it?

A. For the roughly 23 hours of interphase the genetic material stays as extended, uncoiled chromatin, far too fine to resolve as separate bodies. It condenses into short, thick, stainable chromosomes only as M-phase begins — about one hour out of twenty-four. This is exactly why a slide of onion root tip, prepared in the ISC practical on mitosis, shows a large majority of cells in interphase and only a few caught in recognisable dividing stages.

Cells That Step Out of the Cycle

Not every cell keeps cycling. Cells that do not divide further exit G₁ and enter an inactive stage called the quiescent stage (G₀). Such cells remain metabolically active — they carry out their normal work in the tissue — but they do not proliferate. A cell held in this arrested condition is said to be in the G₀-stage. When conditions change and growth is resumed, the cell can re-enter the G₁-phase and continue the cycle.

Definition

G₀ or the quiescent stage is an inactive stage, entered by exit from G₁, in which a cell remains metabolically active but does not divide.

This is not a defect but a normal and necessary arrangement. Mature nerve cells and cardiac muscle cells stay permanently in G₀, which is a large part of why heart and nerve tissue repair themselves so poorly after injury. Liver cells, by contrast, sit in G₀ for long stretches but can be recalled into the cycle when part of the liver is lost, dividing until the lost mass is replaced and then withdrawing again. Whether a cell is cycling or resting is thus a decision the tissue makes according to need.

Numerical 5.1

A cultured mammalian cell completes one cell cycle in 20 hours. Autoradiography shows that S-phase occupies 8 hours and M-phase 1 hour, and G₂ is measured as 4 hours. Calculate the duration of G₁ and express the length of interphase as a percentage of the whole cycle.

5.2 Regulation of the Cell Cycle

For the life processes of a eukaryotic cell to continue properly, the phases of the cell cycle must follow one another in a precisely controlled order. A cell that begins to divide before its DNA has been fully copied, or that separates its chromosomes before they are correctly attached, will hand its daughters an incomplete or unequal set of genetic material. Any loss of regulation therefore shows up as chromosomal abnormalities in the cells that follow.

Two questions have to be answered by any explanation of this control. What sets the timing, and what checks the quality of the work before the cell is allowed to move on?

What Sets the Timing

According to the clock theory, the cell oscillates between interphase and M-phase because of an inbuilt timer rather than an external one. On this view, even if one phase of the cycle is inhibited, the cell proceeds normally to the next phase when its internal clock says so. Entry of a cell from one phase to the next is genetically controlled — the instructions for the timer are in the genome itself.

The molecular machinery behind that timer is a partnership between two kinds of protein: a group of enzymes called cyclin dependent kinases (CdK) and a group of regulatory proteins called cyclins. A kinase is simply an enzyme that attaches a phosphate group to another protein, and attaching that phosphate switches the target protein on or off.

The crucial point is that a CdK molecule is useless on its own. Its concentration in the cell stays more or less steady throughout the cycle, but it has no activity until a cyclin binds to it. Cyclin levels, by contrast, rise and fall — each cyclin is synthesised at a particular stage and destroyed once its job is done. It is this rise and fall of the cyclins that gives the cycle its rhythm, which is exactly why these kinases are called cyclin dependent.

Definition

Cyclin dependent kinases (CdK) are enzymes that drive the cell from one phase of the cell cycle to the next by phosphorylating specific target proteins, and which become active only after binding to a cyclin.

Definition

Cyclins are regulatory proteins whose concentration rises and falls at particular stages of the cell cycle, and which activate the cyclin dependent kinases by binding to them.

Compare Table

How the Kinase Is Switched On

The kinase that regulates the entry of a cell into M-phase is called the M-phase kinase. It has two subunits: one carries the catalytic property and is called the catalytic subunit, while the other exerts regulatory control and is the regulatory subunit, or cyclin.

The catalytic subunit is activated by a reversible cycle of phosphorylation and dephosphorylation at the beginning of M-phase, and this process depends on the bound cyclin. Once active, the cyclin–kinase complex selects the specific proteins that are to be phosphorylated — the cyclin part determines which targets are chosen, so different cyclins send the same kinase after different jobs. Phosphorylating those targets is what actually triggers events such as the condensation of chromatin and the breakdown of the nuclear membrane. The entry of the cell into S-phase is regulated in the same way, by an S-phase kinase.

Flowchart

FC 5.2 – Cyclin–CdK control: cyclin synthesised at a particular stage → cyclin binds the inactive CdK → catalytic subunit activated by phosphorylation/dephosphorylation → complex phosphorylates selected target proteins → cell enters the next phase → cyclin destroyed → CdK returns to the inactive state

Real Incident / Discovery

In the early 1980s, Tim Hunt was working on sea urchin eggs, which divide in a neat, synchronised rhythm after fertilisation. He was tracking the proteins the eggs made, and noticed something odd: one protein built up steadily as the cells approached division and then vanished abruptly each time they divided, only to build up again. A protein appearing and disappearing in step with the cycle was an obvious candidate for controlling it, and Hunt named this class of proteins cyclins. For their work on the genes and molecules that regulate the cell cycle, Hartwell, Hunt and Nurse were awarded the Nobel Prize in 2001. They compared the CdK molecules with an engine and the cyclins with a gear box that controls whether the engine will run in the idling state — in neutral gear — or drive the cell forward through the cycle.

Checkpoints: Quality Control

Timing alone is not enough. At certain fixed points the cell pauses and inspects its own work, and moves on only if the inspection is passed. These control points are called checkpoints, and the cyclin–CdK machinery is held inactive until the check is cleared. Their positions in the cycle are shown in Fig. 5.3.

Diagram

Fig. 5.3 – The cell cycle wheel of Fig. 5.2 with the three checkpoints marked in position: G₁/S at the end of G₁, G₂/M at the end of G₂, and the M checkpoint during metaphase

Checkpoint	Phase	Check-up parameters	Reason or purpose
G₁/S checkpoint	At the end of G₁	Cell size, nutrients, DNA damage	Prevents premature DNA replication
G₂/M checkpoint	At the end of G₂	DNA replication, damage	Ensures readiness for mitosis
M checkpoint	During metaphase	Spindle attachment, alignment	Ensures accurate chromosome separation

Each check is placed exactly where it can still do some good. Damage found at G₁/S can be repaired before it is copied; once S-phase has begun, the error would be duplicated into both chromatids. The M checkpoint, in the same way, is the last moment at which a chromosome that has not been properly attached to the spindle can still be pulled into line — after anaphase begins, the mistake is permanent and is passed to a daughter cell.

Curiosity Corner

Q2. What stops a cell from starting to divide before its DNA has been copied correctly?

A. The G₂/M checkpoint. At the end of G₂ the cell verifies that replication is complete and that the DNA is undamaged; until that check is cleared, the M-phase kinase is not allowed to become active, and the cell simply waits in G₂ rather than entering mitosis.

When Regulation Fails

Because these controls decide whether a cell divides at all, they are also the controls a cancer cell has escaped. A tumour that is growing rapidly typically shows high levels of cyclins and CdK activity, so the brakes provided by the checkpoints are ignored and division continues regardless of damage or crowding. The genetic basis of this loss of control — tumour suppressor genes and oncogenes — is taken up later in your course; for now it is enough to see that the same molecules that keep a normal tissue orderly are the ones that fail in an uncontrolled one.

5.3 Mitosis

Mitosis is the usual form of nuclear division in the body. It takes place in somatic cells — that is, in all the cells of the body except those set aside for producing gametes — and it was first reported by Walther Flemming in 1882. Its defining feature is precision: the genetic material is not merely halved, it is halved exactly, so that each daughter nucleus receives a complete and identical set of chromosomes.

Definition

Mitosis is the division of the nucleus of a somatic cell into two daughter nuclei, each receiving a chromosome set identical in number and kind to that of the parent nucleus.

Remember what the cell has already done before mitosis begins. During S-phase every chromosome was replicated, so each one now consists of two identical sister chromatids joined at the centromere. Mitosis does not copy anything. Its entire job is to sort what has already been copied, sending one chromatid of each pair to each end of the cell.

Mitosis is divided into four successive stages: prophase, metaphase, anaphase and telophase. The stages are continuous — a real cell does not pause between them — but they are easy to tell apart under the microscope because the chromosomes look different in each, as shown in Fig. 5.4 for an animal cell and Fig. 5.5 for a plant cell.

Flowchart

FC 5.3 – Stages of mitosis: prophase (chromatin condenses, nucleolus and nuclear membrane disappear, spindle forms) → metaphase (chromosomes align at the equator, attached to spindle fibres at the centromere) → anaphase (centromere divides, chromatids move to opposite poles) → telophase (chromosomes uncoil, nuclear membrane and nucleoli reappear, two daughter nuclei formed)

Prophase

Chromatin undergoes condensation and takes the form of visible chromosomes. The chromosomes then begin to contract longitudinally, and because of this contraction they become progressively thicker and smoother. The condensation is brought about by tight coiling of the chromatin fibre, so that a thread that was metres long in total becomes a compact body a few micrometres across.

There is a good mechanical reason for this. An extended chromatin fibre is far too long and too fragile to be dragged across a cell without tangling or breaking. Condensing it first is like winding a long cable onto a spool before carrying it — the same material becomes something that can be moved safely.

Towards the end of prophase the nucleolus and the nuclear membrane disappear, and the nucleoplasm is set free. At this stage spindle fibres make their appearance in the cytoplasm, and these fibres form the nuclear spindle. The order here matters: the nuclear membrane must break down before the spindle fibres can reach the chromosomes at all.

Diagram

Fig. 5.4 – Different stages of mitosis in an animal cell: interphase, early prophase, middle prophase, late prophase, metaphase, early anaphase, late anaphase, early telophase, late telophase and daughter cells; label cell membrane, centrosome, aster, centromere, chromosomes, nucleolus, nuclear membrane, chromosomal fibre, interzonal fibre and continuous fibre

Metaphase

Each chromosome gets attached to a spindle fibre at the centromere — more precisely, at a protein plate called the kinetochore that is built on the centromere. Although each chromosome is already divided into two chromatids, the centromere at this stage is still a single undivided structure holding them together. The chromosomes now move quickly and become arranged at the equator of the spindle, forming the equatorial plate.

Exam Corner

Q. Why is it so important that the centromere stays undivided until every chromosome has reached the equator?

A. As long as the sister chromatids are held together, neither of them can be pulled away on its own. The undivided centromere is a safety catch: it keeps separation from starting until the M checkpoint has confirmed that every chromosome is properly attached and aligned. Release the catch too early and one daughter cell gains a chromosome while the other loses one.

This is also the stage at which drugs that interfere with the spindle act. Vinca alkaloids obtained from the periwinkle plant, Catharanthus roseus, prevent spindle fibres from assembling; cells treated with them align nothing and are arrested at metaphase, which is why such compounds are used against rapidly dividing tumour cells.

Anaphase

During this stage the centromere divides into two. Each half centromere carries one chromatid along with it. Due to the shortening of the spindle fibres, the two chromatids separate from one another and move towards the opposite poles of the spindle. Because each chromatid is dragged centromere-first while its arms trail behind, the moving chromosomes frequently assume a V, I, J or L shape — the shape depends on where the centromere sits along the chromosome.

The two groups of chromosomes converge towards the poles, exhibiting a figure of two radiating clusters, which is called the diaster. Anaphase is the shortest stage of mitosis, and it is the stage at which the equal division of the genetic material is actually accomplished.

Telophase

Like prophase, telophase also takes a longer time as compared with the other phases. As the chromosomes reach the opposite poles they begin to absorb water again and gradually become thin and extended, so they are more difficult to observe — this uncoiling, the reverse of the condensation seen in prophase, is called despiralization. At this stage the nuclear membrane and the nucleoli make their appearance once more. Two daughter nuclei are thus produced from one nucleus.

