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Chapter 2: Structure of Chromosomes, Cell Cycle and Cell Division

In the years around 1900, at the marine zoological station in Naples, the German biologist Theodor Boveri spent his working days watching sea urchin eggs divide. Sea urchin eggs are almost transparent, so under a microscope he could follow every stage of a division as it happened.

Normally one sperm enters one egg, and the fertilised egg divides neatly into two, then four, then eight. Boveri found a way to break that pattern. By exposing the eggs to a very heavy concentration of sperm, he obtained eggs that two sperm had entered at the same moment. Such an egg did not divide in two. It was pulled apart in three or four directions at once, and the chromosomes — the thread-like bodies that appear inside a nucleus whenever a cell divides — were shared out unevenly among the new cells.

Boveri then followed those embryos as they grew. Almost all of them developed abnormally. What caught his attention was this: how badly an embryo developed depended on which chromosomes its cells had missed out on, not simply on how many. In 1902 he published the conclusion that makes the rest of this chapter necessary. Chromosomes are not interchangeable beads on a string. Each one carries something different, and a cell can work properly only if it receives one full, correct set.

Almost everything a dividing cell does is a way of making certain that it does.

Diagram

Fig. 2.0-1 — Boveri’s Sea Urchin Experiment Two-part line drawing, side by side. Left: a sea urchin egg entered by one sperm, with a two-poled spindle and the chromosomes shared equally into two daughter cells; label one sperm, spindle with two poles, two daughter cells with equal chromosome sets. Right: an egg entered by two sperm, with a four-poled spindle and the chromosomes shared unevenly into four unequal daughter cells; label two sperm, spindle with four poles, daughter cells with unequal chromosome sets. Use four chromosomes in the normal egg so the unequal sharing is visible at a glance.

NoteCuriosity Corner

Q1. Why must a new cell receive exactly the same set of chromosomes as the cell it came from? Q2. If the DNA in one human cell would stretch about two metres, how does it fit inside a nucleus you cannot even see? Q3. A sperm and an egg each carry 23 chromosomes, yet the baby has 46. Where along the way is the number halved? Q4. Between two divisions a cell looks completely still under a microscope. Is it doing nothing at all? Q5. Children of the same parents are unmistakably related, yet no two are alike. What happens during a cell division to make that possible?

2.1 What Chromosomes Are

Under a microscope, a living cell that has not been treated in any way gives very little away. The nucleus shows up as a rounded region, but nothing inside it can be made out. Add a suitable dye, though, and the picture changes at once. Definite structures appear inside the nucleus, because the dye is taken up by some of the material there and not by the rest.

That material is DNA — deoxyribonucleic acid, the long molecule that carries the instructions for building and running the cell. Most of a cell’s DNA lies inside the nucleus. A small amount sits in the mitochondria, and in a plant cell in the chloroplasts as well.

Inside the nucleus the DNA is not one continuous thread. It is divided among a definite number of separate units, and each of those units is a chromosome. Every chromosome holds one very long DNA molecule with a great many protein molecules attached along its length.

This combination of DNA and protein is called chromatin. Chromatin is the material a chromosome is made of, and the two words are not interchangeable.

Chromatin And Chromosome Chromatin is the DNA-and-protein material itself, spread out as a fine fibre in a cell that is not dividing. A chromosome is that same material coiled up and condensed for division. Same substance, two states — not two different things.

From Fine Fibre To Thick Rod

While a cell is going about its ordinary work and not dividing, its chromatin is stretched out as an extremely fine fibre. That fibre is far too thin to be resolved by a light microscope. Under an electron microscope it appears as a very long, very thin, darkly stained thread filling much of the nucleus. Spread out in this way, the DNA can be reached, read and copied.

When the cell prepares to divide, the chromatin fibres coil on themselves and pack tighter and tighter. As they condense they become shorter and thicker, until each one is a compact body that can be seen and counted under an ordinary school microscope. The stages of this change are shown in Fig. 2.1-1.

There is a reason a cell goes to this trouble. An enormously long, fine thread spread through the nucleus cannot be hauled to one end of a cell without tangling or breaking. Wound into a short, sturdy rod, the same thread can be moved cleanly and set down in the right place.

Diagram

Fig. 2.1-1 — Condensation Of Chromatin Fibres Into Chromosomes Three nuclei drawn in a left-to-right sequence with arrows between them. First: a nucleus filled with a fine, loosely tangled network; label chromatin fibres. Second: the same network partly coiled and thickened. Third: a small number of short, thick, paired bodies clearly separate from one another; label chromosomes. Keep the same number of threads visible in all three so the student can follow one thread through the sequence.

Because these condensed bodies take up certain dyes so strongly, they were named from two Greek words — chroma, meaning coloured, and soma, meaning body. The name records nothing about what they do; it records only how they were first noticed.

NoteDefinition

Chromosome A highly coiled, ribbon-like structure formed by the condensation of chromatin fibres during cell division.

Where A Chromosome Sits Step down from a chromosome and you reach the DNA molecule and the proteins wound with it. Step up and you reach the nucleus holding the full set, then the cell, then the tissue built from those cells. A chromosome is neither a molecule nor a cell — it is one long molecule, packaged.

2.2 The Discovery Of Chromosomes

Chromosomes were not discovered by reasoning. They were discovered by someone looking at the right tissue, at the right moment, through the best lens available at the time.

To see a chromosome at all, three things have to come together. The cells must be dividing, or the chromatin stays uncondensed and invisible. They must be large, or an old microscope cannot separate one thread from the next. And they must be stained, or the threads have no colour to be seen by.

Flemming And The Salamander Larvae In the 1870s the German anatomist Walther Flemming began treating animal tissues with newly available aniline dyes and examining them under a light microscope. Working with the larvae of salamanders — amphibian larvae grow fast, so many of their cells are dividing at any moment, and their cells and threads are unusually large — he saw something no one had followed through properly before. Inside dividing nuclei were minute threads that took up the dye strongly. Watching many cells caught at different moments, Flemming pieced the stages together into a sequence. He noticed that each thread split along its whole length rather than snapping across. He named the stained material chromatin, and he named the division of the nucleus mitosis, from the Greek mitos, a thread. His full account was published in 1882.

Why Lengthwise Splitting Mattered

That one observation carried a great deal. A thread that broke across the middle would give each new cell half a set of instructions. A thread that splits along its length gives each new cell a complete copy, because the two halves lie side by side and are alike from end to end (Fig. 2.2-1).

Flemming could not say what the threads were made of, and he did not use the word chromosome, which came into use only some years later. What he established was the behaviour. The chemistry came afterwards, and the rest of this chapter follows the same order.

Diagram

Fig. 2.2-1 — Lengthwise Splitting Of A Thread, As Flemming Saw It Two simple line drawings side by side. Left: a single stained thread inside a nucleus, drawn as a wavy rod. Right: the same thread split along its entire length into two identical halves lying against each other. Label the whole thread, the split running lengthwise, and the two identical halves. Add a small crossed-out drawing beside them showing a thread broken across its middle into two unequal pieces, labelled as what does not happen.

With stronger microscopes and better staining methods, later workers took the threads apart and found what they were built from. That is where the next section goes.

Who Named The Chromosome Flemming described the threads but never named them. The word chromosome was proposed in 1888 by the German anatomist Wilhelm Waldeyer, from the same Greek roots you met in the last section — coloured body. So the structure was watched for about ten years before it had a name of its own.

2.3 Chromatin — The Material Of Chromosomes

A chromosome, you have seen, is condensed chromatin. So the sensible next question is what chromatin itself is made of. The answer has two parts, and each part does a different job: one substance carries the information, the other packs it away.

2.3.1 The Two Components Of Chromatin

Chromatin is not a single substance but a close combination of two, in roughly fixed proportions. About 40 per cent of its mass is DNA. The remaining 60 per cent or so is protein of one particular kind — the histones.

The division of labour is worth noticing at the start. DNA holds the instructions, but a bare thread of DNA cannot fold itself, cannot keep itself untangled, and cannot be moved about safely. The histones do that part of the work.

DNA belongs to a family of molecules called the nucleic acids, named so because they were first obtained from cell nuclei and behave chemically as acids. There are two kinds of nucleic acid in a cell. One is DNA. The other is RNA, ribonucleic acid, which the cell builds using DNA as the pattern and uses in making proteins. Chromatin contains DNA, not RNA.

2.3.2 The Molecular Structure Of DNA

A single DNA molecule is enormous by the standards of molecules. For this reason it is described as a macromolecule — a very large molecule built by joining a great many small units end to end, like a chain built from links.

DNA is not a single thread. Two strands lie alongside each other and wind around one another in a regular spiral. This shape is called a double helix, and one complete turn of the spiral is shown in Fig. 2.3-1. The two strands are joined to each other all along their length, never at just one or two points.

Diagram

Fig. 2.3-1 — The Double Helix Of DNA Schematic line drawing of a DNA molecule as two ribbon-like strands twisting around each other in a spiral, with short horizontal rungs joining them at regular intervals. Label the two strands, one complete turn of the spiral, and the paired bases forming the rungs. Show at least two full turns so the regularity of the spiral is clear.

How The Shape Of DNA Was Worked Out By the early 1950s chemists knew what DNA was made of but not how those parts were arranged. At King’s College London, Rosalind Franklin was directing beams of X-rays at fibres of DNA and photographing the pattern the rays made as they scattered. In 1952 she obtained an exceptionally clear photograph, and the cross-shaped pattern on it was the fingerprint of a helix; her measurements also fixed the width of the molecule and the spacing of its repeating parts. At Cambridge, James Watson and Francis Crick were building physical models of DNA and testing them against whatever data existed. Working from Franklin’s measurements, in 1953 they arrived at a model that fitted everything known — two strands, wound in a double helix, held together by paired bases. Watson, Crick and Maurice Wilkins were awarded the Nobel Prize in 1962. Franklin had died in 1958, and the prize is not awarded after death.

2.3.2.1 Nucleotides — The Repeating Units Of A Strand

Each strand of DNA is a chain of repeating units called nucleotides. Every nucleotide is built from exactly three parts joined together: a phosphate group, a pentose sugar (a sugar whose molecule contains five carbon atoms), and a nitrogenous base.

The parts are not arranged at random within the strand. The phosphate of one nucleotide joins the sugar of the next, so the sugars and phosphates link up into a long unbroken backbone running lengthwise along the strand.

