Chapter 7: Cell Division
In the early 1880s, a German anatomist named Walther Flemming sat at his microscope with a problem. Cells were nearly transparent. Whatever happened inside them when they multiplied, nobody could see it.
Chemists of that time had just begun making brilliant synthetic aniline dyes for the cloth industry. Flemming borrowed them for biology. When he flooded his cells with dye, something inside the nucleus soaked up the colour far more strongly than the rest. He called this dye-loving material chromatin, from the Greek word for colour.
He then studied the cells of salamander larvae, tissue in which cells divide often. Cell after cell, he saw the same sequence. The stained material tightened into short, distinct threads. The threads lined up neatly across the middle of the cell. Each thread split lengthwise, and the halves were drawn steadily to opposite ends, as if pulled by invisible ropes. Then the cell pinched into two.
Flemming had no idea that these threads carried genes. Yet, by piecing together thousands of still images of dead, stained cells, he had reconstructed a moving process. He named it mitosis, from the Greek word for thread.
Every cell in your body, all of them descended from a single fertilised egg, has been through that same choreography.
Diagram
Fig. 7.0 – A 19th-century microscope beside three stained salamander cells showing threads condensing, aligning at the middle, and separating to opposite poles; label chromatin threads, equator, poles.
Keep these questions in mind as you read. This chapter will answer every one of them.
7.1 Cell Cycle
Where Every Life Begins
The life of every multicellular organism begins as a single cell — the zygote, the cell formed when a male and a female gamete fuse. From that one cell arises an entire body of many types of cells. Growth in every living organism depends on cell division.
This idea is not a guess. It is stated in the cell theory: every cell arises from a pre-existing cell. No cell assembles itself from raw chemicals; it is always produced by the division of a cell that already existed.
The Cell Cycle and Its Two Phases
A dividing cell does not divide continuously. It passes through a fixed, repeating series of events — grow, prepare, divide, grow again. This ordered series of events occurring in the life of a dividing cell is called the cell cycle.
The cell cycle has two phases: interphase and M-phase. During interphase the cell grows or rests as the need may be. During M-phase the cell undergoes actual division. Interphase alternates with the period of division, as shown in Fig. 7.1 and summarised in FC 7.1.
Diagram
Fig. 7.1 – Circular chart of the cell cycle; label G₁ (8 hours or more), S (6–8 hours), G₂ (2–5 hours) inside interphase, and mitosis (1–3 hours) with prophase, metaphase, anaphase, telophase and cytokinesis; mark cell growth, organelle duplication, DNA replication, protein synthesis, and daughter cells.
Flowchart
FC 7.1 – The cell cycle: G₁ phase → S phase → G₂ phase → M-phase (karyokinesis → cytokinesis) → two daughter cells → back to G₁ of the next cycle
Interphase: The Longest, Busiest Phase
Interphase is the stage between two successive cell divisions. It is the longest phase of the cell cycle. Under the microscope, an interphase cell looks quiet — but this appearance is misleading.
During interphase the cell is highly active. It is growing, making proteins, duplicating its organelles and copying its DNA, so that it is fully prepared for the division ahead. Interphase is divisible into three sub-phases: \(G_1\)-phase, S-phase and \(G_2\)-phase.
The Three Sub-Phases of Interphase
The \(G_1\)-phase is also known as the first gap period or first growth period. It starts immediately after cell division. During this phase the cell performs RNA synthesis (mRNA, rRNA and t-RNA), protein synthesis, and synthesis of membranes. In short, the newly formed daughter cell rebuilds itself to full working size.
The S-phase is the synthesis phase, in which DNA is synthesized or replicated, so that the amount of DNA per cell doubles. Histone proteins — the proteins around which DNA is packaged — are also synthesized during this phase, since the new DNA copy needs its own packaging.
An important point for exams: although the DNA content doubles in S-phase, the chromosome number does not change. Each chromosome now simply consists of two identical sister chromatids held together, waiting to be separated later.
The \(G_2\)-phase is the second growth phase, during which the nucleus increases in volume. Metabolic activities essential for cell division occur during this phase, and the various proteins necessary for cell division are synthesized. Besides this, RNA synthesis also occurs during \(G_2\). In animal cells, a daughter pair of centrioles appears near the pre-existing pair.
M-Phase: The Period of Division
M-phase is the period of division. Here ‘M’ stands for mitosis or meiosis. M-phase involves two events that follow one another: karyokinesis and cytokinesis.
Karyokinesis is the division of the nucleus into two daughter nuclei. Cytokinesis is the division of the cytoplasm, resulting in two daughter cells. The nucleus is dealt with first because the genetic material must be sorted accurately before the cell body is split.
In NEET and board papers, this section is most often tested through direct phase-identification questions — which phase DNA doubles in, which phase histones are made in, which is the longest phase — and through assertion–reason statements about interphase being an active rather than a resting stage. Learn the sub-phases in strict order, along with the one event that is unique to each.
7.2 Types of Cell Division
Three Kinds of Cell Division
Not every cell divides in the same way. Three kinds of cell division are found in animal cells: amitosis or direct division, mitosis or indirect division, and meiosis or reductional division. They differ in how carefully the genetic material is handled, and therefore in what kind of daughter cells they produce.