Telophase completes karyokinesis. The cell at this moment has two nuclei but is still a single cell; the division of the cytoplasm that follows is dealt with in Section 5.6.

Diagram

Fig. 5.5 – Different stages of mitosis in a plant cell: interphase, early prophase, middle prophase, late prophase, metaphase, early anaphase, middle anaphase, late anaphase, early telophase and late telophase, drawn within the rigid cell wall

Seeing Mitosis for Yourself

Because mitosis occupies only about an hour of a 24-hour cycle, you need a tissue in which many cells are dividing at once if you are to catch the stages. The growing tip of an onion root is such a tissue, and it is the material used in the ISC practical on mitosis.

Technique

Onion root tip squash (crush smear) preparation Principle: A meristematic tissue contains a high proportion of dividing cells; softening the tissue and pressing it into a single layer lets the stained chromosomes of individual cells be seen clearly. Steps: 1. Cut 2–3 mm from the tip of a growing onion root. 2. Warm it gently in acetocarmine stain over a spirit lamp, without boiling. 3. Transfer to a slide, add a coverslip, and press down firmly to spread the cells into one layer. 4. Examine under the microscope and identify the stages. Used for: Identifying at least four stages of mitosis in a plant cell, and providing the field of cells from which the mitotic index is counted.

Importance and Significance of Mitosis

The fundamental importance of mitosis is the equal quantitative and qualitative division of the nuclear material — equal in amount and identical in kind — so that the genetic constitution of the nuclei is maintained unchanged from one cell generation to the next. This is what allows a single fertilised egg to build a whole organism whose cells all carry the same instructions.

Mitosis provides the opportunity for growth and development of the organism by causing an increase in the number of cells. Even the gonads and the sex cells depend on mitosis to increase their number, after the chromosome number has been halved by meiosis.

The significance of mitosis may be summed up as follows:

The chromosomal number in the daughter cells is maintained.
Mitosis contributes to the growth and development of organisms.
It maintains the DNA and RNA ratio in the cells.
It maintains the proper size and shape of cells.
Old and decaying cells of the body are replaced by new cells produced by mitosis.

This last point is easy to overlook, but it is happening in you continuously. The lining of the intestine is replaced every few days and the outer layer of skin is shed and rebuilt constantly, entirely through mitosis. Wound healing is the same process, locally accelerated: cells at the edge of an injury are recalled from G₀ into the cell cycle and divide until the gap is closed.

5.4 Mitotic Index

Once you can recognise the stages of mitosis on a slide, you can do something more useful than merely naming them — you can count them. The proportion of cells caught in the act of dividing is a direct measure of how actively a tissue is proliferating, and it is expressed as the mitotic index.

Definition

The mitotic index is the ratio of the number of cells undergoing mitosis to the total number of cells observed in the field at a particular time.

\[\text{MI} = \frac{\text{Number of cells showing mitosis}}{\text{Total number of cells in the field}}\]

Here the numerator is the count of cells in prophase, metaphase, anaphase or telophase — any cell in which condensed chromosomes are visible — and the denominator is every cell in the same field, dividing or not. Because both are counts of cells, the mitotic index has no unit. It is a pure ratio, and it is often multiplied by 100 and reported as a percentage.

Why the Value Means What It Means

Recall from Section 5.1 that M-phase occupies only about one hour of a 24-hour cell cycle. If every cell in a tissue were cycling steadily, roughly one cell in twenty-four would be in mitosis at any instant, giving a mitotic index of about 4%. A much higher value tells you that the cells are cycling faster, that a larger share of them are in the cycle at all rather than resting in G₀, or both. A value close to zero tells you that the tissue is made largely of G₀ cells.

Exam Corner

Q. Two slides are prepared from the same root, but one shows 30 cells in the field and the other shows 300. Does the larger field automatically give a higher mitotic index?

A. No. The mitotic index is a ratio, so the size of the field cancels out — a field with three times as many cells will normally also contain about three times as many dividing cells. What a larger count does improve is reliability: a single dividing cell out of 30 swings the value far more than one out of 300, so counting more cells gives a steadier estimate.

Curiosity Corner

Q4. By looking at a stained slide of tissue, how can you tell whether its cells are dividing rapidly or hardly at all?

A. By calculating the mitotic index. Count every cell in the field that shows condensed chromosomes, divide by the total number of cells in that field, and express the result as a percentage. A high mitotic index means rapid cell turnover; a low one means most of the cells are in interphase or have withdrawn into G₀.

Where the Measurement Is Used

In the laboratory this is exactly what you do with the onion root tip preparation from the previous section: choose a field under the high power objective, count the dividing cells and the total cells, and calculate the ratio. Comparing the mitotic index of the extreme root tip with that of a region a few millimetres above it shows at once where the meristem lies, because only the meristematic zone has a high value.

The same measurement is used in medicine. A biopsy from a tumour is scored for the proportion of dividing cells, and a high mitotic index indicates rapid cell turnover — a tumour that is growing and spreading quickly. This is a measure of rate, not of cause; it says how fast the tissue is dividing, not why the control described in Section 5.2 has failed.

Numerical 5.2

A student examines a stained onion root tip preparation under the high power objective and counts 240 cells in one field, of which 9 are in prophase, 4 in metaphase, 3 in anaphase and 2 in telophase. Calculate the mitotic index and express it as a percentage. If a second field from a region 5 mm above the tip contains 300 cells of which 3 are dividing, comment on the difference.

Try Yourself

Using the value you calculated above and the fact that one complete cell cycle in onion root tip cells lasts about 20 hours, estimate roughly how long these cells spend in M-phase. State one assumption your estimate depends on.

5.5 Meiosis

Every species has a fixed number of chromosomes in its body cells, and that number stays the same generation after generation. Ask why, and a problem appears at once. If the egg and the sperm each carried the same number of chromosomes as the vegetative cells, the fertilised egg of each generation would contain twice as many chromosomes as the nuclei of the generation before it. Within a few generations the number would be absurd.

This is prevented by a special kind of division in which the chromosome number is reduced to half that found in the vegetative nuclei, so that fertilisation restores it rather than doubling it. The reduction is achieved by two successive divisions in the life cycle, and the whole process is called meiosis or reduction division. The term was coined by Farmer and Moore in 1905.

Definition

Meiosis is a special type of cell division, occurring in germinal or reproductive cells, in which two successive divisions follow a single round of DNA replication, so that one diploid cell gives rise to four haploid nuclei.

The process of meiosis is fundamentally the same in all plants and animals. The two divisions are called the first meiotic division and the second meiotic division, and they do quite different jobs: the first separates the homologous chromosomes and is therefore reductional, while the second separates sister chromatids and is equational, exactly as mitosis is.

Exam Corner

Q. If meiosis produces four cells, why does the DNA not have to be replicated a second time between the two divisions?

A. Because each division separates a different thing. S-phase before meiosis I gives every chromosome two chromatids. Meiosis I splits the homologous pairs, leaving each nucleus with half the number of chromosomes — but each of those chromosomes is still double. Meiosis II then splits those chromatids. One round of copying is enough to supply both divisions, and a second round would defeat the whole purpose by restoring the diploid number.

First Meiotic Division (or Heterotypic Division)

Meiosis I occurs in four stages: prophase I, metaphase I, anaphase I and telophase I. The whole sequence is shown in Fig. 5.6.

Diagram

Fig. 5.6 – Different stages of meiosis in an animal cell: interphase, leptotene, zygotene, pachytene, diplotene, diakinesis, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II and the four daughter cells; label centrioles, centrosphere, cell membrane, nuclear membrane, chromatin, synapsis of homologous chromosomes and crossing over

Prophase I

Prophase I is the longest stage of meiosis, and it is unlike anything in mitosis. It occurs in five sub-stages.

Leptotene: The nucleus becomes larger and the chromosomes become more distinct. They appear as slender threads bearing bead-like structures called chromomeres.
Zygotene: The chromosomes become shorter and thicker, and homologous chromosomes start pairing. This pairing is called synapsis, and each pair is called a bivalent. Pairing is highly specific — each region of one chromosome comes to lie beside the matching region of its partner. If pairing occurs between non-homologous chromosomes, it is called pseudosynapsis.
Pachytene: The chromosomes become still shorter and thicker, and each is now clearly seen to consist of two chromatids — they were already double from the preceding S-phase, but only now is that doubleness visible. The paired structure, containing four chromatids in all, is called a tetrad. At this stage the chromatids begin to exchange corresponding segments with one another, and this exchange is crossing over.
Diplotene: The chromosomes are further thickened and shortened, and the paired homologues begin to repel each other and move apart. They remain held, however, at certain points where exchange has occurred; these X-shaped points of contact are the chiasmata, and they become clearly visible at this stage. As the homologues continue to separate, the chiasmata slide towards the ends of the chromatids — this movement is called terminalization, and it continues into the next sub-stage.
Diakinesis: This is the final sub-stage of prophase I. Terminalization is completed and the homologous chromosomes get separated. The nuclear membrane and the nucleolus disappear, and spindle fibres appear.

Definition

Crossing over is the mutual exchange of corresponding segments between the non-sister chromatids of homologous chromosomes, occurring during pachytene of prophase I.

Crossing over is the reason meiosis matters to genetics rather than merely to arithmetic. Because the exchanged segments carry genes, a chromatid emerging from prophase I is a new combination of the alleles that entered it — part maternal, part paternal. The distance between two genes on a chromosome can in fact be estimated from how often crossing over separates them, and this is the basis of the gene mapping you will meet later in the course.

Flowchart

FC 5.4 – Prophase I: leptotene (chromosomes appear as threads with chromomeres) → zygotene (synapsis of homologues, bivalents form) → pachytene (tetrads, crossing over occurs) → diplotene (homologues repel, chiasmata visible, terminalization begins) → diakinesis (terminalization complete, nuclear membrane and nucleolus disappear, spindle appears)

Metaphase I, Anaphase I and Telophase I

Metaphase I: The bivalents are arranged along the equatorial plane and attached to the spindle fibres in such a way that the centromeres of the two members of each bivalent face towards opposite poles. Note the difference from mitotic metaphase — here whole pairs line up, not single chromosomes, and it is a matter of chance which member of a given pair faces which pole.

Anaphase I: The spindle fibres contract and the bivalents are pulled towards opposite poles. Each dyad — a chromosome still consisting of two chromatids — appears V, U, J or L shaped as it moves. The separation of homologous chromosomes at this stage is called disjunction. The centromere does not divide here, which is precisely why the number of chromosomes going to each pole is half the original number. Failure of this separation is called non-disjunction, and it leads to daughter cells with abnormal chromosome numbers.

Telophase I: At the opposite poles the chromosomes become uncoiled and greatly elongated. The nucleolus and the nuclear membrane reappear. This division results in a total of two daughter nuclei, each with the haploid number of chromosomes, though each chromosome is still double.

Curiosity Corner

Q3. Mitosis makes two daughter cells that are identical, but meiosis makes four that are all different. Where does that difference come from?

A. From two events, both belonging to meiosis I. Crossing over at pachytene physically exchanges segments between non-sister chromatids, so no chromatid leaves prophase I with quite the combination of alleles it entered with. Then, at metaphase I, each bivalent orients on the spindle independently of every other bivalent, so the maternal and paternal chromosomes are dealt out to the poles in a fresh combination each time. Mitosis has neither event — no pairing, no exchange, and single chromosomes lining up — so its products are identical.

Second Meiotic Division (or Homotypic Division)

Meiosis II occurs in the two nuclei formed after meiosis I. It involves the following four steps.