The nitrogenous base of each nucleotide is attached to the sugar and points inwards — away from its own backbone and towards the base facing it on the other strand. There the two bases are joined to each other, and that joining is what holds the two strands together (Fig. 2.3-2).

Put the two strands together and the whole molecule takes the form of a ladder. The two sugar-phosphate backbones are the uprights, and the pairs of bases meeting in the middle are the rungs. The ladder is then twisted along its length into the helix you saw in Fig. 2.3-1.

Diagram

Fig. 2.3-2 — A Nucleotide, And Two Strands Joined By Their Bases Two-part line drawing. A: a single nucleotide drawn as three joined shapes — a circle for the phosphate group, a pentagon for the pentose sugar and a rectangle for the nitrogenous base; label all three. B: a short length of two parallel DNA strands drawn untwisted as a flat ladder, showing four rungs; label the sugar-phosphate backbone of each strand, the paired bases, and the hydrogen bonds joining each pair. Show two hydrogen bonds on an A–T rung and three on a G–C rung, with the bases lettered A, T, G and C.

2.3.2.2 Complementary Base Pairing

Only four kinds of nitrogenous base occur in DNA — adenine (A), guanine (G), cytosine (C) and thymine (T). Everything DNA records is written using these four.

The four fall into two groups according to the shape of their molecules. The purines, adenine and guanine, have a double-ring structure and are the larger of the two. The pyrimidines, cytosine and thymine, have a single ring and are smaller. RNA carries uracil (U) in place of thymine, and uracil too is a pyrimidine.

Which Bases Are Purines Pure As Gold — PURines are Adenine and Guanine. The other bases are pyrimidines. Note that the sizes run the opposite way to the names: pyrimidine is the longer word but the smaller molecule.

Bases do not pair up at random. Adenine always pairs with thymine, held by two hydrogen bonds — weak attractions that are easily made and easily broken. Guanine always pairs with cytosine, held by three such bonds. This is written as \(A = T\) and \(G \equiv C\), the number of lines standing for the number of hydrogen bonds.

Notice what this rule does to the shape of the molecule. A large purine always faces a small pyrimidine, never purine against purine or pyrimidine against pyrimidine. Every rung is therefore the same width, and the ladder keeps an even thickness from one end to the other.

The rule has a second consequence, and it is the more important of the two. Because each base can face only one partner, the order of bases along one strand completely fixes the order along the other. The two strands are said to be complementary, and each of them carries the whole of the information. When the time comes to make a copy, that is exactly what the cell makes use of, as you will see in Section 2.7.2.

Q. A stretch of one DNA strand reads A–G–C–T–T. What does the matching stretch of the other strand read?

A. T–C–G–A–A. Each base has only one possible partner, so the second strand follows automatically from the first.

2.3.3 Histone Proteins And Nucleosomes

Now to the other half of chromatin. The histones are the proteins that help coil and package the DNA, and they do it in a very regular way.

Eight histone protein molecules group together into a core, called a histone octamer. The DNA strand winds around this core, making one or two turns, and then runs on for a short distance before winding around the next core. The core together with the DNA wound around it is a nucleosome.

An easy picture is a football with a long rope wound twice around it. The ball is the histone core and the rope is the DNA. Repeat this along the whole length of the molecule and the chromatin fibre looks like a string of beads, as in Fig. 2.3-3.

The nucleosome has the same standing in a chromosome that the nucleotide has in a nucleic acid: it is the basic structural unit, the thing the whole is built out of. A single human chromosome may contain roughly a million of them.

Diagram

Fig. 2.3-3 — The Nucleosome Line drawing in two parts. Left: one nucleosome enlarged — a rounded core drawn as eight small spheres packed together, with the DNA double strand wound about twice around it; label the eight histone proteins, the histone core and the DNA. Right: four such nucleosomes in a row along a continuous DNA strand, with short lengths of free DNA between them, giving the beads-on-a-string appearance; label the chromatin fibre.

Nucleotide And Nucleosome A nucleotide is one link of a DNA chain — a phosphate, a sugar and a base. A nucleosome is far bigger: a length of DNA wound around eight histone proteins. Nucleotide is the unit of the nucleic acid; nucleosome is the unit of the chromosome.

2.3.4 Coiling And Supercoiling

Here is the problem the cell has to solve. If all the DNA in one human cell were pulled straight and laid end to end, it would measure about 2 metres. The nucleus it must fit inside is only about 6 micrometres (µm) across — six thousandths of a millimetre.

Two metres of thread cannot simply be stuffed into a space that size. It has to be wound, and then the winding has to be wound again. Look at a coiled telephone cord, or the cord of an old landline handset. The cord is a coil, and if you let it hang it twists into further loops — coils of a coil. Chromatin is packed in the same manner, and a coil formed out of an existing coil is called a supercoil.

The whole sequence of packing runs from a bare double helix to a fully condensed chromosome, as set out in FC 2.1 and drawn in Fig. 2.3-4.

Flowchart

FC 2.1 — Packing Of DNA Into A Chromosome DNA double helix → DNA wound around cores of eight histone proteins, forming nucleosomes → nucleosomes packed close together into a chromatin fibre → chromatin fibre coiled → coiled fibre supercoiled → fully condensed chromosome visible at cell division

Diagram

Fig. 2.3-4 — From DNA To Chromosome Vertical ladder diagram read from bottom to top, each stage joined to the next by an upward arrow. Bottom: a short length of DNA double helix. Next: the same DNA wound around histone cores to form nucleosomes, drawn as beads on a string. Next: the beaded string packed into a thicker chromatin fibre. Next: the fibre coiled. Next: the coiled fibre supercoiled into a still thicker rope. Top: a condensed chromosome, shown inside a nucleus inside a cell. Label every stage, and mark the width of the structure at each stage to show it getting thicker and shorter.

Each round of coiling makes the thread shorter and thicker. By the end, a fibre metres long has been reduced to a body a few micrometres long — short enough to be dragged across a cell without tangling, and thick enough to be seen.

Q2. If the DNA in one human cell would stretch about two metres, how does it fit inside a nucleus you cannot even see?

A. By being wound in stages. The DNA is first wound around histone cores into nucleosomes, these are packed into a chromatin fibre, and that fibre is coiled and then supercoiled. Each stage shortens the thread, until two metres of DNA occupies a nucleus about 6 µm across.

Take a metre of thin string and lay it beside a ruler. Now wind it tightly around a pencil, turn by turn, and measure how much of the pencil it covers. Compare the two lengths. You have carried out one round of coiling; the cell carries out several, one on top of another.

2.4 The Structure Of A Chromosome

Look at any drawing of a chromosome made at the start of cell division and you will notice that it is not a single rod. It appears as two rods lying side by side, pinched together at one point. This section explains why it has that shape, and what the pinch is for.

2.4.1 Chromatids And Sister Chromatids

Before a cell divides, every one of its chromosomes is copied. The copy is not made separately and stored somewhere else in the nucleus. It stays attached to the original from the moment it is made. That is why a condensed chromosome, seen at the start of division, is already double.

One lengthwise half of such a duplicated chromosome is called a chromatid. The two chromatids of one chromosome are exact copies of each other and are joined together, and a pair of this kind is called sister chromatids (Fig. 2.4-1).

Notice that this doubling is not a doubling of the chromosome number. Two sister chromatids joined together still count as one chromosome. A human cell about to divide has 46 chromosomes, made up of 92 chromatids.

Diagram

Fig. 2.4-1 — A Chromosome Before And After Duplication Two line drawings side by side. A: an unduplicated chromosome, drawn as a single condensed rod with a narrow constriction part-way along it; label the single chromatid and the centromere. B: the same chromosome after duplication, drawn as two identical rods lying side by side and joined at the constriction, giving an X-shape; label the two sister chromatids, the centromere and the arms of the chromosome. Draw the constriction at the same position in both so the student can see it is a fixed site.

Chromatid And Chromosome A chromosome is the whole structure. A chromatid is one lengthwise half of it after duplication. Two sister chromatids joined at a centromere are still one chromosome — they become two chromosomes only once they are pulled apart.

When The Division Is Over

Once the sister chromatids of each chromosome have been separated and drawn into different cells, each chromatid is on its own. It is now a chromosome in its own right. In the new cell it uncoils, thins out and returns to being a very long, fine chromatin fibre.

This gives you a useful check on your understanding. If 46 chromosomes appear in a nucleus at division, then between divisions that nucleus holds 46 chromatin fibres — one fibre for every chromosome, not one for every chromatid.

Q. A human cell is about to divide. How many chromosomes and how many chromatids are present in it?

A. 46 chromosomes and 92 chromatids, because each of the 46 chromosomes is made of two sister chromatids at this stage.

2.4.2 The Centromere And Its Functions

The two sister chromatids are held together at one point along their length. There the chromosome is narrowed, and under a microscope this shows as a small pinched-in region. This point of attachment is the centromere.

The centromere of a given chromosome always lies at the same site along it. It may be near the middle, giving two arms of equal length, or well off-centre, giving one long arm and one short one. Its position is a fixed feature of that particular chromosome.

Now for what it is for. When a cell divides, spindle fibres — fine protein threads that form across the dividing cell and act like ropes — take hold of the chromosomes. They do not attach anywhere along the arms. They attach at the centromere and nowhere else.

That single point of grip is what makes an orderly division possible. As the spindle fibres shorten, they pull on the centromere; the centromere splits, the two sister chromatids come apart, and each is drawn away towards an opposite end, or pole, of the cell. Because every chromosome is gripped at one fixed handle, all of them are pulled in the same organised way at the same moment.

The centromere therefore has two functions.

  1. It is the region by which a chromosome is attached to the spindle fibres during cell division.
  2. It splits at the proper moment, allowing the sister chromatids of a chromosome to detach from each other and travel to opposite poles.

Centromere A single narrow point of attachment, positioned identically on both chromatids → the spindle can grip each chromosome at exactly one place → both chromatids are pulled apart cleanly, together, and towards opposite poles, so neither cell is left short.

2.5 Genes

A chromosome is one very long DNA molecule, and along that molecule the four bases follow one another in a definite order. That order is not the same all the way along. It is divided into stretches, and each stretch carries a separate instruction.