There is an important restriction on which cells can perform which division. Mitosis can be performed by haploid as well as diploid cells, but meiosis can be performed by diploid cells only. The reason is simple: meiosis works by pairing homologous chromosomes, and a haploid cell has only one member of each pair, so there is nothing to pair with.
Here it is worth recalling two terms from your earlier study. A diploid cell (written \(2n\)) carries two sets of chromosomes, one inherited from each parent, so its chromosomes exist as homologous pairs. A haploid cell (written \(n\)) carries only one set. Homologous chromosomes are the two chromosomes of a pair that are alike in size and shape and carry genes for the same characters — one of maternal and one of paternal origin.
That haploid cells can divide mitotically is not merely theoretical. In the honey bee, drones develop from haploid unfertilized eggs, and every cell of the drone’s body is produced by mitosis of haploid cells. In Marchantia, a liverwort, haploid spores form the gametophyte by mitosis.
Flowchart
FC 7.2 – Types of cell division in animal cells: cell division → branches into amitosis (direct; haploid or diploid cells) / mitosis (indirect; haploid or diploid cells → 2 identical cells) / meiosis (reductional; diploid cells only → 4 haploid cells)
Amitosis: The Direct Division
Amitosis is the simplest mode of cell division. Nothing elaborate happens — no spindle, no condensed chromosomes, no orderly line-up.
During amitosis, the nucleus elongates and a constriction appears somewhere along its length. This constriction deepens and divides the nucleus unequally into two daughter nuclei. This is followed by the division of the cytoplasm, which results in the formation of two daughter cells. The sequence is shown in Fig. 7.12.
Diagram
Fig. 7.12 – Amitosis in three stages: rounded nucleus → elongated nucleus with a constriction in the middle → two unequal daughter nuclei followed by division of cytoplasm into two daughter cells; label nucleus, constriction, daughter nuclei, daughter cells.
Notice the word unequally. Because there is no spindle to measure out the chromosomes, the two daughter nuclei need not receive identical amounts of genetic material. This division therefore occurs in unicellular organisms, abnormal cells, old cells and in foetal membrane cells — situations in which an exact genetic copy is not essential.
Mitosis: The Indirect Division
Mitosis is a type of cell division in which a cell divides to form two daughter cells that are identical to each other and identical to the parent cell. This is why it is also called equational division — the chromosome number of the daughter cells equals that of the parent.
Mitosis is completed in two steps: karyokinesis and cytokinesis. Karyokinesis is the nuclear division, and it is sub-divided into prophase, metaphase, anaphase and telophase.
Although these four steps are used for the sake of convenience, it must be remembered that mitosis is a continuous process. It starts with the disappearance of the nuclear membrane in prophase and ends with the separation of two fully formed cells after cytokinesis. The stages have no walls between them; a cell slides from one into the next. The complete sequence is set out in FC 7.3.
Flowchart
FC 7.3 – Mitosis: interphase (chromosomes already duplicated) → prophase → metaphase → anaphase → telophase (karyokinesis complete) → cytokinesis → two identical daughter cells → back to interphase
Prophase
This phase involves condensation of chromatin material, migration of centrioles, appearance of the mitotic apparatus and disappearance of the nuclear membrane. Prophase is essentially the packing-up stage: the long, tangled chromatin is folded into short, sturdy bodies that can be moved without breaking.
Due to this condensation, each chromosome becomes visible under the light microscope, and can be seen with its sister-chromatids connected by the centromere. The two chromatids are the two identical copies made during the S-phase of interphase, still joined at the centromere.
The nucleolus starts to disappear. The nuclear membrane disintegrates and disappears gradually — a necessary step, because as long as the nucleus is enclosed, the spindle fibres of the cytoplasm cannot reach the chromosomes. Both early and late prophase are shown in Fig. 7.2.
Diagram
Fig. 7.2 – Early prophase and late prophase; label centrioles, nucleolus, nuclear membrane, chromosome, condensed chromosomes, and the disappearing nuclear membrane.
The centrosome, which had undergone duplication during interphase, begins to move towards opposite poles of the cell. By the end of prophase the mitotic apparatus is almost completely formed.
Metaphase
In this phase, chromosomes are completely condensed, so that they appear very short. The sister-chromatids and the centromere become very prominent. Being at their most compact, this is the stage at which chromosomes are best studied and photographed.
All the chromosomes lie at the equatorial plane of the cell. This is called the metaphase plate. The mitotic spindle is fully formed, and spindle fibres are attached to the centromere. The arrangement is shown in Fig. 7.3.
Diagram
Fig. 7.3 – Metaphase; label spindle fibres, metaphase plate with chromosomes aligned at the equator, and the poles of the spindle.
The centromere of each chromosome divides horizontally (that is, parallel to the long axis) into two, each half being associated with one chromatid. Each chromatid is now attached to its own spindle fibre from its own pole, and is ready to be pulled away.