Prophase II: The nucleolus and the nuclear membrane disappear. Spindle fibres appear.
Metaphase II: The chromosomes get arranged at the equatorial plane of the spindle fibres, attached to the spindle fibre. The attachment is at the region of the centromere.
Anaphase II: The spindle fibres contract and pull the centromeres towards opposite poles. As a result, the sister chromatids go towards the opposite poles.
Telophase II: The nuclear membrane and the nucleolus reappear. This results in a total of four nuclei formed after the complete meiosis.

Flowchart

FC 5.5 – Overall course of meiosis: one diploid cell after S-phase → meiosis I (prophase I → metaphase I → anaphase I → telophase I) → two haploid nuclei, chromosomes still double → meiosis II (prophase II → metaphase II → anaphase II → telophase II) → four haploid nuclei

Significance of Meiosis

It helps in the production of haploid gametes, which maintains the chromosomal number in the offspring after fertilization.
It produces the new generation.
It leads to variation in generations, caused by the crossing over that occurs during meiosis.

That third point is the raw material of evolution and of plant and animal breeding alike. Without meiotic variation, every offspring would be a copy of a mixture of two fixed parental sets, and a population would have far less to select from when its environment changed.

Compare Table

In the ISC practical you study meiosis from permanent slides rather than by preparing your own, because meiosis takes place deep inside anthers and testes and cannot be squashed out as conveniently as an onion root tip. When you examine those slides, use the table above as your guide: paired chromosomes at the equator mean metaphase I, single chromosomes mean metaphase II.

5.6 Cytokinesis

Karyokinesis leaves the cell with two complete nuclei, but still with one cytoplasm and one plasma membrane. The division of the cytoplasm that follows, and that finally produces two separate cells, is known as cytokinesis.

Definition

Cytokinesis is the division of the cytoplasm of a cell, following nuclear division, by which the two daughter nuclei come to lie in two separate daughter cells.

Cytokinesis takes place by either of the following two methods:

By cell-plate formation
By cleavage of the cytoplasm

(a) By Cell-Plate Formation

During telophase, droplets of liquid — vesicles carrying wall material, delivered from the Golgi apparatus — appear in the equatorial region of the spindle. Gradually these droplets unite to form the so-called cell plate. The spindle region along the equatorial plate is called the phragmoplast, and it guides the vesicles into position.

Protoplasmic membranes are then formed by the protoplasm on both sides of this liquid layer, and the new cell walls are laid down between the two protoplasmic membranes. The plate grows outwards from the centre until it meets and fuses with the existing wall of the parent cell, at which point the two daughter cells are fully separated.

This is the method used by plant cells, as you can see in the plant mitosis stages of Fig. 5.5, where the new wall appears as a straight line across the middle of the cell.

(b) By Cleavage of the Cytoplasm

This kind of division takes place in animal cells and in many simple organisms such as fungi. In this case, cell division takes place by the formation of a ring-like furrow in the plasma membrane. The furrow appears at the equator and deepens steadily until it has cut entirely through the cell. The cleavage results in the formation of two plasma membranes, and hence two separate cells, as shown in Fig. 5.7.

Diagram

Fig. 5.7 – Two methods of cytokinesis compared: (a) a plant cell in which a cell plate forms at the centre of the phragmoplast and grows outwards to join the parent cell wall; (b) an animal cell in which a cleavage furrow appears at the surface and deepens inwards until the cell is pinched in two

Compare Table

Flowchart

FC 5.6 – Completion of division: telophase (two daughter nuclei formed) → cytokinesis → plant route: vesicles collect at the equator → cell plate forms → grows outwards → fuses with parent wall; animal route: furrow appears at the equator → deepens → cuts through the cell → two daughter cells in both routes

When Cytokinesis Does Not Follow

Karyokinesis and cytokinesis are separate events, and they can be uncoupled. If the nucleus divides but the cytoplasm does not, the result is a single cell containing two or more nuclei. This happens naturally in some tissues and in many fungal hyphae, where a continuous cytoplasm is shared by many nuclei. It also explains a common observation question: a cell seen with two distinct nuclei but no partition between them has completed telophase and has not yet completed cytokinesis.

5.7 Techniques in Cytology: Microscopy

Everything described so far in this chapter — condensing chromosomes, spindle fibres, chiasmata — is invisible to the unaided eye. Cells range from about 1 nm for their finest structures to about 1 mm at their largest, and the human eye cannot resolve detail at that scale at all. To see the cell and its different components, we need microscopes of different types.

Two quantities decide what a microscope can do, and they are not the same thing.

Definition

Magnifying power (MP) is the ability of a microscope to enlarge the image of an object. It depends on the focal length of the eye-piece, the focal length of the objective and the tube length of the microscope.

Definition

Resolving power (RP) is the ability of a microscope to distinguish two closely situated points as separate structures.

Magnification without resolution is useless. Enlarging a blurred image simply gives a bigger blur — two chromosomes lying close together will appear as one thick smudge no matter how much you magnify them. Resolving power is therefore the real limit on what a microscope can show, and it is set by the wavelength of the radiation used: the shorter the wavelength, the finer the detail that can be resolved. This single principle explains the whole difference between the light microscope and the electron microscope.

Light Microscope

A light microscope uses light as its source of illumination. A simple microscope uses a single lens, as in a hand lens or a dissection microscope, while a compound microscope uses a combination of more than one lens.

The compound microscope works on the principle that the image formed by one lens acts as the object for the second lens, so the two magnifications multiply. The image formed by a compound microscope is enlarged, virtual and inverted — which is why a slide must be moved to the left when you want the image to travel to the right.

Compound microscopes are of two types.

(a) Bright field microscope. This is the most common type of microscope, used on a routine basis in clinical and educational laboratories. The specimen is illuminated from below through a condenser, and light that passes straight through it reaches the objective. In this type of microscope the background appears light and the object appears dark or shaded, so the specimen must usually be stained to be seen at all.

The objectives on the nose piece carry different powers — low power (10 X), high power (40 X) and oil immersion (100 X) — and the ocular lens or eye piece adds its own magnification on top.

(b) Dark field microscope. Developed by Zsigmondy in the early 1900s, this is also called the ultramicroscope. It uses a special condenser fitted with an opaque disc that obstructs light from passing through the centre of the beam. In this way, the specimen is illuminated only by light coming from the sides at an angle, and only the light scattered or reflected by the specimen itself reaches the objective lens to form the image. The specimen therefore appears bright against a dark background, as shown in Fig. 5.8.

Diagram

Fig. 5.8 – Ray paths compared in a bright field microscope and a dark field microscope; label eye, ocular lens, objective lens, specimen, condenser lens, opaque disc, light source, and mark “only light reflected by the specimen is captured by the objective lens” and “unreflected light” on the dark field side

Dark field illumination is particularly useful for specimens that cannot be stained properly — including living cells, which many stains would kill — and it helps in observing ciliary and flagellar movements. Its resolution, however, is poor, so it is a technique for seeing that something is there and moving, rather than for examining its fine structure.

Technique

Light microscopy Principle: Visible light passing through or scattered by a specimen is gathered by an objective lens, and the image it forms is further enlarged by the eye-piece. Steps: Mount and stain the specimen on a slide → focus with the low power objective using the coarse adjustment → switch to high power and focus with the fine adjustment → adjust the condenser and diaphragm for contrast. Used for: Routine examination of cells and tissues — identifying the stages of mitosis in an onion root tip squash, examining a Leishmann-stained blood smear, and distinguishing Gram-positive from Gram-negative bacteria, all of which are ISC practical exercises.

Electron Microscope

With a light microscope, objects smaller than about 200 nm cannot be viewed, because visible light has a wavelength of 400–700 nm and cannot resolve structures much finer than half its own wavelength. To go below that limit, the electron microscope uses a beam of electrons in place of light rays. Electrons behave as waves of far shorter wavelength — about 0.005 nm — and this gives the instrument its much greater power of both resolution and magnification. Since electrons are deflected by air molecules, the whole apparatus must be enclosed in a vacuum chamber, and glass lenses are replaced by electromagnets.

The electron microscope is of two types.

(a) Transmission Electron Microscope (TEM). Invented by Knoll and Ruska in the early 1930s, the TEM works by passing electrons through the specimen. A beam of electrons is emitted by a hot cathode and accelerated by a large potential difference. The beam is focussed by electromagnetic coils and allowed to pass through the specimen; an intermediate image formed by the objective lens is converted into a real image by a projection lens, and this image is projected on a photographic plate or a fluorescent screen. The magnifying power of the TEM is about 1 to 3 lakh and its resolving power is 0.2 to 10 Å.

Because the electrons must pass through it, the specimen has to be cut into extremely thin sections and is killed and dehydrated in preparation. The image is two-dimensional and shows internal structure — this is how the internal membranes of a mitochondrion or the layers of a nuclear envelope are seen.

(b) Scanning Electron Microscope (SEM). Invented by Knoll in the mid-1930s, the SEM works quite differently. A beam of electrons is directed on to the surface of the specimen with the help of electromagnetic lenses; this is called the primary electron beam. It knocks out electrons from the surface of the specimen, and these are called secondary electrons. The secondary electrons are collected by an electron collector, amplified by an amplifying device and finally projected on a photographic plate or a fluorescent screen. It is the secondary beam, not the primary one, that actually forms the image.

The SEM is especially useful in studying surface detail — the sculpturing of a pollen grain is the classic example — because it produces a three-dimensional image. Its magnifying power is about 10,000 and its resolving power is 200 Å.

Diagram

Fig. 5.9 – Electron microscope; label insulator, electron gun, specimen chamber, binocular viewer, photographic plate and control panel

Compare Table

Compare Table

TEM vs SEM | S. No. | TEM | SEM | |—|—|—| | 1 | Resolution about 2.5 nm | Resolution about 20 nm | | 2 | Magnification 10,000 – 1,00,000 X | Magnification 1,000 – 10,000 X | | 3 | The electron beam passes through the specimen and then through an electromagnetic objective lens which magnifies the image. | The electron beam is directed over the surface of the specimen; it knocks out electrons from the surface, which are collected, amplified and used to produce the image. | | 4 | A single electron beam is used throughout to produce the magnified image. | Two electron beams are involved — a primary beam that excites the specimen’s electrons, and a secondary beam of emitted electrons which actually forms the image. | | 5 | Has high resolution and is extremely valuable for examining the different layers of a specimen. | Especially useful for studying the surface structures of intact cells and viruses. | | 6 | The specimen is first killed and then used for viewing. | An intact specimen can be used for viewing. | | 7 | Provides 2-D images. | Provides 3-D images. |

5.8 Karyotype and Karyotyping

Metaphase is the one moment in a cell’s life when its chromosomes are fully condensed, individually distinct and spread out on a single plane. That is exactly what makes it useful. If a cell can be caught and photographed at metaphase, its chromosomes can be counted, measured and arranged — and the resulting picture is characteristic of the species the cell came from.

The term karyotype refers to the group of characteristics that help in identifying a particular chromosome set belonging to a particular species. These features include chromosome number, size, shape and the ratio of the arms on either side of the centromere.

Definition

The karyotype is the complete set of characteristics of the chromosomes of a species — their number, size, shape and arm ratio — by which that chromosome set can be identified.

When a karyotype is represented with the help of a diagram, it is called an idiogram. In an idiogram the chromosomes of the haploid set of an organism are arranged in a series of decreasing size.

Karyotypes are also described by how evenly graded they are. If there is only a small difference in the length or size of the first and the last chromosomes, the karyotype is called symmetrical; if the differences are large, it is called asymmetrical.

Counting the Human Chromosomes

Tjio and Levan first reported, in 1956, that the 2n number of chromosomes in man is 46 — that is, 44 autosomes and two sex chromosomes, which are also called allosomes or heterosomes. This was a correction of a figure that had been accepted for decades, and it was made possible by treating the cells so that the metaphase chromosomes spread apart cleanly enough to be counted without ambiguity.