One such stretch is a gene — a particular sequence of nucleotides at a particular place on a chromosome. A single chromosome carries many hundreds of genes, laid end to end along its length like entries in a list (Fig. 2.5-1).

Each gene codes for one particular protein. The order of bases in the gene fixes the order in which the units of that protein are joined together, so a different sequence gives a different protein. Proteins then do the actual work of the body — haemoglobin, for instance, is the protein that carries oxygen in the blood — and it is through the proteins they specify that genes show up as visible features of an organism.

Genes are also the units of heredity, which means they are the things actually passed on from parents to offspring. A child does not inherit a feature directly; the child inherits the genes, and the features follow from them. That is why particular characteristics run in families.

NoteDefinition

Gene A specific sequence of nucleotides on a chromosome which codes for a particular protein, expressed in the form of a particular feature of the body. Genes are the units of heredity, transferred from parents to offspring.

Diagram

Fig. 2.5-1 — Genes Along A Chromosome Line drawing in three stages, left to right, each an enlargement of the one before. Left: a condensed chromosome with its centromere. Middle: an arrow leading to the uncoiled DNA double helix from part of that chromosome. Right: a straightened length of the DNA drawn as a flat ladder, with four separate stretches shaded and labelled gene 1, gene 2, gene 3 and gene 4, and the unshaded lengths between them left unlabelled. Mark on the right-hand drawing that each gene is a definite sequence of nucleotides at a definite position.

Where A Gene Sits Step down from a gene and you reach the individual nucleotides that spell it out. Step up and you reach the whole DNA molecule, then the chromosome it is packed into, then the nucleus holding the complete set. A gene is not a structure you can see — it is a stretch of a molecule.

DNA Fingerprinting Only about one part in a hundred of human DNA actually codes for proteins. Much of the rest consists of short sequences repeated over and over, and the number of repeats at any given site differs a great deal from one person to another. In 1984 Alec Jeffreys, working at the University of Leicester, found a way to display these repeated regions as a pattern of bands. Because the pattern is close to unique for each person, it can be used to establish identity. DNA fingerprinting is now used to settle questions of parentage, to identify people after accidents and disasters, and in criminal investigations, where biological traces such as blood or hair left at a scene can be compared with a known sample.

Where The Word Came From The word gene is much older than any knowledge of DNA. It was introduced in 1909 by the Danish botanist Wilhelm Johannsen, who needed a name for whatever it was that carried a characteristic from parent to offspring. What that thing was made of stayed unknown for another forty years.

2.6 Why New Cells Are Needed

Every cell in your body came into existence when some other cell divided. Nothing else makes cells. So before studying how a cell divides, it is worth asking why cells need to be made at all.

There are four reasons, and they are not the same as one another. New cells are needed for growth, for replacement, for repair and for reproduction.

2.6.1 Growth

Almost every plant and animal begins life as a single cell — the fertilised egg, or zygote. Everything the organism later becomes is built from that one cell.

The zygote divides into two, the two into four, the four into eight, and so on. By about the fourth day, a human embryo is a ball of roughly sixteen cells, the result of four rounds of division (Fig. 2.6-1).

An organism does not grow mainly by its cells swelling. It grows because the number of cells increases. As the cells multiply, they also begin to differ from one another and take up particular jobs, so that groups of them form tissues, and tissues are built into organs.

Diagram

Fig. 2.6-1 — Repeated Division Of The Zygote Line drawing of five stages left to right, joined by arrows: a single rounded zygote, then two cells, then four, then eight, then a ball of sixteen cells. Under each stage write the number of cells and the number of divisions completed, as \(1 \times 2 = 2\), \(2 \times 2 = 4\), \(4 \times 2 = 8\), \(8 \times 2 = 16\). Show all the cells enclosed within a common outer boundary from the two-cell stage onwards, and mark that the whole ball is no larger than the original zygote.

2.6.2 Replacement

Cells wear out in the ordinary course of living, and the body replaces them without any injury being involved.

The scale of this is easy to underestimate. About two million red blood cells are destroyed in your body every second. Exactly as many are made to take their place, by the division of parent cells in the bone marrow.

Plants do the same in their own way. Old, dried leaves fall away and new ones are produced at the growing points.

What Bone Marrow Is Bone marrow is the soft tissue filling the hollow inside certain bones, such as the ribs, the breastbone and the ends of the long bones of the limbs. It is one of the busiest dividing tissues in the body, and it is where red blood cells, most white blood cells and platelets are produced.

Mark a fine line across your thumbnail close to its base with a pen, and note the date. Look again after two weeks and see how far the mark has travelled. Nothing pushed it except new cells being made underneath, continuously, without your noticing.

2.6.3 Repair

Repair is different from replacement. Replacement is routine; repair follows damage — a cut in the skin, or a bone broken in a fall.

Here too the answer is cell division. Cells at the edges of the damaged region divide, the new cells fill in the gap, and the broken surfaces are gradually joined together.

Why A Broken Bone Is Put In A Cast A fracture is not mended by the cast. It is mended by cells dividing, which lay down new bone tissue across the break until the two ends are continuous again. That takes weeks, and the new tissue is soft and weak at first. The cast does one thing only: it holds the broken ends still and in the correct position while those cells work. If the ends shift, the dividing cells bridge the gap in the wrong alignment, and the bone heals crooked. This is also why a fractured bone in a young child usually mends faster than the same fracture in an older adult — cell division in the body slows with age.

2.6.4 Reproduction

Reproduction, too, depends on dividing cells, though it works differently in different organisms.

In Amoeba, a single-celled animal, reproduction is the division itself. The nucleus divides, the cytoplasm follows, and the parent becomes two independent individuals with nothing left over. Bacteria also divide in two, but a bacterium has no nucleus at all, so its division is a simpler process called binary fission.

In larger plants and animals, ordinary body cells cannot make a new individual. Special cells in the reproductive organs — the testes and the ovaries in humans — undergo a different kind of division called meiosis, which produces the sex cells, or gametes. Sperms and eggs receive only half the number of chromosomes their parent cell had: one chromosome out of each pair. That halving turns out to be one of the most important events in biology, and the next subsection shows why.

2.6.5 Chromosome Number In Body Cells And Sex Cells

The chromosomes of a body cell are not a random collection. They occur in pairs. A human body cell has 46 chromosomes, which is 23 pairs, and this is true of a man and a woman alike.

A cell that carries chromosomes in pairs is called diploid, written as \(2n\). For a human being, \(2n = 46\).

A sex cell is different. A sperm or an egg carries only 23 chromosomes — one member taken from each of the 23 pairs, and never both. A cell carrying a single set in this way is called haploid, written as \(n\), and for a human being \(n = 23\).

Now follow the arithmetic through a fertilisation. A sperm brings 23 chromosomes and an egg brings 23, so the fertilised egg has \(23 + 23 = 46\) — the full diploid number restored, with one member of every pair coming from each parent. Every body cell of the new individual, formed by later divisions of that zygote, carries 46 as well. The comparison is set out in Table 2.1 and the complete cycle in FC 2.2.

Table 2.1 — Body Cells Compared With Sex Cells In Human Beings | Feature | Body cells | Sex cells (sperms and eggs) | |—|—|—| | Number of chromosomes | 46 | 23 | | Arrangement | 23 pairs | one member of each pair, unpaired | | Number in a man | 46 | 23 | | Number in a woman | 46 | 23 | | Written as | \(2n\) (diploid) | \(n\) (haploid) | | Produced by | mitosis | meiosis |

Flowchart

FC 2.2 — The Chromosome Number Through One Generation Body cells of parents (\(2n = 46\)) → meiosis in the reproductive organs → sperm (\(n = 23\)) and egg (\(n = 23\)) → fertilisation → zygote (\(2n = 46\)) → repeated mitosis → body cells of the new individual (\(2n = 46\)), which on reaching maturity again undergo meiosis, returning to the start of the cycle

Notice what would happen without the halving. If gametes carried 46 chromosomes each, a zygote would have 92, its offspring 184, and the number would double in every generation. Meiosis exists to prevent exactly that.

Keep one further point in mind as you go on. In the divisions that serve growth, replacement and repair, the chromosome number does not change: the chromosomes are duplicated and then shared out equally, so each new cell ends with the same 46 the parent cell had. That kind of division is mitosis, and it takes place in all body cells. The only exceptions are the special cells in the ovaries and the testes that give rise to eggs and sperms, and those divide by meiosis instead.

2.7 The Cell Cycle

A cell does not divide the moment it is formed. Before it can divide it has work to do. It must grow to a proper size, build proteins and RNA, and make a complete copy of its DNA. Only then can it split into two.

These tasks are not carried out in a haphazard order. They follow a fixed, predictable sequence, and that sequence is called the cell cycle. When one round is over, the two new cells begin the same sequence again — divide, grow, redivide.

NoteDefinition

Cell Cycle The series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.

The cycle falls into two unequal parts, shown in Fig. 2.7-1. The first is a long non-dividing phase called the interphase, in which all the preparation is done. The second is a much shorter dividing phase, the M-phase, in which the cell actually splits.

Diagram

Fig. 2.7-1 — The Cell Cycle Circular diagram with an arrow running clockwise all the way round to show that the cycle repeats. Divide the circle into two unequal parts: a large arc labelled interphase, and a smaller arc labelled M phase (cell division). Divide the interphase arc further into three labelled parts in clockwise order — first growth phase (G1), synthesis phase (S), second growth phase (G2) — and against each write its main event: G1, RNA and proteins made, mitochondria and chloroplasts divide; S, DNA synthesised and chromosomes duplicated; G2, further RNA and proteins made for division. From the end of G1 draw a branch arrow leaving the circle to a box labelled resting phase (R). Draw the M phase arc leading back into G1 and mark that it makes two cells.

2.7.1 Interphase

Look at a cell just after a division. It is small, with a full-sized nucleus but not much cytoplasm around it. During interphase it will grow until it is as large as the cell it came from, and it will get everything ready for the next division.

For a long time interphase was called the resting phase of the cell, because nothing whatever can be seen changing in the nucleus during it. The name was a mistake. Nothing looks different because the chromatin is uncoiled and far too fine to be resolved; behind that still appearance the cell is doing its heaviest chemical work, including copying the whole of its DNA.

Q4. Between two divisions a cell looks completely still under a microscope. Is it doing nothing at all?