Anaphase
The chromatids of each chromosome separate and form two chromosomes, called daughter chromosomes. From this moment the cell contains twice its usual number of chromosomes, though only for a few minutes.
The formed chromosomes are pulled away in opposite directions by the spindle apparatus. Chromosomes being pulled away appear like a bunch of bananas during anaphase, because the centromere leads the way and the two arms trail behind it. This is visible in Fig. 7.4.
Diagram
Fig. 7.4 – Anaphase; label chromatids moving to opposite poles, spindle fibres, and the V- or J-shaped bending of chromosome arms behind the centromere.
Each set of chromosomes reaching the opposite poles of the cell marks the end of anaphase.
Telophase
Telophase is the final stage of karyokinesis, and it undoes almost everything prophase did. The chromosomes, with their centromeres at the poles, begin to uncoil, lengthen and lose their individuality — they can no longer be told apart, and become the diffuse chromatin of the next interphase.
The nucleolus begins to reappear. The nuclear membrane begins to appear around the chromosomes at each pole. Spindle fibres break down and get absorbed in the cytoplasm. Thus two daughter nuclei are formed in a cell, as seen in Fig. 7.5.
Diagram
Fig. 7.5 – Telophase; label the two newly formed daughter nuclei, reappearing nuclear membrane and nucleolus, and the still-undivided cytoplasm.
Cytokinesis
The division of the cytoplasm into two daughter cells is called cytokinesis. In animal cells, the division starts with a constriction, generally at the equator. This constriction gradually deepens and ultimately joins in the centre, dividing the cell into two daughter cells. This process of division of the cytoplasm is perpendicular to the spindle, which ensures that the cut passes between the two new nuclei rather than through either of them. This mechanism of cytokinesis is characteristic of animal cells.
Plant cells, however, are covered by a relatively non-flexible cell wall. Due to this, a furrow cannot be formed — a rigid wall cannot be squeezed inwards. Instead, a cell wall or partition starts to appear at the centre of the cell and grows outward to meet the existing lateral walls.
The formation of the new cell wall begins with the formation of a simple precursor called the ‘cell-plate’, which represents the middle lamella between the walls of two adjacent cells. Both mechanisms are compared in Fig. 7.6.
Diagram
Fig. 7.6 – Cytokinesis in an animal cell (cleavage furrow, contractile ring of microfilaments, daughter cells) beside cytokinesis in a plant cell (vesicles forming cell plate, wall of parent cell, cell plate, new cell wall, daughter cells).
At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. Since these organelles are present in large numbers, an approximately equal sharing is enough — unlike chromosomes, which must be distributed exactly.
Significance of Mitosis
As mitosis is an equational division, the chromosome number is maintained constant. It ensures equal distribution of the nuclear and the cytoplasmic content between the daughter cells, both quantitatively and qualitatively. The hereditary material (DNA) is also equally distributed.
Mitosis helps in the growth and development of organisms. Old and worn-out cells are replaced through mitosis. It also helps in the asexual reproduction of organisms and in vegetative propagation in plants.
The process of mitosis also maintains the nucleo-cytoplasmic ratio. As a cell grows, its cytoplasm increases far faster than the ability of its nucleus to control that cytoplasm; by dividing, the cell restores a workable balance between nuclear and cytoplasmic volume.
Mitosis is a very reliable process for preserving the genetic make-up of cells or organisms, but it cannot introduce variation or new combinations of existing genes. Reliability is its strength and also its limit.
Death of a Cell
You may think that it is bad for cells in your body to die. In many cases, that is true: it is not good for cells to die because of an injury — for example, due to a scrape or a harmful chemical. Such death is called necrosis.
However, some cells of our body die, not randomly, but in a carefully controlled way. For example, during embryonic development the cells between the embryonic fingers (digits) die naturally, by a process called apoptosis, to give a definite shape to the fingers. Before this, the developing hand is a paddle-shaped plate; the fingers are separated by removing the cells in between them.
This is a common form of programmed cell death, where cells undergo “cellular suicide” when they receive certain signals. Apoptosis involves the death of cells, but it benefits the organism as a whole — for instance, by letting fingers develop, or by eliminating potential cancer cells.
Meiosis: The Reductional Division
Meiosis takes place only in reproductive cells, during the formation of gametes. By this division, the number of chromosomes is reduced to half of the total number, and hence it is also called reductional division. The cells in which meiosis takes place are termed meiocytes.
Meiosis produces four haploid daughter cells from a diploid parent cell. Note the contrast with mitosis at once: one round of DNA replication is followed by two rounds of division, which is exactly how the chromosome number is halved.
Meiosis is of two types of divisions:
- First meiotic division or Heterotypic division — Meiosis I
- Second meiotic division or Homotypic division — Meiosis II
Flowchart
FC 7.4 – Overall course of meiosis: diploid meiocyte (\(2n\), DNA replicated) → Meiosis I (prophase-I → metaphase-I → anaphase-I → telophase-I → cytokinesis-I) → 2 haploid cells (\(n\)) → interkinesis (short, sometimes absent) → Meiosis II (prophase-II → metaphase-II → anaphase-II → telophase-II → cytokinesis-II) → 4 haploid cells (\(n\))
Meiosis I — Prophase I
During the first meiotic division, a diploid cell is divided into two haploid cells. The chromosome number is reduced to half of the total number in the daughter cells. Hence this division is also called heterotypic division — the daughter cells are of a different chromosome type from the parent. It consists of the phases prophase-I, metaphase-I, anaphase-I, telophase-I and cytokinesis-I.