Curiosity Corner

Q5. How do we know that a human body cell carries exactly 46 chromosomes?

A. By preparing a karyotype. Dividing cells are arrested at metaphase, swollen so that the chromosomes separate, spread on a slide, stained and photographed; the chromosomes in the photograph are then cut out and arranged in order. Tjio and Levan did precisely this in 1956 and established the human number as 46 — 44 autosomes and 2 sex chromosomes.

Technique

Karyotyping Principle: Chromosomes are individually distinguishable only when fully condensed, so dividing cells are arrested at metaphase and spread out before being stained and arranged in order of size and centromere position. Steps: 1. Culture dividing cells, usually leucocytes from a blood sample. 2. Add colchicine, which prevents spindle formation and so arrests the cells at metaphase. 3. Treat with a hypotonic solution so that the cells swell and the chromosomes move apart. 4. Fix, drop on to a slide to spread the chromosomes, and stain. 5. Photograph the metaphase plate and arrange the chromosomes in homologous pairs in order of decreasing size. Used for: Determining the chromosome number and sex of an individual, identifying numerical and structural chromosomal abnormalities, prenatal diagnosis, and comparing chromosome sets between species.

In the ISC practical you are shown any metaphasic plate of mitosis and asked to prepare a karyotype from it, which is essentially step 5 done by hand — the chromosomes are cut out of the photograph, matched into pairs and arranged in order.

Diagram

Fig. 5.10 – A normal human male karyotype, with the 22 pairs of autosomes arranged in order of decreasing size into Denver groups A to G, and the X and Y chromosomes placed separately; mark the centromere position on one chromosome of each group

Sex Chromosomes in the Human Karyotype

The human female has 44 autosomes and two identical sex chromosomes, denoted by XX (44 + XX). Because the two members of the pair are alike, every ovum she produces carries an X chromosome as its sex chromosome. Since all her gametes are alike in this respect, the human female is said to be homogametic.

The human male produces two different types of gametes or sperms, one carrying an X chromosome and the other carrying a Y chromosome. Because of this dissimilarity between the two kinds of gamete formed, human males are called heterogametic.

The male carries only one X chromosome, so for the genes on it he has a single copy rather than a pair. He is therefore described as hemizygous for X-linked genes, rather than homozygous or heterozygous as a female may be. This single-copy condition is why X-linked conditions such as colour blindness and haemophilia appear far more often in males — a topic you will return to under sex-linked inheritance.

The Denver System of Classification

According to the Denver system of classification, human chromosomes are classified into seven groups on the basis of their size and the position of the centromere.

Group	Chromosome	Features
A	1, 2 and 3	Metacentric, largest in size
B	4 and 5	Submetacentric, second largest
C	6, 7, 8, 9, 10, 11, 12 and X	Largest group, submetacentric
D	13, 14 and 15	Acrocentric, SAT chromosomes
E	16, 17 and 18	Metacentric and submetacentric
F	19 and 20	Smallest group, metacentric
G	21, 22 and Y	Acrocentric, smallest in size; 21 and 22 are SAT chromosomes

Two points in this table are worth reading carefully. First, the X chromosome is not given a group of its own — it is placed in group C, whose members it resembles in size and centromere position, which is why a karyotype alone does not always make the sex obvious at a glance. Second, the SAT chromosomes of groups D and G carry satellites: small bodies attached to the short arm by a thin thread, and the sites at which the nucleolus is organised.

What a Karyotype Reveals

Because it displays the whole chromosome set at once, a karyotype detects errors of number and of gross structure directly. An extra chromosome in group G, a missing sex chromosome, or a piece of one chromosome attached to another all show up in the arranged photograph. Conditions arising from such errors — Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome — are diagnosed in exactly this way, and their chromosomal composition is dealt with separately when you study errors in cell division.

What a karyotype cannot show is a change in a single gene. The resolution of the method is a whole chromosome band, not a base pair, so a karyotype will look entirely normal in a person carrying a point mutation. Knowing what a technique cannot detect is as important as knowing what it can.

5.9 Summary

The cell cycle is the life period of a cell, during which it grows, replicates its DNA and divides into two daughter cells. Growth of the cytoplasm is continuous, but DNA synthesis occurs only in one specific window.
The duration of the cycle varies with organism and cell type — about 24 hours in a typical human cell and about 90 minutes in yeast.
The cycle has two main parts: interphase, the long phase of growth and preparation, and M-phase, the short phase of division. Interphase is chemically the busiest part of a cell’s life, not a resting stage.
Howard and Pelc divided the cycle into four successive phases: G₁, S, G₂ and M.
G₁ is the pre-DNA synthesis phase, in which RNAs and the enzymes and substrates needed for replication are made. It is the most variable phase in length, and it is the phase at which the duration of the whole cycle is chiefly regulated.
S-phase is the phase of DNA synthesis. Histones are made in this phase and associate with the new DNA. Euchromatic and G–C rich regions replicate earlier than heterochromatic and A–T rich regions, so replication timing reflects chromatin structure and gene function as well as base composition.
G₂ is the post-DNA synthesis phase, in which cytoplasmic growth is completed and the factors needed for chromosome condensation are made. A cell in G₂ contains twice the DNA of the original diploid cell.
The C-value is the amount of DNA, in picograms, in a haploid set of chromosomes; a diploid cell is 2C in G₁ and 4C after S-phase.
Cells that stop dividing leave G₁ and enter the quiescent stage, G₀, where they remain metabolically active but do not proliferate. They can re-enter G₁ when growth is resumed.
M-phase consists of karyokinesis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm.
According to the clock theory, the cell oscillates between interphase and M-phase because of an inbuilt timer, and entry from one phase to the next is genetically controlled.
Cell cycle regulation depends on cyclin dependent kinases (CdK), which are enzymes of steady concentration but no activity on their own, and cyclins, regulatory proteins whose levels rise and fall at particular stages and which activate the CdKs by binding to them.
The M-phase kinase has a catalytic subunit and a regulatory subunit (the cyclin). The catalytic subunit is activated by a reversible cycle of phosphorylation and dephosphorylation, and the bound cyclin selects which proteins are to be phosphorylated. Entry into S-phase is regulated in the same way by an S-phase kinase.
Hartwell, Hunt and Nurse received the Nobel Prize in 2001 for their work on cell cycle regulation, comparing CdKs to an engine and cyclins to the gear box that decides whether the engine idles or drives the cell forward.
Three checkpoints inspect the cell’s work: G₁/S at the end of G₁ (size, nutrients, DNA damage), G₂/M at the end of G₂ (completeness of replication, damage) and the M checkpoint during metaphase (spindle attachment and alignment). Loss of these controls appears as chromosomal abnormalities and as the uncontrolled proliferation seen in tumours.
Mitosis is the usual nuclear division of somatic cells, first reported by Walther Flemming in 1882. It sorts the chromatids already made in S-phase, giving two daughter nuclei identical to the parent.
Prophase: chromatin condenses into visible chromosomes by coiling; the nucleolus and nuclear membrane disappear and spindle fibres appear.
Metaphase: each chromosome attaches to a spindle fibre at its still-undivided centromere, and the chromosomes align at the equator.
Anaphase: the centromere divides, and the chromatids are drawn to opposite poles by the shortening spindle fibres, assuming V, I, J or L shapes; the two clusters form the diaster.
Telophase: the chromosomes uncoil, or despiralize, and the nuclear membrane and nucleoli reappear, producing two daughter nuclei.
Mitosis is important because it divides the nuclear material equally in both quantity and quality, so the genetic constitution of the nuclei is maintained. It provides for growth and development, maintains chromosome number, the DNA–RNA ratio and cell size and shape, and replaces old and decaying cells.
The mitotic index is the ratio of cells undergoing mitosis to the total number of cells in the field:

\[\text{MI} = \frac{\text{Number of cells showing mitosis}}{\text{Total number of cells in the field}}\]

It is a pure ratio, often expressed as a percentage. A high value indicates rapid cell turnover, as in a meristem or a fast-growing tumour.

Meiosis, a term coined by Farmer and Moore in 1905, occurs in germinal cells. Two successive divisions follow one round of DNA replication, so one diploid cell gives four haploid nuclei and the chromosome number of the species stays constant across generations.
Prophase I is the longest stage and has five sub-stages: leptotene (threads with chromomeres), zygotene (synapsis, bivalents), pachytene (tetrads, crossing over), diplotene (homologues repel, chiasmata visible, terminalization begins) and diakinesis (terminalization complete, nuclear membrane and nucleolus disappear).
Crossing over is the mutual exchange of corresponding segments between non-sister chromatids of homologous chromosomes, and it occurs at pachytene.
In metaphase I whole bivalents align at the equator; in anaphase I the centromere does not divide and homologous chromosomes separate, which is called disjunction; telophase I gives two haploid nuclei whose chromosomes are still double.
Meiosis II — prophase II, metaphase II, anaphase II and telophase II — separates the sister chromatids and is equational, giving four nuclei in all.
Meiosis produces haploid gametes, maintains the chromosome number of the offspring after fertilization, and generates variation through crossing over and the independent orientation of bivalents.
The chief differences between mitosis and meiosis lie in the cells they occur in, the number and genetic constitution of the products, the presence of synapsis and crossing over, and the behaviour of the centromere at anaphase.
Cytokinesis divides the cytoplasm by either of two methods. In plant cells, droplets unite in the phragmoplast to form a cell plate that grows outwards and new walls are laid between two protoplasmic membranes. In animal cells and simple organisms such as fungi, a ring-like furrow forms in the plasma membrane and deepens until it cuts through the cell.
Magnifying power is the ability to enlarge an image; resolving power is the ability to distinguish two close points as separate. Resolution, set by the wavelength used, is the real limit on what a microscope can show.
A compound microscope works on the principle that the image formed by one lens acts as the object for the next, giving an enlarged, virtual and inverted image. In a bright field microscope the background is light and the object dark; in a dark field microscope, developed by Zsigmondy, an opaque disc in the condenser lets only scattered light reach the objective, so the specimen appears bright against a dark background — useful for unstainable specimens and for observing ciliary and flagellar movement.
The electron microscope uses electrons of about 0.005 nm wavelength in place of light, and so resolves detail far below the 200 nm limit of the light microscope. The TEM, invented by Knoll and Ruska, passes electrons through a thin, killed specimen and gives 2-D images of internal structure. The SEM, invented by Knoll, directs a primary beam on to the surface and forms the image from secondary electrons, giving 3-D images of surface detail such as pollen sculpturing.
The karyotype is the set of characteristics — number, size, shape and arm ratio — that identifies the chromosome set of a species; its diagrammatic representation, with the haploid set arranged in decreasing size, is the idiogram. A karyotype is symmetrical if the first and last chromosomes differ little in size, and asymmetrical if they differ greatly.
Karyotyping arrests dividing cells at metaphase with colchicine, swells them in hypotonic solution, spreads and stains them, and arranges the photographed chromosomes in order. It reveals chromosome number, sex, and numerical or gross structural abnormalities, but cannot detect changes in a single gene.
Tjio and Levan reported in 1956 that the human 2n number is 46 — 44 autosomes and two sex chromosomes. Females are XX and homogametic; males are XY and heterogametic, and are hemizygous for X-linked genes because only one X is present.
Under the Denver system, human chromosomes fall into seven groups, A to G, on the basis of size and centromere position, with the X placed in group C and the Y in group G.