A. Far from it. It looks still only because its chromatin is uncoiled into fibres too thin to be seen. During this interphase the cell is growing, making proteins and RNA, and copying every one of its DNA molecules. This is the busiest part of the cell cycle, not a rest from it.

Interphase itself is made of three phases, which always occur in the same order. Two of them are growth phases and the third, in between, is the phase in which the DNA is copied. The letter G in their names stands for gap, from the days when they were thought of as gaps between one visible event and the next.

2.7.1.1 First Growth Phase (G1)

This is the phase in which the cell grows. RNA and proteins are synthesised in quantity and the volume of the cytoplasm increases, so the small new cell builds itself up to full size.

Two organelles multiply during G1 as well. Mitochondria divide in animal cells, and mitochondria and chloroplasts divide in plant cells, so that each daughter cell will receive a proper share of them.

Towards the end of G1 the cell reaches a fork in the road, and every cell must take one path or the other. It may withdraw from the cell cycle altogether and enter a resting phase (R), in which it goes on doing its ordinary work but no longer prepares to divide. Or it may commit itself to dividing and pass into the synthesis phase.

Organelles With Their Own DNA Mitochondria and chloroplasts are unlike other organelles: each contains a small amount of its own DNA and its own ribosomes, and each makes some of its own proteins. This is why they are not built afresh by the cell but divide in two on their own, and are then shared between the daughter cells.

2.7.1.2 Synthesis Phase (S)

Synthesis here means the synthesis of DNA. In this phase every DNA molecule in the nucleus is copied, so that each chromosome comes to consist of two identical sister chromatids joined at the centromere (Fig. 2.7-2).

Be careful about what has and has not changed. The amount of DNA in the nucleus has doubled. The number of chromosomes has not changed at all, because the two copies stay attached to each other and count as one chromosome until they are pulled apart.

Diagram

Fig. 2.7-2 — What Happens To A Chromosome In The S Phase Two drawings side by side joined by an arrow. Left, before S phase: three chromosomes of different lengths, each drawn as a single rod with a marked constriction; label chromosome containing one DNA molecule and label the centromere. Right, after S phase: the same three chromosomes, each now drawn as two identical rods joined at the constriction; label sister chromatids. Below the pair of drawings write: DNA content doubled, chromosome number unchanged.

DNA Amount And Chromosome Number After the S phase a human cell contains twice as much DNA but still has 46 chromosomes, not 92. Each chromosome is simply now made of two chromatids. Count centromeres, not rods: one centromere means one chromosome.

2.7.1.3 Second Growth Phase (G2)

G2 is a shorter growth phase. The RNA and proteins that the cell will need for the act of division itself continue to be made in it, including the protein from which the spindle fibres are later built.

By the end of G2 the cell is fully prepared: grown to size, with its DNA copied and its equipment ready. It now enters the M phase and divides. The two daughter cells that result pass straight into interphase again, and in this way the cell cycle goes on and on.

2.7.2 Formation Of New DNA

The copying that happens in the S phase is called replication, and the way the DNA molecule is built makes it possible. You saw in Section 2.3.2.2 that each base can pair with only one partner. That single rule is the whole basis of the copy.

The double helix opens at one end, the hydrogen bonds between the paired bases break, and the two strands come apart like the two halves of a zip. Each strand now has its bases exposed.

Free nucleotides are present in the nucleus. As each base is exposed, the nucleotide carrying its correct partner comes and takes its place against it — adenine against thymine, guanine against cytosine — and these are joined up into a new strand. A new strand is built against each of the two old strands at the same time, and the process runs along the whole length of the molecule.

The outcome is two complete DNA molecules where there was one, as set out in FC 2.3 and drawn in Fig. 2.7-3. Notice that neither of them is entirely new. Each is made of one old strand and one newly built strand, and because the pairing rule allows no choice, the two molecules are identical to each other and to the original. The full process is a good deal more complicated than this outline, and you will meet it properly in higher classes.

Flowchart

FC 2.3 — Replication Of DNA DNA double helix → hydrogen bonds between the paired bases break → the two strands separate from one end, like a zip opening → bases on each old strand lie exposed → free nucleotides pair with them, each base taking only its own partner → a new strand is built against each old strand at the same time → two identical double helices, each made of one old strand and one new strand

Diagram

Fig. 2.7-3 — Replication Of DNA Three drawings in a row, joined by arrows and lettered A, B and C. A: a complete DNA double helix, labelled double helix. B: the same molecule opened at one end into a Y-shape, with a new strand being built alongside each separated old strand; label the old strands, the new strands and the point at which the two strands are separating. C: two complete double helices side by side; label on each one the old strand and the new strand. Use two clearly different line styles for old and new strands and keep those styles the same across all three drawings.

2.7.3 Can The Cell Cycle Go On Endlessly?

It cannot, and it does not. Cell division is controlled, and the control differs from one tissue to another. In some places the cycle stops permanently, in some it stops for a time and starts again when needed, and in some it continues throughout life.

How Long Different Human Cells Last Read this set of figures for adults, and look for the pattern rather than memorising the numbers.

  • Epithelial cells lining the gut: about 5 days
  • Skin cells: about 2 weeks
  • Red blood cells: about 120 days
  • Liver cells: about 1 to 2 years
  • Bone cells: about 10 years
  • Cells of the eye lens, nerve cells of the cerebral cortex, most muscle cells: formed in the embryo, never replaced

Start at the top. The gut lining is scraped by food and bathed in digestive juices all day, and skin is rubbed, washed and exposed to sunlight. These are the most heavily worn surfaces in the body, and they are the ones replaced fastest. Red blood cells are squeezed through narrow capillaries millions of times over and wear out in about four months. Bone is protected inside the body and is replaced slowly.

Now look at the last line, which breaks the pattern. Here the cells are not replaced at all. In each of these tissues, what the cell does depends on exactly where it sits — a lens cell must lie in a perfectly ordered row for the lens to stay transparent, and a nerve cell of the cortex is useful only because of the particular connections it has formed with other nerve cells. A brand-new replacement cell would not carry those connections. In such tissues the body’s answer is not to replace cells but to make them last.

So the rate is not random. Broadly, the greater the wear a tissue takes, the faster its cells are replaced; and where the exact arrangement of cells matters more than their freshness, division stops permanently.

Plants show the same principle of controlled division in a different form. Their cells do not divide everywhere. Division is concentrated at the growing points, the meristems, where cells divide rapidly and produce new leaves, buds and flowers.

A further group of cells is set aside for reproduction. These are the germinal cells, found in the testes and the ovaries in animals and in the anthers and the ovary in plants, and they do not divide by mitosis at all. They undergo meiosis, which is taken up in Section 2.10.

The control can also fail. If the cell cycle in a group of cells runs on without stopping, those cells go on dividing when they should not, and the excess cells form a swelling called a tumour. Some tumours stay where they are and cause little harm; others spread and are cancerous.

Finally, the balance between cells made and cells lost changes over a lifetime. In childhood, new cells are produced faster than old ones die, and the body grows. In adult life the two rates match, and the number of cells stays about the same. In old age, cells are produced more slowly than they are lost, and tissues gradually thin.

2.8 The Two Types Of Cell Division

You now know why cells divide and how a cell prepares itself for dividing. What remains is the division itself, and here biology has not one process but two.

The two are not small variations on a theme. They differ in where they take place, in how many cells they produce, and above all in the number of chromosomes those cells end up with.

Cell division by mitosis takes place in body cells. It produces diploid cells carrying the same set of chromosomes the parent cell had, and it is what the body uses for growth, replacement and repair.

Cell division by meiosis takes place only in the germinal cells of the reproductive organs. It produces the gametes, which are haploid, and it is used for one purpose only — the making of sex cells. FC 2.4 sets the two paths side by side.

NoteDefinition

Cell Division The process by which a parent cell, after duplicating its chromosomes, divides to produce daughter cells.

Flowchart

FC 2.4 — The Two Types Of Cell Division Cell division → branches into two types Branch 1: mitosis → takes place in body cells → serves growth, replacement and repair → daughter cells diploid (\(2n\)), chromosome number unchanged Branch 2: meiosis → takes place in the germinal cells of the reproductive organs → serves gamete formation → daughter cells haploid (\(n\)), chromosome number halved

Mitosis is taken up in Section 2.9 and meiosis in Section 2.10, and the two are then compared feature by feature in Section 2.11.

Mitosis And Meiosis Two words that differ by three letters and by nearly everything else. Mitosis keeps the chromosome number the same in the daughter cells; meiosis makes it less. If the daughter cells match the parent, it is mitosis; if they carry half as many chromosomes, it is meiosis.

2.9 Mitosis

Mitosis is the division in which one parent cell gives rise to two daughter cells that are exact copies of it. Whatever the parent cell had, each daughter cell has — the same kinds of chromosomes, in the same number.

That last point is the essential one. If a human cell with 46 chromosomes divides by mitosis, each of the two new cells has 46, not 23 and not 92. The chromosome number is maintained at every division, which is why every one of the millions of cells in your body carries the same 46 chromosomes as the zygote you began as.

NoteDefinition

Mitosis The cell division in which one parent cell divides to produce two daughter cells identical to it and to each other, the chromosome number remaining the same.

Mitosis is completed in two steps, one after the other. First the nucleus divides, which is called karyokinesis. Then the cytoplasm divides, which is called cytokinesis. FC 2.5 gives the sequence as a whole, and Fig. 2.9-1 shows it drawn out in an animal cell. Keep the figure in view while you read the next few pages.

Flowchart

FC 2.5 — The Course Of Mitosis Interphase (chromosomes duplicated in the S phase) → karyokinesis begins → prophase → metaphase → anaphase → telophase → karyokinesis ends → cytokinesis → two daughter cells, each with the parent’s chromosome number → each daughter cell enters interphase, returning to the start of the cycle

Diagram

Fig. 2.9-1 — Mitosis In An Animal Cell Six drawings of a rounded animal cell arranged in a vertical column with downward arrows between them, in a cell where the chromosome number is 4. A, interphase: nucleus with nuclear membrane, nucleolus and a fine chromatin network; label the centrosome containing centrioles, the cytoplasm, the nucleus and the nucleolus. B, early prophase: chromatin condensed into four short thick chromosomes each of two chromatids, the two centrosomes moving apart, radiating fibres around each; label chromatids, centromere and aster. C, late prophase: nuclear membrane and nucleolus gone, centrioles at opposite poles, spindle fibres stretching between them; label spindle fibres, achromatic spindle and poles. D, metaphase: all four chromosomes lying in one plane across the middle of the cell, each attached to a spindle fibre at its centromere; label the equator and the point of attachment. E, anaphase: centromeres split, the separated chromatids being drawn towards opposite poles as V shapes, a shallow furrow appearing in the cell membrane at the middle; label daughter chromosomes and the furrow. F, telophase: two groups of chromosomes at the poles thinning into chromatin threads, a nuclear membrane re-forming around each group, nucleolus reappearing, furrow deepened; label the two daughter nuclei and the cleavage furrow. Show the same four chromosomes throughout so that one of them can be followed from A to F.