Prophase-I has a longer duration than any other phase of meiosis. The significant features which are peculiar to meiosis occur in this phase. This phase can be sub-divided into five sub-stages: leptotene, zygotene, pachytene, diplotene and diakinesis, shown in order in Fig. 7.7 and FC 7.5.
Diagram
Fig. 7.7 – Prophase I in five panels, labelled leptotene, zygotene, pachytene, diplotene and diakinesis; show long thread-like chromosomes, paired homologues, tetrads with four chromatids, chiasmata, and terminalised chiasmata.
Flowchart
FC 7.5 – Sub-stages of prophase-I: leptotene (bouquet stage) → zygotene (synapsis, synaptonemal complex) → pachytene (tetrads, crossing-over at chiasmata) → diplotene (desynapsis, chiasmata visible) → diakinesis (terminalization of chiasmata; nucleolus and nuclear membrane disappear)
In leptotene, the volume of the nucleus increases. The chromosomes become distinct, long, thread-like and coiled. They take up a specific orientation — the ‘bouquet stage’ inside the nucleus. This is characterised by the ends of the chromosomes being converged towards that side of the nucleus where the centrosome lies, so that the chromosomes look like the gathered stems of a bouquet of flowers. The centrosome duplicates and migrates to opposite poles.
In zygotene, intimate pairing of homologous chromosomes takes place by the formation of a synaptonemal complex. This pairing is called synapsis. Each pair consists of a maternal chromosome and a paternal chromosome. The pairing is precise, gene for gene, along the whole length of the two chromosomes.
In pachytene, each individual homologue (chromosome) has two similar chromatids. At this stage, tetrads (tetravalents) become clearer in appearance because of the presence of four visible chromatids. The homologous chromosomes of each pair begin to separate from each other. However, they do not completely separate but remain attached together at one or more points.
These points appear like a cross (X) and are known as chiasmata. Chromatids break at these points, and the broken segments are exchanged between non-sister chromatids of homologous chromosomes. This is called crossing-over or recombination.
Crossing-over is the single most important event of meiosis from the point of view of heredity. It takes genes that entered the organism from the mother and genes that entered from the father, and produces chromosomes that carry a mixture of both. No such chromosome existed in either parent.
In diplotene, although chiasmata are formed in pachytene, they become clearly visible in diplotene, due to the beginning of repulsion between the synapsed homologous chromosomes. This is called desynapsis. It involves the disappearance of the synaptonemal complex. The homologues now try to move apart, and remain held only where the chiasmata are — which is precisely why the chiasmata become easy to see.
In diakinesis, the chiasmata begin to move along the length of the chromosomes, from the centromere towards the ends of the chromosomes. This displacement of chiasmata is termed terminalization. The terminal chiasmata exist till metaphase, holding the homologues loosely together until the moment they are required to separate.
In this sub-stage the nucleolus disappears and the nuclear membrane also begins to disappear. Spindle fibres start to appear in the cytoplasm, so that the cell is ready to arrange its chromosomes at the equator.
Meiosis I — The Later Phases
In metaphase-I, the spindle fibres become well developed. The tetrads move towards the equator and orient themselves on the equator in such a way that the centromeres of homologous tetrads lie towards the poles and the arms towards the equator. Due to increasing repulsive forces between the homologous chromosomes, they are ready to separate from each other. This arrangement is shown in Fig. 7.8.
Diagram
Fig. 7.8 – Metaphase-I; label the pole, a tetrad at the equator, recombined chromosomes, and spindle fibres running from the poles to the centromeres.
Compare this carefully with metaphase of mitosis. In mitosis, single chromosomes line up on the plate; in metaphase-I, whole pairs line up, one member of each pair facing each pole. This one difference is the reason the chromosome number is halved in the division that follows.
In anaphase-I, homologous chromosomes are pulled away from each other and carried towards opposite poles by the spindle apparatus. This separation is called disjunction. The two sister chromatids of each chromosome do not separate in meiosis-I, as seen in Fig. 7.9.
Diagram
Fig. 7.9 – Anaphase-I; label astral rays, spindle apparatus, interpolar fibres, chromosomal fibres, and chromosomes with sister chromatids still attached moving to opposite poles.
This is the reductional division. The sister chromatids of each chromosome are connected by a common centromere. Both sister chromatids of each chromosome are now different in terms of genetic content, as one of them has undergone recombination.
In telophase-I, the haploid number of chromosomes, after reaching their respective poles, become uncoiled and elongated. The nuclear membrane and the nucleolus reappear, and thus two daughter nuclei are formed.
In cytokinesis-I, after the karyokinesis, cytokinesis occurs and two haploid cells are formed.