Flowchart

FC 5.7 – The chapter at a glance: G₁ (growth; G₁/S checkpoint) → S (DNA replication) → G₂ (preparation; G₂/M checkpoint) → M-phase, controlled throughout by cyclin–CdK → in somatic cells: mitosis (prophase → metaphase, M checkpoint → anaphase → telophase) → cytokinesis → two identical diploid cells; in germinal cells: meiosis I (prophase I with five sub-stages → metaphase I → anaphase I → telophase I) → meiosis II → four genetically different haploid cells → studied by microscopy, mitotic index counting and karyotyping

5.10 Key Terms

Cell cycle the life period of a cell, from its formation to the completion of its own division, during which it grows, replicates its DNA and divides into two daughter cells.
Interphase the long preparatory part of the cell cycle, made up of G₁, S and G₂, during which the cell grows and copies its DNA but does not divide.
M-phase the phase of division, in which the nucleus and then the cytoplasm divide.
G₁-phase the pre-DNA synthesis phase, in which RNAs and the enzymes and substrates needed for replication are made; the most variable phase in length.
S-phase the phase of DNA synthesis, at the end of which every chromosome consists of two sister chromatids.
Histones the proteins around which DNA is packaged, synthesised during S-phase and associated with the newly replicated DNA.
Euchromatin loosely packed, gene-rich chromatin, which replicates earlier in S-phase.
Heterochromatin tightly packed, gene-poor chromatin, which replicates later in S-phase.
G₂-phase the post-DNA synthesis phase, in which cytoplasmic growth is completed and the factors needed for chromosome condensation are made.
C-value the amount of DNA, in picograms, present in a haploid set of chromosomes of a cell.
Quiescent stage (G₀) an inactive stage entered by exit from G₁, in which a cell stays metabolically active but does not divide.
Karyokinesis the division of the nucleus during M-phase.
Cytokinesis the division of the cytoplasm following nuclear division, which places the two daughter nuclei in two separate cells.
Clock theory the proposal that a cell oscillates between interphase and M-phase because of an inbuilt, genetically controlled timer.
Kinase an enzyme that attaches a phosphate group to another protein, thereby switching that protein on or off.
Cyclin dependent kinase (CdK) an enzyme that drives the cell from one phase of the cycle to the next by phosphorylating target proteins, and which becomes active only after binding a cyclin.
Cyclin a regulatory protein whose concentration rises and falls at particular stages of the cell cycle and which activates a CdK by binding to it.
M-phase kinase the cyclin–CdK complex that regulates the entry of the cell into M-phase, having a catalytic subunit and a regulatory subunit.
Checkpoint a control point at which the cell inspects its own work and is allowed to proceed only if the check is passed.
Mitosis the division of the nucleus of a somatic cell into two daughter nuclei with chromosome sets identical to that of the parent nucleus.
Somatic cells all the cells of the body except those set aside for the production of gametes.
Centromere the constricted region at which the two sister chromatids of a chromosome are held together and at which the spindle fibre attaches.
Kinetochore the protein plate built on the centromere, to which the spindle fibre is actually attached.
Spindle the framework of fibres, formed in the cytoplasm after the nuclear membrane breaks down, along which chromosomes are moved.
Equatorial plate the plane across the middle of the spindle at which chromosomes become arranged during metaphase.
Diaster the figure of two radiating clusters formed as the two groups of chromosomes converge towards the poles in anaphase.
Despiralization the uncoiling of chromosomes at telophase, by which they become thin and extended again.
Mitotic index (MI) the ratio of the number of cells undergoing mitosis to the total number of cells observed in the field at a particular time.
Meiosis a division of germinal cells in which two successive divisions follow one round of DNA replication, so that one diploid cell gives four haploid nuclei.
Chromomere a bead-like thickening seen along the slender chromosome threads at leptotene.
Synapsis the precise pairing of homologous chromosomes during zygotene.
Bivalent a pair of synapsed homologous chromosomes.
Tetrad a bivalent seen at pachytene, in which all four chromatids of the two homologues are distinguishable.
Crossing over the mutual exchange of corresponding segments between non-sister chromatids of homologous chromosomes, occurring at pachytene.
Chiasma the X-shaped point of contact at which exchanged chromatids remain joined, becoming visible at diplotene.
Terminalization the sliding of chiasmata towards the ends of the chromatids as the homologues separate.
Dyad a chromosome still consisting of two chromatids, as it moves to a pole during anaphase I.
Disjunction the separation of homologous chromosomes towards opposite poles at anaphase I.
Cell plate the partition formed at the equator of a dividing plant cell by the union of droplets, between which the new cell walls are laid.
Phragmoplast the spindle region along the equatorial plate that guides the formation of the cell plate.
Furrow the ring-like constriction of the plasma membrane by which an animal cell is cleaved into two.
Magnifying power (MP) the ability of a microscope to enlarge the image of an object, depending on the focal lengths of the eye-piece and objective and on the tube length.
Resolving power (RP) the ability of a microscope to distinguish two closely situated points as separate structures.
Compound microscope a light microscope using more than one lens, in which the image formed by one lens acts as the object for the next, giving an enlarged, virtual and inverted image.
Bright field microscope a compound microscope in which the background appears light and the stained object appears dark.
Dark field microscope a microscope, also called the ultramicroscope, in which an opaque disc in the condenser allows only light scattered by the specimen to reach the objective, so the specimen appears bright against a dark background.
Transmission Electron Microscope (TEM) an electron microscope in which the beam passes through a thin, killed specimen to give a two-dimensional image of internal structure.
Scanning Electron Microscope (SEM) an electron microscope in which a primary beam directed at the specimen surface releases secondary electrons that form a three-dimensional image of surface detail.
Primary electron beam the beam of electrons directed on to the specimen surface in an SEM.
Secondary electrons the electrons knocked out of the specimen surface by the primary beam, which are collected and amplified to form the SEM image.
Karyotype the complete set of characteristics of a species’ chromosomes — number, size, shape and arm ratio — by which that chromosome set can be identified.
Idiogram the diagrammatic representation of a karyotype, with the chromosomes of the haploid set arranged in decreasing size.
Symmetrical karyotype a karyotype in which the first and last chromosomes differ only slightly in length or size.
Asymmetrical karyotype a karyotype in which the differences in length or size between the first and last chromosomes are large.
Autosome any chromosome other than a sex chromosome; humans have 44.
Allosome a sex chromosome, also called a heterosome.
Homogametic producing gametes that are all alike in their sex chromosome, as the human female does.
Heterogametic producing two different kinds of gamete with respect to the sex chromosome, as the human male does.
Hemizygous having only a single copy of a gene rather than a pair, as a male is for genes on the X chromosome.
Denver system the classification of human chromosomes into seven groups, A to G, on the basis of size and centromere position.
SAT chromosome a chromosome bearing a satellite, a small body attached to the short arm by a thin thread.
Colchicine a chemical that prevents spindle formation and so arrests dividing cells at metaphase, used in preparing karyotypes.

5.11 Points to Ponder

A cell spends about twenty-three of every twenty-four hours in interphase and only about one hour dividing. Yet almost every diagram of the cell cycle devotes most of its space to mitosis. What does this tell you about how textbooks decide what is “important”, and what might a student miss because of it?
The checkpoints do not repair anything themselves; they only decide whether the cell may proceed. Is a system that halts work when something is wrong more valuable than one that tries to fix the fault immediately? Consider where each approach would be preferable, in a cell and in a laboratory.
Mature nerve and cardiac muscle cells stay permanently in G₀, which is why heart and nerve tissue heal so poorly. What advantage might an organism gain from having certain tissues give up the ability to divide altogether?
Crossing over deliberately breaks and rejoins DNA molecules — the very kind of damage that the G₂/M checkpoint is supposed to detect and act against. How can the same event be a fault in one context and a necessity in another?
Anti-cancer drugs that disrupt spindle formation kill dividing cells indiscriminately, which is why they also damage hair follicles, the gut lining and bone marrow. If a treatment’s selectivity depends only on how fast a cell divides, what does that imply about the side effects a patient must be counselled about before consenting to it?
A high mitotic index in a tumour biopsy is treated as bad news, while a high mitotic index in a root tip is treated as a sign of healthy growth. What extra information, beyond the number itself, does an observer need before a measurement can be interpreted at all?
A karyotype can reveal an unborn child’s sex as an incidental result of a test carried out to look for chromosomal abnormalities. Given that prenatal sex determination is a criminal offence in India, what responsibilities does this place on the laboratory that performs the test and on the doctor who receives the report?
Tjio and Levan corrected a chromosome count that had been printed in textbooks and accepted by researchers for over thirty years. How could a plainly countable number stay wrong for so long, and what does the episode suggest about the difference between an observation and an established fact?
Preparing a karyotype requires cells to be arrested, swollen, fixed and stained — the cell is destroyed in the course of being examined. In what sense is the picture that results a true representation of a living cell, and what should a biologist keep in mind when drawing conclusions from it?
Meiosis generates variation by shuffling existing alleles, not by creating new ones. If a breeder wants a genuinely new trait in a crop, what does this limitation mean for the value of conventional breeding compared with the biotechnological methods you will study later in the course?

5.12 Objective Questions

A cell that has just completed telophase but not cytokinesis differs from a normal interphase cell in that it has:

Twice the chromosome number
Two nuclei in one cytoplasm
No spindle fibres
Condensed chromosomes at the equator

The C-value of a diploid cell in G₁ is 2C. Its C-value at the beginning of prophase will be:

Histones are synthesised chiefly during:

G₁-phase
S-phase
G₂-phase
Telophase

G–C rich regions of the genome replicate earlier than A–T rich regions in some cells mainly because they are:

Held together by three hydrogen bonds
Part of active, open, gene-rich chromatin
Located nearer the centromere
Free of associated histones

The concentration that rises and falls rhythmically through the cell cycle is that of:

Cyclin dependent kinases
Cyclins
Histones
Spindle fibres

In the engine-and-gear-box comparison used by Hartwell, Hunt and Nurse, the gear box corresponds to:

The CdK
The cyclin
The centromere
The checkpoint

Which of the following is checked at the G₁/S checkpoint but not at the M checkpoint?

Spindle attachment
Chromosome alignment
Availability of nutrients
Position of the centromere

Despiralization of chromosomes is characteristic of:

Prophase
Metaphase
Anaphase
Telophase

The V, I, J or L shape assumed by chromosomes during anaphase depends on:

The length of the spindle fibres
The position of the centromere on the chromosome
The number of chromatids present
The stain used on the slide

The figure of two radiating clusters of chromosomes seen at the poles in anaphase is called the:

Bivalent
Tetrad
Diaster
Phragmoplast

Acetocarmine is used in the ISC practical on mitosis in order to:

Soften the root tip so that cells spread apart
Arrest the cells at metaphase
Stain the chromosomes so that stages can be identified
Dissolve the cell wall

In a field of 200 cells, 12 cells show condensed chromosomes. The mitotic index is:

0.6%
6%
12%
16.7%

Two fields are counted from the same slide, one containing 50 cells and the other 500. Compared with the first, the mitotic index calculated from the second field is:

Certainly ten times larger
Certainly ten times smaller
Expected to be similar but more reliable
Impossible to calculate

Bead-like structures seen along the slender chromosome threads at leptotene are called:

Chiasmata
Chromomeres
Kinetochores
Satellites

Chiasmata first become clearly visible during:

Zygotene
Pachytene
Diplotene
Diakinesis

Terminalization refers to the:

Pairing of homologous chromosomes
Sliding of chiasmata towards the ends of the chromatids
Disappearance of the nuclear membrane at diakinesis
Separation of sister chromatids at anaphase II

A cell at the end of telophase I contains nuclei that are:

Haploid, with each chromosome made of one chromatid
Haploid, with each chromosome made of two chromatids
Diploid, with each chromosome made of one chromatid
Diploid, with each chromosome made of two chromatids

The phragmoplast is associated with:

Furrow formation in animal cells
Cell plate formation in plant cells
Attachment of spindle fibres to the centromere
Organisation of the nucleolus

Cytokinesis in a plant cell proceeds:

From the periphery inwards
From the centre outwards
From one pole towards the other
Simultaneously at all points on the membrane