2.9.1 Karyokinesis — Division Of The Nucleus

All the changes that take place in the nucleus during a cell division are together called karyokinesis, from the Greek karyon, a nucleus. It is completed in four phases: prophase, metaphase, anaphase and telophase.

These four are named for convenience, but they are not four separate operations. Each merges into the next without any break, so that the whole of karyokinesis is one continuous process. A cell caught on a slide is simply caught somewhere along it.

Two words are needed before the phases can be described. The poles of a cell are the two extremities of an imaginary axis running through it — think of the north and south of a globe. The equator is the plane midway between the poles, dividing the cell into two similar halves.

The Order Of The Four Phases PMAT — Prophase, Metaphase, Anaphase, Telophase. Prophase comes first because pro means first; metaphase means after; anaphase means back or up, when the chromatids are drawn back to the poles; telophase means end. The Greek roots give you the order without a mnemonic once you know them.

2.9.1.1 Prophase

Prophase is the phase of preparation, and it is the longest of the four. Seven things happen in it.

  1. The chromosomes become short and thick as the chromatin condenses, and are now clearly visible inside the nucleus.
  2. Each chromosome is seen to be already duplicated. The copying happened earlier, in the S phase of interphase, so each chromosome now consists of two chromatids.
  3. The two sister chromatids of a chromosome remain attached to each other at the centromere.
  4. In an animal cell, the centrosome — a small body lying just outside the nucleus, containing a pair of rod-like centrioles — divides into two. The centrioles are duplicated at the same time. The two daughter centrosomes then move apart and take up positions at opposite poles of the cell. Around each, fine fibres radiate outwards in all directions like the rays of a star, and this radiating structure is called the aster, from the Greek aster, a star.
  5. A number of fibres appear stretching between the two poles. These are the spindle fibres, and together they form the spindle. It is also called the achromatic spindle, because unlike the chromosomes it does not take up stain and stays colourless.
  6. The nuclear membrane and the nucleolus break down and disappear. They have to. While the nuclear membrane is in place, the spindle fibres cannot reach the chromosomes at all.
  7. The duplicated chromosomes begin moving towards the equator of the cell.

Centromere And Centrosome The centromere is part of a chromosome — the constriction where the two chromatids are joined and where spindle fibres attach. The centrosome is not part of a chromosome at all. It lies in the cytoplasm of an animal cell, contains the centrioles, and organises the spindle.

2.9.1.2 Metaphase

Metaphase is short, and only two things happen in it.

  1. The duplicated chromosomes arrange themselves on the equatorial plane, lying in a single plane across the middle of the cell.
  2. Each chromosome becomes attached to a spindle fibre by its centromere.

The arrangement is worth a moment’s thought. Lying at the equator, every chromosome is the same distance from both poles, and each is held by fibres running to both sides. When the pull comes, it comes on all of them at once and equally in both directions. This is what makes the sharing out that follows exact rather than approximate.

2.9.1.3 Anaphase

Anaphase is the shortest phase of all, and it is the moment the actual separation happens.

  1. The centromere holding the two chromatids together divides, so the chromatids are no longer joined.
  2. The two sister chromatids of every chromosome move apart and are drawn towards opposite poles. The pulling is done by the spindle fibres, which shorten and so haul the chromatids in.

Because a chromatid is dragged by its centromere, with its two arms trailing behind, the separating chromatids often appear V-shaped or J-shaped as they move.

In an animal cell, a shallow furrow begins to appear in the cell membrane around the middle of the cell towards the end of this phase.

Q. A human cell is in anaphase. How many chromosomes are travelling towards each pole?

A. 46 towards each pole. Once the centromere splits, each chromatid becomes a chromosome in its own right, so the cell momentarily contains 92 chromosomes — 46 for each of the two cells about to be formed.

2.9.1.4 Telophase

Telophase reverses prophase. Everything that was taken apart at the beginning is now rebuilt, in two places at once.

  1. The two groups of daughter chromosomes reach the opposite poles. Each chromosome lengthens, becomes thinner and turns back into a network of fine chromatin threads.
  2. A nuclear membrane forms around each of the two groups, so there are now two nuclei in the one cell.
  3. A nucleolus reappears inside each daughter nucleus, and the spindle fibres disappear.

The cytoplasm is dealt with far more casually than the nucleus. Its contents, including the mitochondria and, in a plant cell, the chloroplasts, are simply distributed at random between the two halves of the cell. Only the chromosomes are shared out exactly, because only the chromosomes have to be.

Following One Chromosome Through Mitosis Fix your attention on a single chromosome — say the longest one in the cell — and follow it right through.

  1. Interphase. It is not visible. It lies uncoiled as a long chromatin fibre, and during the S phase it is copied, so that it is now two identical chromatids joined at one centromere.
  2. Prophase. It coils up, shortens and thickens until it can be seen. Both its chromatids are visible, still joined at the centromere. The nuclear membrane around it disappears.
  3. Metaphase. It moves to the equator and takes its place in the single plane there. A spindle fibre from each pole attaches to its centromere.
  4. Anaphase. Its centromere splits. Its two chromatids are pulled apart, one towards each pole, and each is now a chromosome in its own right.
  5. Telophase. Each of the two uncoils again into a long chromatin fibre and is enclosed in a new nuclear membrane.
  6. After cytokinesis. The two lie in two different cells — and each is an exact copy of the chromosome you started with.

2.9.2 Cytokinesis — Division Of The Cytoplasm

Karyokinesis has produced two nuclei, but still inside one cell. Cytokinesis is the division of the cytoplasm that turns that one cell into two.

In an animal cell, the furrow that appeared at the end of anaphase deepens. It is a constriction of the cell membrane at the equator, and it presses inwards from all round the cell until the two sides meet at the centre. The cytoplasm is then split in two, with one nucleus in each half, and two separate cells exist where there was one. This is called the cleavage furrow.

A plant cell cannot do this. Outside its cell membrane it has a rigid wall of cellulose, and a rigid wall cannot be pinched inwards. So a plant cell divides in the opposite direction: from the inside out.

A cell plate is laid down in the cytoplasm at the equatorial plane. It begins at the centre of the cell and grows outwards towards the periphery until it reaches and joins the existing cell wall. The plate then becomes the new wall between the two daughter cells.

Parent And Daughter Both words are inexact, and biologists use them only out of habit. The parent cell does not survive its own division as a parent does — it loses its identity entirely in the process. And there is nothing feminine about a daughter cell. The terms are conventions, not descriptions.

2.9.3 Mitosis In Animal Cells And Plant Cells

What happens to the nucleus and the chromosomes is the same in both. The four phases occur in the same order, the chromosomes behave in the same way, and the outcome is the same. The differences lie in the cytoplasm and in where the division takes place, and there are four of them (Table 2.2). Fig. 2.9-2 shows the same six stages in a plant cell for comparison with Fig. 2.9-1.

Table 2.2 — Mitosis In An Animal Cell Compared With Mitosis In A Plant Cell | Feature | Mitosis in an animal cell | Mitosis in a plant cell | |—|—|—| | Asters | Asters are formed | Asters are not formed | | Organisation of the spindle | Formed with the help of the centrioles and asters | Formed without centrioles, which the cell does not possess | | Cytokinesis | By furrowing of the cytoplasm; the furrow starts at the cell membrane and proceeds inwards to the centre | By the formation of a cell plate; the plate is laid down at the centre and extends outwards to the cell membrane | | Where it occurs | In most tissues throughout the body, for growth and replacement | Mainly in the meristematic tissue — at the growing tips, for lengthening, and at the sides, for increase in girth |

Diagram

Fig. 2.9-2 — Mitosis In A Plant Cell Six drawings of a rectangular plant cell arranged in a vertical column with downward arrows between them, matching stage for stage with Fig. 2.9-1 and using the same chromosome number of 4. Draw a thick outer cellulose cell wall on every stage and label it on the first. A, interphase: nucleus with nuclear membrane, nucleolus and chromatin threads; mark that there is no centrosome. B, early prophase: four condensed chromosomes of two chromatids each; mark that no asters are formed. C, late prophase: nuclear membrane and nucleolus gone, spindle fibres formed between the two poles without centrioles; label the spindle. D, metaphase: chromosomes in one plane at the equator, attached by their centromeres. E, anaphase: chromatids separated and moving to opposite poles. F, telophase and cytokinesis: two nuclei re-forming, and a cell plate laid down across the centre of the cell, drawn as a broken line growing outwards from the centre towards the wall; label the cell plate and show arrows marking the direction of its growth.

Plant tissue is in fact the easiest place to watch mitosis, because in a growing root tip a great many cells are dividing at the same moment. A thin section of such a tissue, seen under a microscope, shows cells caught in every phase at once (Fig. 2.9-3). The cells are not lined up neatly; they lie at all angles, so the equator of one cell may run in quite a different direction from the equator of its neighbour.

Diagram

Fig. 2.9-3 — Cells Of A Root Tip In Different Phases Of Mitosis Line drawing of a section of plant tissue, showing about twenty rectangular cells packed together with their walls drawn in. Six of the cells are numbered 1 to 6 and show, in this order: 1 a cell in interphase with a fine chromatin network; 2 a cell in prophase with condensing chromosomes; 3 a cell in metaphase with chromosomes in one plane at the equator; 4 a cell in anaphase with two groups of chromosomes moving apart; 5 a cell in telophase with two nuclei re-forming; 6 two small young cells newly formed after division. Deliberately draw the dividing cells at different angles to one another so that the spindles do not all run the same way. Give a key beneath the drawing naming the phase against each number.

2.9.4 Significance Of Mitosis

Gather together what mitosis achieves.