In many cases these daughter cells pass through a short resting phase, or interphase / interkinesis. It is important to note that no DNA replication occurs in interkinesis — that is why the second division can reduce nothing further and simply separates chromatids. In some cases the changes of the telophase may not occur at all, and anaphase-I leads directly to the prophase of meiosis II.
Diagram
Fig. 7.10 – Cytokinesis-I; label the two daughter cells and the reappearing nuclear membrane around each haploid nucleus.
Meiosis II
During this division, the two haploid cells formed during the first meiotic division divide further into four haploid cells. This division is similar to mitosis. The daughter cells formed in the second meiotic division are identical to their parent cells with respect to the chromosome number formed in meiosis-I. Hence this division is called homotypic division — the daughter cells are of the same chromosome type as the cells that produced them.
It consists of the following phases: prophase-II, metaphase-II, anaphase-II, telophase-II and cytokinesis-II. The whole of meiosis II is shown in Fig. 7.11.
Diagram
Fig. 7.11 – Meiosis II; show both cells from telophase-I (and cytokinesis) passing through prophase II, metaphase II, anaphase II and telophase II, ending in four haploid cells.
In prophase-II, the chromosomes are distinct, with two chromatids. Each centriole divides into two, resulting in the formation of two centrioles which migrate to opposite poles and form asters. Spindle fibres are formed between the centrioles. The nuclear membrane and nucleolus disappear.
In metaphase-II, chromosomes get arranged at the equator. The two chromatids of each chromosome are separated by the division of the centromere. Some spindle fibres are attached to the centromeres, and some are arranged end to end between two opposite centrioles.
In anaphase-II, the separated chromatids become daughter chromosomes and move to opposite poles, due to the contraction of the spindle fibres attached to the centromeres. This is the moment the sister chromatids — which stayed together throughout meiosis I — are finally parted.
In telophase-II, the daughter chromosomes uncoil. The nuclear membrane surrounds each group of chromosomes and the nucleolus reappears.
In cytokinesis-II, cytokinesis occurs after nuclear division. Two haploid cells are formed from each haploid cell. Thus, in all, four haploid daughter cells are formed. These cells undergo further changes to develop into gametes.
Significance of Meiosis
Meiotic division produces gametes or spores. If meiosis did not operate, the number of chromosomes would double or quadruple with every generation, resulting in the formation of monstrosities (abnormal forms).
The constant number of chromosomes in a given species across generations is maintained by meiosis and fertilization together. Meiosis halves the number, fertilization restores it — the two processes balance each other exactly.
Because of crossing over, exchange of genetic material takes place, leading to genetic variations, which are the raw materials for evolution. Natural selection can only act on differences that already exist in a population; meiosis is the process that keeps producing them.
Flowchart
FC 7.6 – Constancy of chromosome number across generations: diploid parent (\(2n\)) → meiosis → gametes (\(n\)) → fertilization → zygote (\(2n\)) → mitosis → diploid body cells of the offspring (\(2n\)) → back to meiosis in the next generation
In examinations, this section is tested most often through stage identification from a diagram, through sequence-ordering of the sub-stages of prophase-I, through match-the-following items pairing each sub-stage with its event (bouquet stage, synapsis, crossing over, desynapsis, terminalization), and through assertion–reason questions on why meiosis-I is reductional while meiosis-II is not. Learn each stage by the one event that occurs only in it.
7.3 Summary
Every multicellular organism begins as a single cell, the zygote, and grows by cell division. As the cell theory states, every cell arises from a pre-existing cell.
The cell cycle is the sequence of events occurring in the life of a dividing cell. It has two phases: interphase (growth or rest) and M-phase (actual division). Interphase alternates with the period of division.
Interphase is the longest phase of the cell cycle, and the cell is highly active during it. It has three sub-phases: \(G_1\), S and \(G_2\).
\(G_1\)-phase, the first gap or first growth period, begins immediately after cell division. RNA synthesis (mRNA, rRNA, t-RNA), protein synthesis and membrane synthesis occur in it.
S-phase is the synthesis phase, in which DNA is replicated so that the amount of DNA per cell doubles. Histone proteins are also synthesized in this phase.
\(G_2\)-phase is the second growth phase. The nucleus increases in volume, metabolic activities and proteins essential for division are produced, RNA synthesis continues, and in animal cells a daughter pair of centrioles appears near the pre-existing pair.
M-phase, in which ‘M’ stands for mitosis or meiosis, involves karyokinesis (division of the nucleus into two daughter nuclei) followed by cytokinesis (division of the cytoplasm into two daughter cells).
Flowchart
FC 7.7 – Whole-chapter sequence: cell cycle → interphase (\(G_1\) → S → \(G_2\)) → M-phase → branches into amitosis (unequal nuclear constriction → 2 cells) / mitosis (prophase → metaphase → anaphase → telophase → cytokinesis → 2 identical cells) / meiosis (meiosis I: prophase-I → metaphase-I → anaphase-I → telophase-I → cytokinesis-I → 2 haploid cells → meiosis II: prophase-II → metaphase-II → anaphase-II → telophase-II → cytokinesis-II → 4 haploid cells)
Three kinds of cell division occur in animal cells: amitosis (direct), mitosis (indirect) and meiosis (reductional). Mitosis can be performed by haploid as well as diploid cells, but meiosis can be performed by diploid cells only. Drones of the honey bee develop from haploid unfertilized eggs, and in Marchantia haploid spores form the gametophyte by mitosis.