The property that finally limits how much fine detail a microscope can show is its:

Magnifying power
Resolving power
Tube length
Number of objectives

In a dark field microscope, the specimen appears bright against a dark background because:

The specimen is stained with a fluorescent dye
Only light scattered by the specimen reaches the objective
The objective lens is coated with an opaque film
The light source is placed above the specimen

The electron microscope resolves finer detail than the light microscope essentially because electrons have:

Greater mass
A negative charge
A much shorter wavelength
Higher energy of illumination

A researcher wishes to examine the surface sculpturing of a pollen grain in three dimensions. The suitable instrument is the:

Bright field microscope
Dark field microscope
Transmission electron microscope
Scanning electron microscope

In a scanning electron microscope, the image is actually formed by:

The primary electron beam
The secondary electrons emitted from the surface
Electrons transmitted through the specimen
Light emitted by the fluorescent screen

A karyotype represented as a diagram, with the chromosomes of a haploid set arranged in decreasing size, is called a/an:

Idiogram
Bivalent
Diaster
Tetrad

Colchicine is used in karyotype preparation because it:

Stains the chromosomes deeply
Prevents spindle formation and arrests cells at metaphase
Causes the cells to swell so chromosomes separate
Fixes the cells on the slide

Under the Denver system, the X chromosome is placed in group:

A human male is described as hemizygous for X-linked genes because he:

Has two identical sex chromosomes
Carries only one copy of each gene on the X chromosome
Produces only one type of gamete
Has an inactivated Y chromosome

A karyotype prepared from a patient’s blood sample would fail to detect:

An extra chromosome in group G
A missing sex chromosome
A change in a single base of one gene
A large piece of one chromosome attached to another

Assertion (A): Interphase is described as the resting phase of the cell. Reason (R): The cell is chemically inactive throughout interphase.
Assertion (A): A cell in G₂ has twice the DNA of a cell in G₁ but the same number of chromosomes. Reason (R): Replication produces two chromatids per chromosome without separating them.
Assertion (A): A cyclin dependent kinase has no activity of its own. Reason (R): The catalytic subunit becomes active only when a cyclin is bound to it.
Assertion (A): The G₁/S checkpoint is placed before rather than after DNA replication. Reason (R): Damage detected before S-phase can be repaired, whereas damage copied during S-phase is passed to both chromatids.
Assertion (A): Chromatin condenses into thick, short chromosomes before the chromosomes are moved. Reason (R): An extended chromatin fibre would tangle or break if dragged across the cell.
Assertion (A): The centromere remains undivided until metaphase is complete. Reason (R): Sister chromatids held together cannot be separated before the M checkpoint is cleared.
Assertion (A): Crossing over occurs during pachytene of prophase I. Reason (R): The chromosomes replicate into two chromatids each for the first time at pachytene.
Assertion (A): DNA is not replicated between meiosis I and meiosis II. Reason (R): Meiosis I separates homologous chromosomes while meiosis II separates sister chromatids that are already present.
Assertion (A): A plant cell divides its cytoplasm by forming a cell plate rather than a furrow. Reason (R): The rigid cell wall prevents constriction of the plasma membrane from outside.
Assertion (A): A high magnification is not by itself enough to show two closely placed chromosomes as separate. Reason (R): Enlarging an unresolved image only enlarges the blur.
Assertion (A): Karyotypes are usually prepared from cultured white blood cells. Reason (R): Mature red blood cells lack a nucleus and cannot be brought to metaphase.
Which of the following cell divisions produces genetically identical daughter cells?

Mitosis
Meiosis I
Meiosis II
Binary fission

The process of crossing over occurs during:

Metaphase I
Prophase I
Anaphase I
Telophase I

Which of these is not a phase of mitosis?

Prophase
Metaphase
Cytokinesis
Anaphase

A cell with 46 chromosomes undergoes mitosis. The number of chromosomes in each daughter cell will be:

Which structure ensures proper separation of chromosomes during mitosis?

Ribosome
Spindle fibre
Lysosome
Centrosome

Meiosis is essential for:

Healing wounds
Growth
Asexual reproduction
Sexual reproduction

What is the role of CdKs in the cell cycle?

DNA synthesis
Protein breakdown
Cell cycle regulation
Energy production

The phase where chromosomes line up at the equator is:

Prophase
Anaphase
Metaphase
Telophase

The mitotic index is a measure of:

Cell size
DNA content
The proportion of dividing cells
Chromosome number

In meiosis, the chromosome number is reduced to half during:

Prophase I
Anaphase I
Telophase II
Cytokinesis II

Which cell cycle phase precedes mitosis?

G₁
S
G₂
All of these

Which checkpoint ensures DNA has been replicated correctly before mitosis?

G₁
S
G₂
M

Which cells in the human body typically undergo meiosis?

Skin cells
Liver cells
Gamete-forming cells
Muscle cells

A cell observed with two distinct nuclei but no complete cell wall is in:

Anaphase
Prophase
Telophase
Metaphase

What ensures the specificity of CdK action in the cell cycle?

Their number
Their size
Their interaction with cyclins
Their RNA content

What happens if spindle fibres fail to attach to kinetochores during metaphase?

DNA damage
Chromosomes misalign
Non-disjunction
All of these

Which of the following distinguishes meiosis from mitosis?

Chromosome duplication
Cytokinesis
Pairing of homologous chromosomes
Nuclear membrane breakdown

A cell in metaphase I of meiosis has:

Chromatids aligned singly
Homologous chromosomes aligned in pairs
Random chromatid alignment
Separated chromatids

Cyclin-dependent kinases (CdKs) become active when bound to:

DNA
Cyclins
ATP
RNA

During anaphase of mitosis:

Homologous chromosomes separate
Chromatids move to opposite poles
Cytoplasm divides
Nuclear envelope reforms

In a tissue sample, 20 cells out of 100 are undergoing mitosis. The mitotic index is:

2%
20%
0.2%
80%

Meiosis results in:

Four diploid cells
Four haploid cells
Two haploid cells
Two diploid cells

DNA replication takes place during:

G₁-phase
G₂-phase
S-phase
M-phase

A high mitotic index usually indicates:

Ageing cells
Non-dividing cells
Rapid cell division
Differentiated cells

Which of the following is true for metaphase II of meiosis?

Homologous pairs align
Sister chromatids align
Crossing over occurs
Chromosomes condense

CdK activity is regulated by:

Cyclin levels
Ribosome concentration
Mitochondrial ATP
RNA synthesis

Which process leads to genetic variation in meiosis?

Spindle formation
Crossing over
Cytokinesis
Telophase

In meiosis II, sister chromatids separate during:

Metaphase II
Anaphase II
Prophase II
Telophase II

Which checkpoint ensures that the spindle fibres are properly attached?

G₁
G₂
S
M

In a dividing cell, failure of CdK inhibition leads to:

Delayed division
Uncontrolled progression
Apoptosis
DNA repair

Which event leads to formation of genetically unique gametes?

Anaphase II
Crossing over and independent assortment
Prophase II
Telophase I

Which of the following best explains how the mitotic index is useful in cancer diagnosis?

A high index indicates slow division
A high index suggests rapid cell turnover
A low index means high DNA content
It measures genetic mutation

If a meiotic cell fails to undergo cytokinesis after meiosis I, what is the result?

Two cells with diploid nuclei
One cell with two haploid nuclei
One diploid cell
Four cells with unequal chromosomes

CdKs push the cell from G₁ into S-phase by:

Activating p53
Degrading DNA polymerase
Phosphorylating target proteins
Inhibiting ATP production

What might happen if the M checkpoint is bypassed?

Arrest in S-phase
Proper chromatid separation
Unequal chromosome distribution
DNA damage repair

Telophase I of meiosis is followed by:

Prophase II
Metaphase II
Cytokinesis I
Anaphase I

Why does DNA not replicate again between meiosis I and meiosis II?

The cell does not enter interphase
S-phase is skipped
To maintain chromosome number
Because spindle formation occurs

Inhibition of CdK activity would directly result in:

Enhanced transcription
Cell cycle arrest
Cell wall rupture
ATP increase

Which of the following is a key feature of anaphase I but not anaphase II?

Separation of sister chromatids
Nuclear envelope formation
Separation of homologous chromosomes
Chromosome duplication

Which cellular component helps organise spindle fibres during mitosis?

Ribosome
Nucleolus
Centrosome
Lysosome

In meiosis, the reduction in chromosome number occurs during:

S-phase
Anaphase I
Telophase II
G₁-phase

A rapidly growing tumour shows high expression of CdK and cyclins. What does this suggest?

Reduced mitotic index
Enhanced cell cycle regulation
Uncontrolled cell proliferation
Lack of cell division

For each of the following assertion-reason questions, select the correct option:

Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
Assertion is true, but Reason is false.
Both Assertion and Reason are false.

Assertion (A): Meiosis leads to the formation of genetically diverse gametes. Reason (R): Crossing over and independent assortment occur during meiosis.
Assertion (A): Mitotic index helps estimate the number of dying cells in a tissue. Reason (R): Mitotic index is the ratio of dividing cells to total cells in a population.
Assertion (A): CdKs require binding with cyclins to become active. Reason (R): Cyclins act as inhibitors of cell cycle progression.
Assertion (A): Homologous chromosomes separate during meiosis II. Reason (R): Meiosis II resembles mitosis in the type of division.
Assertion (A): A cell in metaphase has duplicated chromosomes aligned on the equator. Reason (R): Spindle fibres help pull chromosomes towards the equator during metaphase.
Assertion (A): In mitosis, daughter cells have half the chromosome number of the parent cell. Reason (R): Mitosis includes one round of DNA replication followed by two divisions.
Assertion (A): Cells remain in the G₀-phase when they exit the cell cycle. Reason (R): G₀-phase is a resting stage where cells permanently lose the ability to divide.
Assertion (A): Meiosis contributes to maintaining a constant chromosome number across generations. Reason (R): Meiosis reduces the chromosome number to half in gametes.
Assertion (A): Cytokinesis is part of mitosis. Reason (R): Cytokinesis occurs immediately after telophase to divide the cytoplasm.
Assertion (A): CdK activity is regulated only by the presence of cyclins. Reason (R): Cyclin levels fluctuate during the cell cycle.

Fill in the blank: The phase of the cell cycle in which DNA replication occurs is called the ______ phase.
Fill in the blank: Cells that exit the cell cycle and remain metabolically active enter the ______ phase.
Fill in the blank: The division of the nucleus during cell division is known as ______.
Fill in the blank: The stage of mitosis where chromosomes align at the equatorial plane is called ______.
Fill in the blank: The regulatory proteins that control progression of the cell cycle are called ______.
Fill in the blank: The process by which cytoplasm divides to form two daughter cells is known as ______.
Fill in the blank: The exchange of genetic material between homologous chromosomes occurs during ______ of meiosis I.