  1. Growth. New cells are formed in the tissues, and the body increases in size.
  2. Repair. Damaged and wounded tissues are made good by the renewal of lost cells.
  3. Replacement. Old and dead cells are replaced by new ones — blood cells, for instance, and the cells of the epidermis of the skin.
  4. Asexual reproduction. In many single-celled organisms, division of the cell is reproduction. Yeast, Saccharomyces cerevisiae, reproduces by budding: a small outgrowth forms on the parent cell, the nucleus divides by mitosis, one nucleus passes into the bud, and the bud separates as a new individual.
  5. Maintenance of the chromosome number. Every daughter cell receives the same number and the same kinds of chromosomes as the parent cell had.

The fifth of these underlies the other four.

Q1. Why must a new cell receive exactly the same set of chromosomes as the cell it came from?

A. Because the chromosomes carry the genes, and the genes specify the proteins that make the cell work. A cell that received only some of them would lack the instructions for whatever the missing chromosomes carried, and could not function normally. Mitosis exists to prevent that: it copies every chromosome and shares the copies out exactly.

2.10 Meiosis

The name says what the division does. It comes from the Greek meion, meaning to lessen, and meiosis is the division in which the chromosome number is lessened — halved, to be exact.

Meiosis does not occur in ordinary body cells. It takes place only in the reproductive organs: in the testes and the ovaries in human beings, where it gives rise to the sperms and the ova. In flowering plants it takes place in the anthers, leading to the pollen grains, and in the ovules within the ovary, leading to the female gametes.

NoteDefinition

Meiosis The cell division that takes place in the reproductive organs to produce gametes, in which the chromosome number of the daughter cells is reduced to half that of the parent cell.

2.10.1 Halving Of The Chromosome Number

A human body cell has 23 pairs of chromosomes, that is \(2n = 46\). When it divides by meiosis to form gametes, only one member of each pair is passed on, so a gamete receives 23 single chromosomes, that is \(n = 23\). It never receives both members of a pair.

Why must this happen? Because of what fertilisation does. The sperm and the egg fuse, and their chromosomes are added together. If each gamete carried the full 46, the zygote would have 92, and every generation would double the number again. By halving it first, meiosis makes sure the diploid number is restored at fertilisation and no more — \(23 + 23 = 46\), with one member of every pair coming from each parent.

The halving is not done in a single division. Meiosis is completed in two divisions, one after the other, and the DNA is copied only once, before the first of them. The first division separates the members of each pair, so the two cells produced already have the haploid number. The second division then separates the chromatids of each chromosome, as mitosis does. The outcome is four cells from one, each haploid (FC 2.6 and Fig. 2.10-1).

Flowchart

FC 2.6 — The Outline Of Meiosis Diploid cell in the reproductive organ (\(2n\)) → chromosomes duplicate, each becoming two chromatids → the members of each pair come together and lie side by side → material is exchanged between them → first division: the members of each pair are separated → two cells, each haploid (\(n\)) → second division: the chromatids of each chromosome are separated → four haploid (\(n\)) gametes, no two of them alike

Diagram

Fig. 2.10-1 — The Outcome Of Meiosis Schematic line drawing running from top to bottom, in a cell where the chromosome number is 4, that is two pairs. Top: one cell labelled \(2n\), containing two long chromosomes and two short ones; draw one long and one short as solid and the other long and short as outlined, and give a key stating that solid chromosomes came from the mother and outlined ones from the father. Second row: the same cell after duplication, each chromosome now two chromatids joined at a centromere, with the long pair lying together and the short pair lying together. Third row: two cells side by side, labelled first division, each containing one long and one short chromosome of two chromatids, and each carrying a different combination of solid and outlined. Bottom row: four cells labelled \(n\), each containing one long and one short single chromosome. Mark against the bottom row that four haploid cells have been formed from one diploid cell.

Q3. A sperm and an egg each carry 23 chromosomes, yet the baby has 46. Where along the way is the number halved?

A. In meiosis, the division that forms the gametes in the testes and the ovaries. The 23 pairs of a body cell are separated, and each gamete receives only one member of every pair. Fertilisation then adds the two haploid sets together and restores the diploid 46 in the zygote.

2.10.2 Homologous Chromosomes And Crossing Over

Return to the idea of a pair. The 46 chromosomes of a human body cell are not 46 unrelated structures. They are 23 matched pairs, and the two members of any one pair are called homologous chromosomes.

Homologous chromosomes are alike in length and in shape, and their centromeres lie at the same position. They carry genes for the same set of characters, in the same order along their length. What they are not is identical, because one of them came from the mother and the other from the father, and the two may carry different forms of the same gene.

During meiosis, and only during meiosis, the two members of each pair come together and lie closely side by side along their whole length. This pairing is called synapsis. Since each chromosome has already been duplicated, four chromatids now lie together in a bundle.

While they lie paired like this, the chromatids may cross over one another. At the point where two chromatids cross, they break and rejoin — but each rejoins to the other one, so that a length of one chromatid has been swapped for the matching length of the other. The X-shaped point at which they cross and stay attached is called a chiasma (plural: chiasmata).

This exchange takes place between non-sister chromatids — that is, between a chromatid of the maternal chromosome and a chromatid of the paternal chromosome. It does not take place between the two sister chromatids of one chromosome, and there would be no point in it if it did, since those two are identical copies.

The exchange is called crossing over, and Fig. 2.10-2 shows it stage by stage. Its result is that chromosomes come out of meiosis carrying combinations of genes that neither parent chromosome had before — part maternal, part paternal, in one continuous length. This reshuffling is called genetic recombination.

NoteDefinition

Crossing Over The exchange of genetic material during meiosis between two non-sister chromatids of a pair of homologous chromosomes, resulting in genetic recombination.

Diagram

Fig. 2.10-2 — Crossing Over Four drawings in a row, joined by arrows, showing one pair of homologous chromosomes only. 1: the two homologous chromosomes drawn separately, each of two chromatids joined at a centromere, one solid and one outlined; label maternal chromosome, paternal chromosome and sister chromatids. 2: the two lying side by side along their length; label pairing of homologous chromosomes, or synapsis, and label the non-sister chromatids. 3: two non-sister chromatids crossing over each other at one point, forming an X; label the chiasma as the point of crossing over. 4: the two chromosomes drawn separately again, each now with one chromatid partly solid and partly outlined where the exchange took place; label chromosomes with exchanged portions, carrying a new combination of genes. Keep the solid and outlined shading consistent across all four drawings so the exchanged segment can be traced.

Sister Chromatids And Homologous Chromosomes Sister chromatids are two identical copies of one chromosome, joined at a centromere. Homologous chromosomes are two separate chromosomes, one from each parent, alike in size and shape but not identical in the genes they carry. Crossing over happens between homologous chromosomes, never between sisters.

2.10.3 Significance Of Meiosis

Meiosis does two things, and both matter.

  1. It halves the chromosome number in the gametes, so that when the gametes fuse at fertilisation the normal diploid number is restored. Without meiosis the chromosome number would double in every generation.
  2. It mixes up the genes, so that the gametes produced are not copies of the parent cell or of each other.

The mixing happens in two separate ways during the first division.

The first is the separation of the pairs. When the homologous chromosomes are pulled apart, it is settled quite independently for each pair which member goes to which side. A gamete therefore ends up with some chromosomes that came originally from the mother and some from the father, in a combination that is its own.

The second is crossing over. Even a single chromosome passed on is often no longer purely maternal or purely paternal, because lengths of it have been exchanged.

Put together, these two make every gamete different from every other, and so the progeny of the same two parents are never identical. Children of the same parents resemble each other and their parents, because the genes all came from the same two sets. They differ from each other because no two gametes carried the same selection.

Every Gamete Is A New Deal A human being has 23 pairs of chromosomes. Each gamete takes one member of every pair, and which member it takes is settled independently for each pair. That alone allows \(2^{23}\) different combinations — 8,388,608 of them — before crossing over has contributed anything at all.

Q5. Children of the same parents are unmistakably related, yet no two are alike. What happens during a cell division to make that possible?

A. Meiosis. As the homologous pairs separate, each gamete receives an independently chosen mixture of maternal and paternal chromosomes; and crossing over exchanges lengths of material between the pairs before they separate. Every gamete therefore carries a different combination of genes, and every child begins from a different pair of gametes.

These differences between individuals are called variations, and they are the raw material of heredity. You will meet them again, and in far more detail, when you study how characters are inherited.

2.11 Mitosis And Meiosis Compared

Both divisions begin the same way. In each, the DNA is copied once, so that every chromosome enters the division as two chromatids. From that point onwards they part company completely.

The whole of the difference follows from what the two divisions are for. Mitosis has to produce more cells of the kind that already exists, so it copies and repeats. Meiosis has to produce gametes, so it must halve the chromosome number and, in doing so, it also shuffles the genes.

Before reading Table 2.3, note one term used in it. Somatic cells are the ordinary body cells — the word comes from soma, body, the same root you met in the name chromosome. In a plant the corresponding term is vegetative cells. They are all the cells of an organism other than the germinal cells that give rise to gametes.

Table 2.3 — Differences Between Mitosis And Meiosis | Feature | Mitosis | Meiosis | |—|—|—| | Where it occurs | In the somatic or vegetative (body) cells | In the germinal cells of the reproductive organs | | What it is for | Growth, repair and replacement | Formation of gametes only | | When it occurs | Continuously throughout life | Only during the reproductively active period of life | | Number of nuclear divisions after the chromosomes are duplicated | One | Two | | Number of daughter cells produced | Two | Four | | Chromosome number in each daughter cell | The full set is passed on — the diploid number, \(2n\) | Only half the set is passed on, one member of each pair — the haploid number, \(n\) | | Pairing of homologous chromosomes | Does not occur | Occurs, and is followed by crossing over | | Chromosomes and genes in the daughter cells | Identical to those of the parent cell | Randomly assorted between the gametes, giving genetic variation |

Read the last two rows together, because they are connected. Homologous chromosomes never pair in mitosis, so there is no opportunity for an exchange, and the daughter cells come out identical. In meiosis they do pair, an exchange takes place, and the daughter cells come out different. Sameness and variation are not accidents of the two processes; each is produced deliberately by the way the chromosomes are handled.