In amitosis, the nucleus elongates, a constriction deepens and divides it unequally into two daughter nuclei, after which the cytoplasm divides. It occurs in unicellular organisms, abnormal cells, old cells and foetal membrane cells.
Mitosis produces two daughter cells identical to each other and to the parent cell. It is completed in two steps — karyokinesis and cytokinesis — and is a continuous process, beginning with the disappearance of the nuclear membrane in prophase and ending with the separation of two fully formed cells after cytokinesis.
Prophase: chromatin condenses, chromosomes become visible with sister chromatids joined by the centromere, centrioles migrate, the mitotic apparatus appears, and the nucleolus and nuclear membrane disappear.
Metaphase: chromosomes are fully condensed and lie at the equatorial plane, forming the metaphase plate. The spindle is fully formed and spindle fibres are attached to the centromeres. The centromere of each chromosome divides horizontally into two, each half associated with one chromatid.
Anaphase: the chromatids of each chromosome separate to form daughter chromosomes, which are pulled to opposite poles by the spindle apparatus. Kinetochore fibres contract while polar fibres elongate. Anaphase ends when each set reaches its pole.
Telophase: chromosomes uncoil, lengthen and lose their individuality; the nucleolus and nuclear membrane reappear; spindle fibres break down. Two daughter nuclei are formed.
Cytokinesis in animal cells occurs by a constriction at the equator, perpendicular to the spindle, that deepens and joins at the centre. In plant cells, the non-flexible cell wall prevents a furrow, so a cell-plate — representing the middle lamella — forms at the centre and grows outward to meet the lateral walls. Organelles such as mitochondria and plastids are distributed between the daughter cells.
Significance of mitosis: it is equational, so chromosome number stays constant; nuclear and cytoplasmic contents and the hereditary material are distributed equally, both quantitatively and qualitatively; it supports growth and development, replaces old and worn-out cells, enables asexual reproduction and vegetative propagation, and maintains the nucleo-cytoplasmic ratio. It preserves genetic make-up reliably but cannot introduce variation or new gene combinations.
Cell death may be necrosis, caused by injury such as a scrape or a harmful chemical, or apoptosis, a controlled programmed cell death. Apoptosis shapes the embryonic fingers by removing the cells between the digits and also eliminates potential cancer cells, benefiting the organism as a whole.
Meiosis, a term coined by J. B. Farmer in 1905, occurs only in reproductive cells (meiocytes) during gamete formation. It halves the chromosome number and produces four haploid daughter cells from one diploid parent cell, through meiosis I and meiosis II.
Meiosis I (heterotypic division) reduces a diploid cell to two haploid cells. Its phases are prophase-I, metaphase-I, anaphase-I, telophase-I and cytokinesis-I.
Prophase-I is long and has five sub-stages: leptotene (nucleus enlarges; chromosomes become long, thread-like and coiled in the bouquet stage; centrosome duplicates and migrates), zygotene (synapsis of homologous chromosomes through the synaptonemal complex), pachytene (tetrads of four chromatids visible; crossing-over between non-sister chromatids at the chiasmata), diplotene (desynapsis, disappearance of the synaptonemal complex, chiasmata clearly visible) and diakinesis (terminalization of chiasmata; nucleolus and nuclear membrane disappear; spindle fibres appear).
Metaphase-I: tetrads move to the equator with centromeres of homologous tetrads towards the poles and arms towards the equator, and repulsive forces prepare them to separate.
Anaphase-I: homologous chromosomes are pulled to opposite poles — this is disjunction — while the two sister chromatids of each chromosome do not separate. The sister chromatids now differ genetically because one has undergone recombination.
Telophase-I: the haploid sets uncoil and elongate, and the nuclear membrane and nucleolus reappear, forming two daughter nuclei. Cytokinesis-I then produces two haploid cells, which may pass through a short interphase or interkinesis; sometimes telophase is skipped and anaphase-I leads directly into prophase of meiosis II.
Meiosis II (homotypic division) is similar to mitosis and divides the two haploid cells into four. Prophase-II: chromosomes with two chromatids become distinct, centrioles divide and migrate, asters and spindle fibres form, and the nuclear membrane and nucleolus disappear. Metaphase-II: chromosomes arrange at the equator and the centromere divides to separate the two chromatids. Anaphase-II: the separated chromatids become daughter chromosomes and move to opposite poles. Telophase-II: chromosomes uncoil, the nuclear membrane surrounds each group and the nucleolus reappears. Cytokinesis-II gives four haploid daughter cells in all, which develop further into gametes.
Significance of meiosis: it produces gametes or spores; without it, chromosome number would double or quadruple, giving monstrosities. Meiosis together with fertilization keeps the chromosome number of a species constant across generations, and crossing over produces genetic variation, the raw material for evolution.