5.13 Check Your Concepts

Cytoplasmic growth is described as continuous, while DNA synthesis is confined to one window of the cycle. Explain why it would be dangerous for a cell to replicate small stretches of its DNA continuously through the cycle instead.
The enzymes and nucleotides needed for replication are made in G₁, before S-phase begins. What would be the consequence if a cell entered S-phase with only half the required supply?
A yeast cell completes its cycle in about 90 minutes and a human cell in about 24 hours, yet both copy an entire genome in the process. What does this tell you about the difference between the amount of work done and the rate at which it is done?
Interphase is sometimes called the “resting phase”. Argue for or against the accuracy of this name, using specific events of G₁, S and G₂ as evidence.
Two cells are examined. One has a 2C DNA content and the other 4C, but both have the same number of chromosomes. Identify the phase of each and explain how the same chromosome number can carry different DNA amounts.
A nerve cell and a cell of the intestinal lining are both taken from the same person. Explain why one is permanently in G₀ while the other cycles every few days, and what this difference means for the repair of each tissue after injury.
A drug is developed that blocks the destruction of cyclins after each division, so that cyclin levels stay permanently high. Predict its effect on the cell cycle and explain your reasoning.
Compare and contrast cyclins and cyclin dependent kinases with respect to their chemical nature, their concentration through the cycle and their activity when alone.
The three checkpoints are placed at the end of G₁, at the end of G₂ and during metaphase. Explain, for any two of them, why that particular position is the last useful moment at which the fault they detect can still be corrected.
Match the following: | Column A | Column B | |—|—| | a. G₁/S checkpoint | i. Verifies spindle attachment and alignment | | b. G₂/M checkpoint | ii. Regulates entry of the cell into DNA synthesis | | c. M checkpoint | iii. Assesses cell size, nutrients and DNA damage | | d. S-phase kinase | iv. Confirms that replication is complete and undamaged |
Why is chromatin condensation placed at the beginning of mitosis rather than at the end? Answer in terms of what would go wrong if the order were reversed.
Arrange the following in the correct sequence and name the stage at which each occurs: chromatids move to opposite poles; nuclear membrane reappears; nuclear membrane disappears; chromosomes align at the equator; centromere divides.
The centromere is a single undivided structure at metaphase but divides at anaphase. Explain why the timing of this single event decides whether the daughter cells receive equal chromosome sets.
Fig. 5.4 shows the stages of mitosis in an animal cell. Redraw any three stages and label the following where they appear: centromere, spindle fibre, aster, nuclear membrane and nucleolus.
Vinca alkaloids prevent spindle fibres from assembling. Predict the stage at which treated cells accumulate, and explain why such drugs damage the gut lining and hair follicles as well as tumour cells.
A student prepares an onion root tip slide as follows: cuts 2 cm from the root, boils it hard in acetocarmine for five minutes, places it on a slide and views it without pressing down the coverslip. Identify three errors in this protocol and state what each would do to the result.
Explain why a slide of onion root tip shows a large majority of cells in interphase even though the tissue is one of the fastest-dividing in the plant.
Two students count the same slide. One reports a mitotic index of 0.08 and the other reports 8%. Explain whether they disagree, and state why the mitotic index has no unit.
A tumour biopsy and a growing root tip both show a high mitotic index. Explain why the same measurement is interpreted as a warning in one case and as normal in the other.
If the whole population of cells in a tissue were cycling steadily with a 24-hour cycle, roughly what mitotic index would you expect? Explain how you arrive at the figure and what an observed value of 20% would then indicate.
Explain why two successive divisions, rather than one, are needed to reduce the chromosome number to half and still give every gamete a complete set of genes.
Arrange the sub-stages of prophase I in order and state, for each, the one event that identifies it: diakinesis, pachytene, leptotene, diplotene, zygotene.
Distinguish between synapsis and pseudosynapsis, and explain why pairing between non-homologous chromosomes would cause problems at anaphase I.
A student writes that “chromosomes divide into two chromatids during pachytene”. Correct this statement and explain what actually happens at pachytene that makes the error understandable.
Crossing over and independent orientation of bivalents both generate variation, but in different ways. Distinguish between the two, and state the stage of meiosis at which each occurs.
Why does the centromere not divide during anaphase I, although it does during anaphase II? Explain what would result if it did divide at anaphase I.
Compare metaphase of mitosis, metaphase I and metaphase II with respect to what lines up on the equatorial plate in each.
Explain why a cell entering meiosis II does not pass through an S-phase first, and what the chromosome number and chromatid number of its nucleus are at that moment.
Under the microscope, how would you tell a cell in metaphase I from a cell in metaphase II of the same organism? Give two observable differences.
Compare and contrast cell plate formation and cleavage of the cytoplasm with respect to the organisms in which each occurs, the direction of division and the structure formed at the equator.
A cell completes karyokinesis but not cytokinesis, and then goes through another round of nuclear division. Describe the cell that results, and name a natural situation in which such a condition is normal.
A student proposes to view an unstained living protozoan and see its flagella moving. Explain why a bright field microscope will serve poorly and which type of light microscope should be used instead, giving the principle involved.
Distinguish between magnifying power and resolving power, and explain why increasing the first without improving the second gains a microscopist nothing.
A researcher must decide between a TEM and an SEM for two tasks: counting the internal membrane layers of a chloroplast, and examining the surface of an intact virus particle. Assign the instrument to each task and justify both choices.
Explain why the entire electron microscope must be enclosed in a vacuum chamber, and why its specimens are always non-living.
Why is metaphase, rather than any other stage, chosen as the moment at which chromosomes are photographed for a karyotype? Give two reasons.
In the karyotyping procedure, state the purpose of each of the following steps and predict what would happen if it were omitted: treatment with colchicine, treatment with hypotonic solution, and dropping the fixed cells on to the slide.
Explain why the sex of an individual is not always obvious at a glance from a karyotype, with reference to the Denver group in which the X chromosome is placed.
A person carries a mutation that changes a single base in one gene. Explain why the karyotype prepared from this person’s cells will appear entirely normal, and what this shows about the resolution of the technique.
Distinguish between homogametic, heterogametic and hemizygous, and state which of the three describes a condition rather than a type of gamete production.
Match the following: | Column A | Column B | |—|—| | a. Group A | i. Acrocentric, smallest, includes the Y chromosome | | b. Group C | ii. Metacentric, largest in size | | c. Group D | iii. Acrocentric SAT chromosomes, numbers 13 to 15 | | d. Group G | iv. Largest group, submetacentric, includes the X chromosome |
What is the cell cycle?
What is the duration of a typical human cell cycle?
Name the phase in which DNA replication occurs.
What is G₀-phase?
Why is interphase called the preparation phase?
Why is S-phase important in the cell cycle?
Why are checkpoints necessary in the cell cycle?
Why is mitosis called equational division?
Why is meiosis called reduction division?
Why is G₁-phase considered the most variable?
In which phase does the DNA content double in a cell?
In which phase of cell division do chromosomes align at the equator?
Which checkpoint prevents damaged DNA from being replicated?
Identify the type of division that produces haploid cells.
Why is prophase I the longest stage of meiosis?
Why does crossing over increase genetic variation?
Why is spindle formation essential for cell division?
Why does failure of checkpoints lead to abnormalities?
Why are homologous chromosomes separated in meiosis I?
Why is the mitotic index important in cell studies?
What happens if the G₂ checkpoint fails?
What is the effect of the absence of cyclins?
Evaluate the effect of uncontrolled cell cycle regulation.
How can a cell repair DNA damage before division? Explain.
During a lab session, a student observes that DNA replication has just been completed in a cell. Identify the phase and state one key event of this phase.
A researcher notes that a cell has just completed division and is synthesising proteins for growth. Name this phase and mention its significance.
A student observes spindle fibres forming in a dividing cell. Name the phase in which this first becomes visible.
A scientist identifies a stage where chromosomes are clearly visible and condensed. Name this stage and one characteristic feature.
A cell is prevented from entering S-phase due to insufficient nutrients. Explain why this checkpoint control is necessary.
A mutation causes a delay in G₁-phase. Explain how this affects the overall cell cycle.
During an experiment, DNA replication is found to occur earlier in euchromatin than in heterochromatin. Explain the reason.
A dividing cell shows disappearance of the nuclear membrane. Explain why this is necessary for mitosis.
A cell fails to enter M-phase despite completing DNA replication. Explain the role of regulatory molecules in this process.
A tissue sample shows many cells in G₀-phase. Predict the type of tissue and justify your answer.
A scientist observes equal distribution of chromosomes in daughter cells. Identify the division type and explain its significance.
A cell exhibits abnormal chromosome numbers after division. Analyse which stage of cell division might have been affected.
In an experiment, crossing over is inhibited during meiosis. Analyse its effect on genetic variation.
A cell shows incomplete separation of chromatids. Analyse the possible defect in cellular structures.
A mutation causes failure of homologous chromosome pairing. Analyse its effect on meiosis I.
A scientist observes unequal distribution of cytoplasm in daughter cells. Analyse the possible cause during cytokinesis.
A cell bypasses the G₁ checkpoint despite DNA damage. What are the consequences that will be faced by the organism?
A defect in the spindle checkpoint allows misaligned chromosomes to separate. Evaluate the consequences on daughter cells.
A scientist observes that certain cells remain metabolically active but do not divide. Explain the concept of G₀-phase and its biological significance.
During an experiment, a delay is observed between DNA replication and mitosis. Explain the importance of G₂-phase in preparing the cell for division.
A study shows that chromatin condenses before cell division. Explain why chromatin condensation is necessary during mitosis.
A researcher notes that meiosis results in genetic variation among offspring. Explain the mechanisms responsible for this variation.
A cell shows different durations of phases under varying environmental conditions. Explain how external factors influence regulation of the cell cycle.
A tissue sample shows rapid cell division after injury. Explain how mitosis contributes to tissue repair and regeneration in this case.
A plant root tip is examined under a microscope and shows actively dividing cells. Identify the stages likely to be observed and explain their significance.
A scientist introduces a chemical that blocks DNA replication in a cell. Explain how this affects progression through the cell cycle.
A cell shows an abnormal chromosome number after fertilization. Explain how errors during meiosis could have led to this condition.
In a classroom discussion, learners like Rohit, Fatima Hasan, Harpreet Singh and Daniel study mitosis and meiosis. They observe that mitosis produces identical daughter cells while meiosis results in genetic variation. Explain four basic differences between mitosis and meiosis, and write their biological significance.
A healthcare trainee group including Christopher, Ahaan and Charanjeet studies tissue repair after injury. They observe that cells in the damaged area divide rapidly to replace lost cells. Explain how mitosis helps in tissue repair and why it is essential for growth and maintenance.
During a genetic study, researchers like Sunita Mishra, Farhan Ali and David Jacob observe that homologous chromosomes fail to separate during meiosis. This leads to abnormal chromosome numbers in the resulting cells. Analyse this phenomenon and explain its consequences on gamete formation.
A biotechnology group including Priya Gupta, Abdul Rahman and Sonika Dolkar is designing a study to control abnormal cell growth. They plan to target regulatory mechanisms such as cyclins and CdKs. Design a strategy explaining how controlling these factors can regulate the cell cycle.

5.14 Case-Based Questions

A student prepares two slides from the same onion. The first is made from the extreme tip of a growing root, the second from a region 6 mm above the tip. Under the high power objective she counts 220 cells in a field of the first slide, of which 11 show condensed chromosomes, and 250 cells in a field of the second slide, of which 2 show condensed chromosomes.

Calculate the mitotic index for each slide and express both as percentages.
Account for the difference between the two values in terms of the tissue each slide came from.
In both fields the great majority of cells show no visible chromosomes at all. Explain why.
Suggest one change to her counting method that would make her values more reliable, and justify it.

A pathology laboratory receives two biopsy samples from the same patient — one from a suspected tumour and one from adjacent healthy tissue. The report records a mitotic index of 24% for the tumour sample and 3% for the healthy sample. Further tests show unusually high levels of cyclins and CdK in the tumour cells.

Explain what the difference in mitotic index indicates about the two tissues.
Relate the high cyclin and CdK levels to the observed mitotic index, naming the control that has been lost.
The mitotic index tells the pathologist how fast the tissue is dividing but not why. Explain what this measurement cannot establish.
Suggest, with reasoning, why a drug that inhibits CdK activity might be considered for this patient.

A patient undergoing treatment with a drug that prevents spindle fibres from assembling reports hair loss, mouth ulcers and a fall in blood cell counts within two weeks. Cells examined from a tumour sample during treatment are found accumulated at one particular stage of mitosis.