Reductional And Equational The first meiotic division is called reductional, because it reduces the chromosome number from \(2n\) to \(n\). The second is called equational, because the number stays equal to what it was — as it does in mitosis, which is why mitosis is also an equational division.

2.12 Summary

  1. Most of a cell’s DNA lies in the nucleus, divided among a definite number of separate units called chromosomes.
  2. Each chromosome holds one long DNA molecule with many protein molecules attached to it. This combination of DNA and protein is chromatin.
  3. In a cell that is not dividing, chromatin is an extremely fine, uncoiled fibre. As the cell prepares to divide it coils and condenses into short, thick chromosomes that can be stained and seen.
  4. Walther Flemming, working on the larvae of salamanders, published in 1882 the first careful account of these threads. He saw that they split along their length, and he named the division of the nucleus mitosis.
  5. Chromatin is about 40 per cent DNA and about 60 per cent histone protein. DNA and RNA are the two kinds of nucleic acid; chromatin contains DNA.
  6. DNA is a macromolecule made of two complementary strands wound around each other in a double helix. The shape was established by Watson and Crick in 1953, working from Rosalind Franklin’s X-ray photographs.
  7. Each strand is a chain of repeating nucleotides. A nucleotide has three parts: a phosphate group, a pentose sugar and a nitrogenous base.
  8. The sugars and phosphates form a backbone running lengthwise; the bases point inwards and pair with the bases of the other strand, making a ladder in which the paired bases are the rungs.
  9. DNA has four nitrogenous bases. Adenine and guanine are purines, with a double ring; cytosine and thymine are pyrimidines, with a single ring. RNA carries uracil, a pyrimidine, in place of thymine.
  10. Adenine pairs only with thymine, held by two hydrogen bonds, and guanine only with cytosine, held by three. The order of bases on one strand therefore fixes the order on the other.
  11. Histones package the DNA. A length of DNA wound around a core of eight histone proteins forms a nucleosome, which is the basic structural unit of a chromosome.
  12. About 2 metres of DNA is fitted into a nucleus about 6 µm across by successive coiling: DNA is wound into nucleosomes, these are packed into a chromatin fibre, and the fibre is coiled and then supercoiled.
  13. A duplicated chromosome consists of two identical sister chromatids joined at a centromere. Two chromatids joined at one centromere still count as one chromosome.
  14. The centromere is the point at which spindle fibres attach to a chromosome, and it splits at the proper moment so that the sister chromatids can be drawn to opposite poles.
  15. A gene is a specific sequence of nucleotides at a definite site on a chromosome, coding for a particular protein that is expressed as a feature of the body. Genes are the units of heredity.
  16. New cells are needed for growth, for replacement of worn-out cells, for repair of damage and for reproduction.
  17. A human body cell is diploid, with 46 chromosomes in 23 pairs, written \(2n\). A gamete is haploid, with 23 chromosomes, written \(n\). Fertilisation restores the diploid number.
  18. The cell cycle is the series of events leading to the duplication of a cell’s DNA and its division into two daughter cells. It consists of a long interphase and a short M phase.
  19. Interphase has three phases in order. In G1 the cell grows, RNA and proteins are made, and mitochondria and chloroplasts divide; at the end of G1 a cell may instead withdraw into a resting phase. In S the DNA is replicated, so the DNA content doubles while the chromosome number stays the same. In G2 further proteins needed for division are made.
  20. In replication the two strands of the double helix separate, and a new strand is built against each of them by complementary base pairing. Each of the two molecules produced contains one old strand and one new strand.
  21. The cell cycle is regulated. In some tissues division stops permanently, in some it stops until it is needed, and in some it continues throughout life. Uncontrolled division produces a tumour, which may or may not be cancerous.
  22. There are two types of cell division. Mitosis occurs in body cells and produces diploid cells; meiosis occurs in the germinal cells of the reproductive organs and produces haploid gametes.
  23. Mitosis is completed in two steps — karyokinesis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm.
  24. Karyokinesis passes through four continuous phases. In prophase the chromosomes condense and become visible as pairs of chromatids, the centrosome divides and moves to the poles forming asters, the spindle appears, and the nuclear membrane and nucleolus disappear.
  25. In metaphase the chromosomes line up in one plane at the equator and each attaches to a spindle fibre by its centromere.
  26. In anaphase the centromeres divide and the sister chromatids are drawn to opposite poles by the shortening of the spindle fibres.
  27. In telophase the chromosomes reach the poles and uncoil, the spindle disappears, and a nuclear membrane and nucleolus re-form around each set.
  28. Cytokinesis occurs by furrowing in an animal cell, the furrow deepening from the membrane inwards, and by the formation of a cell plate in a plant cell, the plate growing from the centre outwards.
  29. Mitosis in a plant cell differs from that in an animal cell in that no asters are formed, the spindle is formed with the help of microtubules, cytokinesis is by cell plate, and the division is confined mainly to meristematic tissue.
  30. Mitosis provides for growth, repair, replacement and asexual reproduction, and maintains the same chromosome number in the daughter cells.
  31. Meiosis is completed in two divisions with only one duplication of DNA, producing four haploid cells from one diploid cell. The first division is the reduction division, in which the members of each homologous pair are separated.
  32. Homologous chromosomes are the two chromosomes of a pair, one from each parent, carrying genes for the same characteristics. During meiosis they pair closely, an X-shaped chiasma forms between non-sister chromatids, and segments are exchanged.
  33. This exchange is crossing over, and it produces new combinations of genes.
  34. Meiosis halves the chromosome number so that the diploid number is restored at fertilisation, and it mixes genes in two ways — by the chance separation of maternal and paternal chromosomes and by crossing over. Together these produce the variations found among offspring.

Flowchart

FC 2.7 — The Chapter In One Chain Nucleotide → chain of nucleotides forms a DNA strand → two complementary strands form a DNA molecule → DNA wound around histones forms nucleosomes → nucleosomes packed into a chromatin fibre → fibre coiled and supercoiled into a chromosome → DNA replicated in the S phase, each chromosome becoming two sister chromatids → branch 1: mitosis in body cells → two diploid daughter cells identical to the parent; branch 2: meiosis in germinal cells → four haploid gametes carrying new combinations of genes → fertilisation restores the diploid number, returning to the start in the next generation

2.13 Key Terms

  1. Aster — the star-shaped set of rays that radiates around each centriole in a dividing animal cell.
  2. Cell cycle — the series of events in a cell leading to the duplication of its DNA and its division into two daughter cells.
  3. Cell division — the process by which one cell gives rise to new cells, either by mitosis or by meiosis.
  4. Cell plate — the partition laid down across the middle of a dividing plant cell, growing from the centre outwards to separate the two daughter cells.
  5. Centriole — a small cylindrical body within the centrosome of an animal cell, around which the aster forms.
  6. Centromere — the constricted point at which the two sister chromatids of a chromosome are joined and to which the spindle fibres attach.
  7. Centrosome — the region just outside the nucleus of an animal cell, containing the centrioles, which divides in two before division and moves to the poles.
  8. Chiasma — the X-shaped structure formed where two non-sister chromatids of a homologous pair cross during meiosis; plural chiasmata.
  9. Chromatid — one lengthwise half of a duplicated chromosome.
  10. Chromatin — the material of which chromosomes are made, being DNA together with histone proteins, seen as a fine fibre when the cell is not dividing.
  11. Chromosome — a highly coiled, ribbon-like structure formed by the condensation of chromatin fibres during cell division.
  12. Cleavage furrow — the inward-deepening groove in the cell membrane by which the cytoplasm of an animal cell is divided in two.
  13. Crossing over — the exchange of segments between two non-sister chromatids of a pair of homologous chromosomes during meiosis.
  14. Cytokinesis — the division of the cytoplasm that follows the division of the nucleus and completes a cell division.
  15. Diploid — having chromosomes in pairs, that is the full set, written \(2n\); the condition of a body cell.
  16. DNA — deoxyribonucleic acid, the long molecule that carries the instructions for building and running the cell.
  17. Double helix — the shape of a DNA molecule, in which two complementary strands wind around each other in a regular spiral.
  18. Equator — the plane midway between the two poles of a dividing cell, at which the chromosomes line up during metaphase.
  19. Gene — a specific sequence of nucleotides at a definite site on a chromosome, coding for a particular protein and expressed as a feature of the body.
  20. Haploid — having only one member of each pair of chromosomes, written \(n\); the condition of a gamete.
  21. Histones — the proteins around which DNA is wound, which coil and package it into nucleosomes.
  22. Homologous chromosomes — the two chromosomes of a pair, one from each parent, alike in length and centromere position and carrying genes for the same characteristics.
  23. Hydrogen bond — the weak bond that joins a nitrogenous base on one DNA strand to its partner base on the other.
  24. Interphase — the long non-dividing phase of the cell cycle, made up of G1, S and G2, during which the cell grows and copies its DNA.
  25. Karyokinesis — the division of the nucleus, completed in four continuous phases.
  26. Macromolecule — a very large molecule built by joining a great many small units end to end.
  27. Meiosis — the cell division, occurring in the germinal cells of the reproductive organs, that halves the chromosome number and produces four haploid gametes.
  28. Mitosis — the cell division in which one parent cell divides into two identical daughter cells, the chromosome number being maintained.
  29. M-phase — the dividing phase of the cell cycle, in which karyokinesis and cytokinesis take place.
  30. Nitrogenous base — the part of a nucleotide that points inwards and pairs with a base on the opposite strand; adenine, guanine, cytosine or thymine in DNA.
  31. Nucleic acid — either of the two kinds of molecule, DNA and RNA, built from chains of nucleotides.
  32. Nucleosome — a length of DNA wound around a core of eight histone proteins; the basic structural unit of a chromosome.
  33. Nucleotide — the repeating unit of a nucleic acid, made of a phosphate group, a pentose sugar and a nitrogenous base.
  34. Poles — the two extreme ends of a dividing cell, towards which the chromatids are drawn.
  35. Purine — a nitrogenous base with a double-ring structure; adenine and guanine.
  36. Pyrimidine — a nitrogenous base with a single-ring structure; cytosine and thymine in DNA, and uracil in RNA.
  37. Replication — the copying of a DNA molecule during the S phase, each new molecule containing one old strand and one new one.
  38. Resting phase (R) — the state of a cell that has withdrawn from the cell cycle at the end of G1 and no longer prepares to divide.
  39. RNA — ribonucleic acid, the second kind of nucleic acid, built by the cell using DNA as the pattern and used in making proteins.
  40. Sister chromatids — the two identical chromatids of one duplicated chromosome, joined at a single centromere.
  41. Somatic cells — the ordinary body cells of an organism, as distinct from the germinal cells that give rise to gametes.
  42. Spindle fibres — the fine protein threads that stretch between the poles of a dividing cell and draw the chromosomes apart.
  43. Synapsis — the close side-by-side pairing of homologous chromosomes during meiosis.
  44. Zygote — the fertilised egg, the single diploid cell from which a new organism develops.