7.4 Points to Ponder
A cell spends most of its cycle in interphase and only a short while in actual division. If almost all of a cell’s life is preparation, is “the cell cycle” better thought of as a cycle of division, or as a cycle of growth interrupted by division?
Amitosis is quick and needs no spindle, no condensation and no elaborate machinery. Why, then, do the cells that build and repair a multicellular body always take the longer, costlier route of mitosis?
In meiosis, DNA is replicated once but the cell divides twice. What would result if a cell replicated its DNA once and divided only once? What if it replicated twice and divided twice?
Crossing-over involves deliberately breaking chromatids and rejoining the pieces to a different chromatid. In almost any other situation, breaking a chromosome would be an injury. Why is a cell willing to take this risk during pachytene?
Animal cells divide their cytoplasm by pinching inwards, plant cells by building a partition outwards. The outcome is the same, the method is opposite. What does this tell you about how the structure of a cell decides the mechanism it can use?
Mitosis copies faithfully and cannot introduce variation; meiosis introduces variation but cannot, by itself, build a body. Could a species survive in the long run with only one of the two processes?
The chromosome number of a species is described as constant across generations, and yet it is halved in every gamete and restored at every fertilization. In what sense is such a number really “constant”?
Apoptosis shapes the fingers of an embryo by removing cells rather than adding them. Where else in living systems does building something depend on taking something away?
If a cell lost all control over when it may enter S-phase and M-phase, what would happen to the tissue it belongs to? Should every cell of a body be free to divide whenever it can?
Meiosis cannot occur in a haploid cell. Think carefully about which single event of prophase-I makes this impossible, and what that event demands of the cell.
Under the microscope, most cells on any slide of dividing tissue appear to be in interphase. Does this mean the tissue is mostly inactive, or is the observation telling us something about time rather than about activity?
7.5 Check Your Concepts
Interphase is often described as the “resting phase” of the cell cycle. Explain why this description is misleading, giving three activities that occur during interphase.
Why must DNA replication be completed during interphase and not during M-phase? Explain the consequence for the daughter cells if a cell entered mitosis with its replication only half finished.
Arrange the following in the correct sequence and write the sequence: anaphase, cytokinesis, \(G_1\)-phase, \(G_2\)-phase, metaphase, prophase, S-phase, telophase.
Distinguish between karyokinesis and cytokinesis. What kind of cell would be produced if karyokinesis occurred but cytokinesis did not?
Why can mitosis occur in both haploid and diploid cells, while meiosis can occur only in diploid cells? Base your answer on a single event of prophase-I.
A drone honey bee develops from an unfertilized egg. Which type of cell division builds its body, and why can that division work in its cells?
What happens if the nuclear membrane fails to disintegrate during prophase of mitosis? Explain your reasoning.
Why are chromosomes best studied and photographed at metaphase rather than at any other stage?
Name the two types of spindle fibres described in this chapter, state where each is attached, and state what each does during anaphase.
A plant cell cannot divide its cytoplasm by forming a furrow. Explain why, and describe the alternative mechanism it uses.
Why is it acceptable for organelles such as mitochondria and plastids to be distributed only approximately equally between daughter cells, while chromosomes must be distributed exactly?
Mitosis is described as highly reliable, and yet this reliability is also a limitation. Explain this statement with reference to variation.
All the plants of a banana orchard are produced by mitosis from one parent plant. State one advantage and one serious risk of raising a crop in this way.
Differentiate between necrosis and apoptosis. Give one example of a situation in the body where apoptosis is essential.
Why is crossing over said to occur between non-sister chromatids and not between sister chromatids? What would be the result if it occurred between sister chromatids?
Explain why meiosis-I is called reductional division while meiosis-II is called equational division, referring to what separates in the anaphase of each.
No DNA replication occurs during interkinesis. Explain why this fact is essential for the final chromosome number of the four daughter cells.
What would happen to the chromosome number of a species across generations if meiosis did not occur, but fertilization continued as usual?
A homologous pair fails to separate during anaphase-I. Describe the chromosome content of the gametes that will be formed, and state the consequence if such a gamete takes part in fertilization.
Draw and label a diagram of anaphase-I of meiosis, and mark clearly one feature that distinguishes it from anaphase of mitosis.
Identify the sub-stage of prophase-I from each description and write them in their correct order: (i) the synaptonemal complex disappears and the homologues begin to repel each other; (ii) the ends of the chromosomes converge towards the side of the nucleus where the centrosome lies; (iii) chiasmata move towards the ends of the chromosomes; (iv) tetrads of four chromatids become clearly visible; (v) homologous chromosomes pair point by point.
In a slide of dividing tissue, most of the cells appear to be in interphase. Does this mean the tissue divides rarely? Justify your answer using the duration of each phase of the cell cycle.
While observing a slide, a student observed many cells with nuclei. Some of the nuclei were bigger as compared to others, but their nuclear membrane was not so clear. The teacher inferred it as one of the phases in cell division. Which phase may be inferred by the teacher?