Name the stage at which the treated cells accumulate and explain why they cannot proceed beyond it.
Explain why hair follicles, the lining of the mouth and the bone marrow are affected along with the tumour.
Name one tissue in the body that would be largely unaffected by this drug and justify your choice.
Comment on what a doctor is obliged to explain to a patient before beginning such a treatment.

During a study of gamete formation, a researcher observes that in a small proportion of cells a pair of homologous chromosomes moves to the same pole instead of separating at anaphase I. Meiosis then continues normally.

Name the normal process that has failed at anaphase I.
Describe the chromosome content of the four gametes that will be produced by such a cell.
Explain what would happen to the chromosome number of a zygote formed if one of these gametes were fertilised by a normal gamete.
State the technique by which such an abnormality could be detected in the cells of the resulting individual.

A cytology laboratory is asked to examine three specimens: a stained section of liver tissue, a living unstained pond-water organism whose flagella are to be observed in motion, and an ultrathin section of a mitochondrion in which the internal membranes must be counted.

Assign the most suitable instrument to each specimen.
State the principle on which your choice for the second specimen depends.
Explain why the third specimen cannot be examined alive.
If a fourth specimen — an intact pollen grain whose surface pattern is to be studied in three dimensions — were added, which instrument would you choose and why?

A technician prepares a karyotype from a blood sample. The cells are cultured, treated with colchicine, swollen in a hypotonic solution, fixed, dropped on to slides, stained and photographed. In one preparation the chromosomes appear heaped on top of one another and cannot be counted; in another, no metaphase cells are found at all.

Identify the step most likely to have been faulty in each of the two failed preparations, and explain your reasoning.
Explain why white blood cells rather than red blood cells are used as the starting material.
State two kinds of abnormality that a successful karyotype from this sample would reveal.
State one kind of genetic change that the karyotype would fail to detect, and explain why.

A couple is referred for prenatal chromosomal testing after a routine scan raises a concern. The report will show the complete chromosome set of the foetus, and will therefore also reveal its sex, although sex is not the reason for the test. Prenatal sex determination is a criminal offence in India.

Explain why the sex of the foetus is revealed by this test even though it is not being looked for.
State what information the test is actually intended to provide.
Discuss the responsibilities this places on the laboratory and on the doctor who receives the report.
Explain why a regulation of this kind is directed at the use of a technique rather than at the technique itself.

In a culture of dividing plant cells, a chemical is added that prevents vesicles from collecting at the equator of the spindle. Nuclear division continues normally in the treated cells. After two further rounds of division, the culture is examined.

Name the stage of division that has been blocked and the structure that fails to form.
Describe what the treated cells will look like after two further rounds of nuclear division.
Explain why an animal cell treated with a chemical that prevented furrow formation would show a comparable result by a different route.
State one situation in nature in which a cell with many nuclei in a common cytoplasm is normal.

A student prepares a timeline showing the stages from cell formation to division.

Define cell cycle.
Name its two main phases.
Describe the key events of interphase.
What happens during M-phase?

During a lab demonstration, slides of mitosis are shown.

Name the four stages of mitosis.
Describe any two stages with their key features.
What happens to the nuclear membrane during mitosis?
State one significance of mitosis.

A biology model displays cell cycle checkpoints.

What is a checkpoint?
Name two checkpoints.
State their functions.
Why are checkpoints important?

A student studies DNA content during the cell cycle.

In which phase does DNA replication occur?
What happens to DNA content after S-phase?
Describe the DNA status during G₂.
Why is this change important?

A researcher studies somatic cell division.

Define mitosis.
What type of cells undergo mitosis?
Describe chromosome behaviour in anaphase.
State one importance of mitosis.

Spindle fibre formation is disrupted.

In which phase are spindle fibres important?
What is their function?
What happens to chromosome movement?
Predict the outcome in daughter cells.

A comparison between mitosis and meiosis is made.

State one difference in chromosome number.
Compare the behaviour of chromosomes.
Which produces variation?
Why is this difference important?

Unequal genetic distribution is observed.

Which stage ensures equal distribution?
What normally happens in this stage?
What could cause the imbalance?
State one consequence.

Some cells divide uncontrollably.

What regulates the cell cycle?
Name two regulatory molecules.
Analyse the failure of regulation.
What condition may result?

Cell cycle checkpoints fail due to mutation.

What is the role of checkpoints?
What happens if DNA damage is ignored?
Evaluate the long-term effects.
Name one related disorder.

Cyclin-dependent kinases are inhibited.

What is their role?
How do they regulate the cycle?
Evaluate the effect of inhibition.
How can this be useful medically?

A drug affects rapidly dividing cells.

Which cells are targeted?
Why are they affected?
Evaluate its advantages.
Evaluate its disadvantages.

Meiosis produces abnormal gametes.

What is meiosis?
What error leads to the abnormality?
Evaluate the effect on offspring.
What is its evolutionary impact?

During a biology practical session, students such as Akshay, Sunil, Feroz and Danny observe cells at different stages under a microscope. They notice that some cells are actively dividing while others appear to be growing and preparing for division. The teacher explains that the cell cycle consists of interphase and M-phase, with specific events occurring in each stage.

Define cell cycle.
Name the phases of interphase.
What happens during S-phase?
Why is interphase called the preparatory phase?

In a laboratory experiment, students such as Anjinkya, Bumrah, Samson and Rituraj treat dividing cells with a chemical that blocks DNA replication. After some time, they observe that the cells are unable to proceed further in the cell cycle. They try to identify which phase is affected and predict the consequences.

In which phase does DNA replication normally occur?
Which checkpoint will prevent further progression?
What will happen if DNA replication is incomplete?
Why are checkpoints important in the cell cycle?

In a research lab, scientists such as Ramesh Menon, Atif, Baldev Singh and Peter D’Costa observe that a mutation has disrupted spindle fibre formation in dividing cells. As a result, chromosomes fail to move properly during cell division. The team analyses the stage affected and predicts the consequences on daughter cells.

During which stage are spindle fibres most important?
What is the role of spindle fibres?
What happens if chromosomes fail to separate properly?
Predict the effect on daughter cells.

Ishan is part of a research team including Dr. Kapoor, Dr. Khan, Dr. Varghese and Dr. Norbu. The team is designing an experiment to study the effect of cell cycle regulation on cell division. Ishan is asked to propose a structured plan for the study.

Which molecules regulate the cell cycle?
Suggest one method to observe the different stages of cell division.
How will Ishan identify actively dividing cells?
Propose one way to detect abnormal cell division.

Observe the figure showing the cell cycle. It shows all the phases of the cell cycle; then answer the questions that follow.

Label all the parts.
What is the significance of phase “C” in the cell cycle?
If the C value in phase “B” is 2C, what will be the C value in phase “C” and phase “D”?
What is the importance of phase “Y”?

Observe the figures showing mitotic cell division and answer the questions that follow.

Arrange the figures showing mitosis in the correct sequence.
What is the importance of the stage represented in figure “d”?

5.15 Practice With Data

A student examines one field of a stained onion root tip preparation and records the following counts.

Appearance of cell	Number of cells
No visible chromosomes	264
Prophase	14
Metaphase	6
Anaphase	4
Telophase	12

Calculate the mitotic index and express it as a percentage.
The cell cycle in these cells lasts 20 hours. Estimate the duration of M-phase from your answer to part a, stating the assumption on which the estimate rests.
Anaphase shows the fewest cells of the four dividing stages. What does this suggest about the duration of anaphase relative to the other stages, and why?

The DNA content of a single cell was measured at intervals through one complete cycle and plotted against time. The curve stays flat at 6 pg for the first 10 hours, rises steadily from 6 pg to 12 pg between 10 and 18 hours, stays flat at 12 pg until the 23rd hour, and then falls abruptly to 6 pg.

Identify the phase of the cell cycle represented by each of the four portions of the curve.
State the C-value of this cell in G₁ and in G₂.
Explain why the fall at the 23rd hour is abrupt rather than gradual.
Sketch how the curve for chromosome number against time would differ from this curve for DNA content.

In a rapidly dividing tissue with a cycle of 24 hours, the proportion of cells found in each phase at any instant is taken to be proportional to the duration of that phase. A count of 2000 cells gives: G₁ 833 cells, S 667 cells, G₂ 417 cells, M 83 cells.

Calculate the duration of each phase in hours.
Calculate the mitotic index of this tissue as a percentage.
State one physiological condition under which the G₁ figure alone would change greatly while the others stayed nearly the same.

Mitotic indices were determined for four tissues taken from the same adult mammal.

Tissue	Mitotic index (%)
Bone marrow	12.5
Intestinal lining	9.0
Liver	0.5
Cardiac muscle	0.0

Arrange the tissues in order of decreasing rate of cell turnover.
Explain the value recorded for cardiac muscle in terms of the phase in which its cells are held.
The liver value would rise sharply if part of the liver were surgically removed. Explain how this is possible when the value is normally so low.
Predict which two tissues in this table would be most severely affected by a drug that blocks spindle formation, and justify your answer using the data.

Copy and complete the following table for a cell of an organism with a diploid number of 2n = 16, taking the DNA content in G₁ as 2C.

Stage	Number of chromosomes	Number of chromatids per chromosome	DNA content
G₁
G₂
Metaphase of mitosis
End of anaphase, at one pole
End of telophase I of meiosis, in one nucleus
Metaphase II
A gamete

The following counts were made from a haemocytometer-free field of a cell culture treated with a spindle inhibitor. Samples were taken every two hours and the percentage of cells in metaphase was recorded: 0 h — 1.5%; 2 h — 6.0%; 4 h — 14.5%; 6 h — 23.0%; 8 h — 28.5%; 10 h — 29.0%.

Describe the trend shown by the data.
Explain, in terms of the action of the drug, why the percentage rises.
Suggest why the value almost stops rising between 8 and 10 hours.
Predict what the readings would show if the drug were washed out at 6 hours and counting continued.

In an organism, three pairs of homologous chromosomes orient independently of one another at metaphase I. The number of different chromosome combinations possible in the gametes from independent orientation alone is given by \(2^n\), where \(n\) is the number of homologous pairs.

Calculate the number of possible combinations for this organism.
Calculate the corresponding number for a human gamete, taking \(n = 23\), and express the answer in standard form.
Explain why the actual number of genetically different gametes a person can produce is far greater than the figure calculated in part b.

A compound microscope is fitted with a 10 X eye-piece and three objectives of 10 X, 40 X and 100 X. The maximum resolution obtainable with it in practice is 200 nm, while an electron microscope achieves 0.5 nm.

Calculate the total magnification obtained with each of the three objectives.
Calculate how many times better the resolution of the electron microscope is than that of the light microscope.
A structure 80 nm across is present in a specimen. State, with a reason, which of the two instruments can show it as a distinct object.
Explain why fitting a 200 X eye-piece to the light microscope would not solve the problem in part c.

A karyotype prepared from a patient’s cells is counted and the chromosomes are sorted by the Denver system. The totals recorded are: group A — 6; group B — 4; group C — 15; group D — 6; group E — 6; group F — 4; group G — 6. No Y chromosome is present.

Calculate the total chromosome number in this karyotype.
Compare each group total with that expected in a normal female karyotype and identify the group in which the count is abnormal.
State the meiotic error that could produce a gamete leading to this karyotype.
Explain why the absence of a Y chromosome must be read together with the group C count rather than on its own.

Two students count the mitotic index of the same slide independently. Student P examines one field of 40 cells and finds 4 dividing, reporting 10.0%. Student Q examines eight fields totalling 400 cells and finds 26 dividing.

Calculate Student Q’s mitotic index.
Calculate what Student P’s value would have become if she had found one dividing cell more, and what it would have become with one fewer.
Repeat the calculation in part b for Student Q’s count.
Using your figures, explain why the number of cells counted matters even though the mitotic index is a ratio.