2.14 Points To Ponder

  1. A cell keeps its chromatin uncoiled while it is working and tightly coiled while it is dividing. Each state has a drawback: the uncoiled fibre takes up a great deal of room and tangles easily, while the coiled chromosome cannot have its genes read. Why does the cell go to the trouble of switching between the two, instead of settling on something halfway that would do both jobs moderately well?

  2. Meiosis produces variation, and variation is clearly valuable to a species. Why, then, do organisms not use meiosis for growth and repair as well? What would go wrong in a wound if the cells closing it divided by meiosis instead of mitosis?

  3. Section 2.7.3 showed that the most heavily worn tissues are replaced fastest, while the nerve cells of the cerebral cortex are not replaced at all. What does that difference suggest about why a deep cut in the skin heals completely within weeks, while damage to the brain or spinal cord is repaired so much less readily?

  4. Crossing over takes place between non-sister chromatids and never between sister chromatids. Work out for yourself why an exchange between sister chromatids would produce no variation whatever. Then take the question one step further: if crossing over can bring together a new and useful combination of genes, can it also break apart a combination that was already useful?

  5. A plant cell divides by laying down a cell plate that grows from the centre outwards, while an animal cell is pinched in two from the outside inwards. One feature of the plant cell makes the animal method impossible. Which feature is it, and what else about the way plants grow — and about where in a plant growth happens at all — follows from that same feature?

  6. In mitosis the centromere splits at anaphase and not a moment earlier. Suppose that in just one chromosome out of a human cell’s 46, the centromere were to split during metaphase, before the chromosomes had been arranged at the equator. What would the two daughter cells end up with, and why would the error not be corrected at the next division?

2.15 Objective Questions

  1. Which of the following is the basic structural unit of a chromosome? (1 mark)
    1. Nucleotide
    2. Nucleosome
    3. Gene
    4. Chromatid
  2. One complete nucleosome consists of: (1 mark)
    1. a phosphate group, a pentose sugar and a nitrogenous base
    2. two complementary strands of DNA wound in a helix
    3. a length of DNA wound around a core of histone proteins
    4. two identical chromatids joined at a centromere
  3. Adenine is joined to thymine in a DNA molecule by: (1 mark)
    1. one hydrogen bond
    2. two hydrogen bonds
    3. three hydrogen bonds
    4. four hydrogen bonds
  4. During the S phase of interphase: (1 mark)
    1. the amount of DNA doubles but the number of chromosomes does not change
    2. the number of chromosomes doubles but the amount of DNA does not change
    3. both the amount of DNA and the number of chromosomes double
    4. neither the amount of DNA nor the number of chromosomes changes
  5. A human cell that has completed the G2 phase and is about to divide by mitosis contains: (1 mark)
    1. 23 chromosomes and 46 chromatids
    2. 46 chromosomes and 46 chromatids
    3. 46 chromosomes and 92 chromatids
    4. 92 chromosomes and 92 chromatids
  6. In which phase of mitosis is a moving chromosome usually seen bent into a V or a J shape? (1 mark)
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase
  7. Asters are formed during mitosis in: (1 mark)
    1. animal cells only
    2. plant cells only
    3. both animal and plant cells
    4. neither animal nor plant cells
  8. Crossing over during meiosis takes place between: (1 mark)
    1. the two sister chromatids of one chromosome
    2. non-sister chromatids of a pair of homologous chromosomes
    3. any two chromosomes lying near each other
    4. the chromosomes and the spindle fibres
  9. Choose the correct option. (1 mark) Assertion (A): The nuclear membrane disappears during prophase. Reason (R): The spindle fibres cannot reach the chromosomes while the nuclear membrane is intact.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  10. Choose the correct option. (1 mark) Assertion (A): Interphase was once described as the resting phase of the cell. Reason (R): The cell carries out very little chemical activity between two divisions.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  11. Choose the correct option. (1 mark) Assertion (A): Only one duplication of DNA takes place during the whole of meiosis. Reason (R): Meiosis is completed in two successive divisions and produces four haploid cells from one diploid cell.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  12. Match each structure in Column A with its correct description in Column B. (5 marks)
Column A Column B
a. Nucleosome i. Radiating rays around a centriole in a dividing animal cell
b. Centromere ii. X-shaped structure formed between two non-sister chromatids
c. Chiasma iii. DNA wound around a core of eight histone proteins
d. Cell plate iv. The region at which spindle fibres attach to a chromosome
e. Aster v. Partition laid down across the middle of a dividing plant cell
  1. Fill in the blanks. (5 marks)
    1. The two strands of a DNA molecule are held together by ______ bonds.
    2. A phosphate group, a pentose sugar and a nitrogenous base together make one ______.
    3. Mitochondria and chloroplasts divide during the ______ phase of interphase.
    4. The chromosome number is halved during the ______ division of meiosis.
    5. In a plant cell, the cytoplasm is divided by the formation of a ______.
  2. State whether each statement is true or false. (4 marks)
    1. Two sister chromatids joined at one centromere are counted as two chromosomes.
    2. The number of chromatin fibres in a resting nucleus equals the number of chromosomes seen at division.
    3. Asters take up stain readily and are therefore easily seen.
    4. Uracil is a pyrimidine.
  3. Name the following. (5 marks)
    1. The stage of mitosis in which the centromere splits.
    2. The X-shaped structure formed between two chromatids during meiosis.
    3. The plane along which the chromosomes arrange themselves during metaphase.
    4. The phase of the cell cycle in which DNA is replicated.
    5. The close side-by-side pairing of homologous chromosomes during meiosis.
  4. In each set, pick the odd one out and give the category to which the remaining terms belong. (4 marks)
    1. Adenine, guanine, cytosine, uracil
    2. Testis, ovary, anther, bone marrow
    3. Centriole, aster, cleavage furrow, cell plate
    4. Growth, repair, replacement, gamete formation

2.16 Check Your Concepts

  1. Define a gene. (2 marks)

  2. State the two functions of the centromere. (2 marks)

  3. What is meant by the terms diploid and haploid? Give the value of each for a human being. (2 marks)

  4. Name the two steps into which mitosis is divided, and state what is divided in each. (2 marks)

  5. Give a reason why the two strands of a DNA molecule are described as complementary. (2 marks)

  6. Name the two kinds of nucleic acid, and state one difference between the nitrogenous bases they carry. (2 marks)

  7. Why is it wrong to call interphase the resting phase of a cell? (3 marks)

  8. Differentiate between a centromere and a centrosome, stating the basis of difference. (3 marks)

  9. Explain why a purine always pairs with a pyrimidine, and state what would happen to the shape of the DNA molecule if two purines were to pair with each other. (3 marks)

  10. Describe the events that take place during metaphase of mitosis, and explain why the arrangement of the chromosomes at this stage is important. (3 marks)

  11. Give a reason for each of the following. (3 marks)

    1. Chromatin condenses into thick, short chromosomes before a cell divides.
    2. Mitochondria are shared out at random between the daughter cells, while chromosomes are not.
    3. A cell plate, and not a furrow, divides the cytoplasm of a plant cell.

  12. Describe four events that take place during prophase of mitosis in an animal cell. (4 marks)

  13. Differentiate between mitosis and meiosis with regard to each of the following. (4 marks)

    1. The cells in which each occurs
    2. The number of nuclear divisions after the chromosomes are duplicated
    3. The number of daughter cells produced
    4. The chromosome number of the daughter cells

  14. What would happen if the spindle fibres failed to form in a cell that had entered mitosis? Describe the effect on the two daughter cells that would result. (4 marks)

  15. The cell cycle is regulated, and does not run on endlessly. Explain what this means, giving one example of a tissue in which division stops permanently and one in which it continues throughout life, and state what may happen if the regulation fails. (4 marks)

  16. Describe the process by which a DNA molecule is copied, and explain why each of the two molecules produced contains one old strand and one new one. (5 marks)

  17. Explain, using the terms homologous chromosomes, non-sister chromatids and crossing over, how meiosis makes every gamete produced by one individual different from every other. (5 marks)

  18. Differentiate between mitosis in an animal cell and mitosis in a plant cell, stating four points of difference and the basis of each. (5 marks)

2.17 Figure-Based Questions

  1. Study Fig. 2.3-2 and answer the questions that follow. (4 marks)

    1. How many strands are shown in part B of the figure?
    2. Name the three parts of which a single nucleotide is made.
    3. Name the bond by which the two bases of a rung are joined.
    4. State which two bases are joined by three such bonds.

  2. Study panel 4 of Fig. 2.9-1 and answer the questions that follow. (4 marks)

    1. Identify the stage of mitosis shown, and give one reason in support of your answer.
    2. Name the structure that has just divided at the start of this stage.
    3. Name the structures that are drawing the chromosomes towards the poles.
    4. Name the stage that follows this one, and state one event that identifies it.

  3. Study Fig. 2.10-2 and answer the questions that follow. (4 marks)

    1. Name the process that the figure represents.
    2. Name the X-shaped structure shown in the third drawing.
    3. Between which two chromatids does the exchange take place?
    4. State one result of this process for the offspring.

  4. Study Fig. 2.7-1 and answer the questions that follow. (3 marks)

    1. Name the three phases of interphase in the order in which they occur.
    2. State the main event of the phase marked S.
    3. Name the phase from which a cell may leave the cell cycle, and name the state it enters.

  5. Draw a labelled diagram of a single nucleosome as seen in Fig. 2.3-3. Label the histone core, the DNA and the number of histone proteins present. (3 marks)

  6. Draw a labelled diagram to show the anaphase stage of mitosis in an animal cell in which the chromosome number is 4. Label the daughter chromosomes, the spindle fibres, the aster and the poles, using straight lead lines that do not cross. (5 marks)