Students prepared a slide of onion root tip. Many cells were seen under the microscope. There was a cell with two groups of chromosomes at opposite ends of the cell. This cell is in which phase of mitosis?
Students were shown some slides of cancerous cells. The teacher commented that if there had been control at one of the cell cycle phases, there would not have been a condition like this. Which phase was the teacher referring to?
Some Mendelian hybridization experimental results were shown to the students. The teacher informed them that there are two genes located on the same chromosome. He enquired whether they will ever be separated from each other. What is your answer?
Students were observing a film on Paramoecium. It underwent a process of reproduction. The teacher said it is due to cell division. But the students objected, saying that there was no disappearance of the nuclear membrane and no spindle formation, so how can it be cell division? Can you clarify?
Is meiosis responsible for evolution? Justify your answer.
Why are mitosis and meiosis-II called homotypic division?
Write the significance of mitosis.
Enlist the different stages of prophase-I.
With the help of a suitable diagram, describe the cell cycle.
Distinguish between mitosis and meiosis.
Draw the diagram of metaphase.
Is the given figure correct? Why?
Diagram
Fig. 7.13 – A cell with a spindle and centrioles at the two poles, and chromosomes drawn lying one below another along the equator with their arms parallel to the spindle axis; to be examined by the student for correctness.
- Identify the following phases of mitosis and label ‘A’ and ‘B’ given in the diagrams.
Diagram
Fig. 7.14 – A dividing cell with spindle fibres labelled and chromosomes at the equator; the structure at the equator marked ‘A’ for the student to identify and label.
Diagram
Fig. 7.15 – A dividing cell with two sets of chromosomes moving apart towards the poles; the moving structures marked ‘B’ for the student to identify and label.
7.6 Practice With Numericals
A diploid animal cell has \(2n = 46\). State the number of chromosomes and the number of chromatids present in one such cell at (i) the end of \(G_1\)-phase, (ii) the end of \(G_2\)-phase, and (iii) the end of telophase of mitosis.
A single cell of a plant with \(2n = 12\) undergoes four successive mitotic divisions. How many cells are formed, and how many chromosomes does each of them contain?
How many meiocytes must undergo meiosis to produce 60 pollen grains, and how many meiotic divisions in all are involved?
In a certain cell, \(G_1\) lasts 9 hours, S lasts 7 hours, \(G_2\) lasts 3 hours and M-phase lasts 2 hours. Calculate the total duration of the cell cycle and the percentage of the cycle occupied by interphase.
A diploid cell has \(2n = 16\). State the number of chromosomes and the number of chromatids present at (i) metaphase-I, (ii) the end of telophase-I in one daughter cell, and (iii) the end of telophase-II in one daughter cell.
An organism has 8 chromosomes in each of its gametes. State the chromosome number in its zygote, in a cell of its root tip, and in one of its meiocytes before meiosis begins.
If an onion has 16 chromosomes in its leaf cell, how many chromosomes will there be in its root cell and in its pollen grain?
7.7 Objective Questions
The connecting link between Meiosis-I and Meiosis-II is ……………
- interphase-I b. interphase-II c. interkinesis d. anaphase-I
Synapsis is pairing of ………….
- any two chromosomes b. non-homologous chromosomes c. sister chromatids d. homologous chromosomes
Spindle apparatus is formed during which stage of mitosis?
- Prophase b. Metaphase c. Anaphase d. Telophase
Chromosome number of a cell is almost doubled up during ……………
- \(G_1\)-phase b. S-phase c. \(G_2\)-phase d. \(G_0\)-phase
How many meiotic divisions are necessary for formation of 80 sperms?
- 80 b. 40 c. 20 d. 10
How many chromatids are present in anaphase-I of meiosis-I of a diploid cell having 20 chromosomes?
- 4 b. 6 c. 20 d. 40
In which of the following phases of mitosis are chromosomes arranged at the equatorial plane?
- Prophase b. Metaphase c. Anaphase d. Telophase
Find the incorrect statement:
- Condensation of chromatin material occurs in prophase.
- Daughter chromatids are formed in anaphase.
- Daughter nuclei are formed at metaphase.
- Nuclear membrane reappears in telophase.
Histone proteins are synthesized during ……
- \(G_1\) phase b. S-phase c. \(G_2\) phase d. Interphase
Assertion: The amount of DNA per cell doubles during S-phase. Reason: The number of chromosomes per cell also doubles during S-phase. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: In anaphase, chromosomes appear like a bunch of bananas. Reason: The centromere leads the movement towards the pole while the arms of the chromosome trail behind. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Match the following column-A with column-B and rewrite.
| Column-A (phases) | Column-B (their events) |
|---|---|
| a. Leptotene | i. Crossing over |
| b. Zygotene | ii. Desynapsis |
| c. Pachytene | iii. Synapsis |
| d. Diplotene | iv. Bouquet stage |
- Match the following and rewrite: (a) bouquet stage, (b) synaptonemal complex, (c) chiasmata, (d) terminalization, (e) disjunction — with (1) diakinesis, (2) leptotene, (3) anaphase-I, (4) zygotene, (5) pachytene.