Cell Cycle and Cell Division Class 11 Notes

• G₁ phase (Gap 1): cell grows, organelles duplicate, proteins synthesised. The cell decides whether to divide or enter G₀ (quiescent phase — non-dividing, e.g. neurons, cardiac muscle cells).
• S phase (Synthesis): DNA replication occurs. Each chromosome is duplicated to form two sister chromatids joined at the centromere. The centrosome also duplicates. DNA content doubles (2C → 4C) but chromosome number stays the same.
• G₂ phase (Gap 2): final preparations — tubulin and other proteins for the mitotic spindle are synthesised.
• M phase (Mitosis + Cytokinesis): nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).

Mitosis is equational division — each daughter cell receives the same number and kind of chromosomes as the parent cell (2n → 2n). Mitosis is responsible for growth, tissue repair, and asexual reproduction. It produces two genetically identical daughter cells.

Stages of mitosis

• Prophase: chromatin condenses into visible chromosomes (each with two sister chromatids), the nuclear envelope breaks down, the nucleolus disappears, and the mitotic spindle begins to form from centrosomes.
• Metaphase: chromosomes align at the metaphase plate (equatorial plane). Spindle fibres from opposite poles attach to the kinetochores of sister chromatids.
• Anaphase: centromeres split; sister chromatids are pulled to opposite poles by shortening of spindle fibres. Each chromatid is now an independent chromosome.
• Telophase: chromosomes decondense, nuclear envelopes re-form, nucleoli reappear, and the spindle disassembles. This is essentially the reverse of prophase.

Cytokinesis follows karyokinesis. In animal cells a cleavage furrow pinches the cell in two; in plant cells a cell plate forms from the centre outward because of the rigid cell wall.

Meiosis is a special type of cell division that reduces the chromosome number by half (2n → n) to produce gametes (spores in plants). It involves two successive divisions — Meiosis I and Meiosis II — with no DNA replication between them. A single diploid cell produces four haploid daughter cells.

Meiosis I — Reduction division

Meiosis I separates homologous chromosomes (not sister chromatids) and halves the chromosome number. Prophase I is the longest and most complex phase of meiosis, divided into five substages:

• Leptotene: chromosomes begin to condense and become visible as thin threads.
• Zygotene: homologous chromosomes pair up (synapsis) to form bivalents (tetrads). The synaptonemal complex holds them together.
• Pachytene: crossing over (recombination) occurs — non-sister chromatids of a bivalent exchange segments at chiasmata. This generates genetic variation.
• Diplotene: the synaptonemal complex dissolves; homologues begin to separate but remain attached at chiasmata (visible as X-shaped intersections).
• Diakinesis: chromosomes are fully condensed; chiasmata terminalise; the nuclear envelope breaks down.

In metaphase I, bivalents align at the metaphase plate (not individual chromosomes). The orientation of each bivalent is random — independent assortment. In anaphase I, homologous chromosomes separate to opposite poles; sister chromatids remain joined. In telophase I, two haploid cells form.

Meiosis II — Equational division

Meiosis II resembles mitosis: sister chromatids separate. In prophase II the chromosomes condense again; in metaphase II individual chromosomes align at the plate; in anaphase II centromeres split and chromatids move to opposite poles; in telophase II four haploid nuclei are formed, followed by cytokinesis.

• Reduction: halves the chromosome number from diploid (2n) to haploid (n), ensuring that when two gametes fuse at fertilisation, the zygote restores the diploid number.
• Crossing over in prophase I creates new combinations of alleles on a single chromosome — genetic recombination.
• Independent assortment of bivalents at metaphase I means each gamete gets a unique mix of maternal and paternal chromosomes.
• Together, crossing over and independent assortment are the basis of genetic variation in sexually reproducing organisms — the raw material for evolution.

• Mitosis: one division → two daughter cells; Meiosis: two divisions → four daughter cells.
• Mitosis: daughter cells are diploid (2n) and genetically identical to parent; Meiosis: daughter cells are haploid (n) and genetically different.
• Mitosis: no crossing over; Meiosis: crossing over occurs in prophase I.
• Mitosis: occurs in somatic cells for growth and repair; Meiosis: occurs in germ cells to produce gametes/spores.
• Mitosis: homologous chromosomes do not pair; Meiosis I: homologues pair as bivalents (synapsis).

Ploidy refers to the number of sets of chromosomes. Diploid (2n) cells have two sets — one from each parent. Haploid (n) cells have one set. In humans 2n = 46, so n = 23. After S phase the DNA content doubles (4C) but the chromosome count remains 2n because sister chromatids share a centromere. In anaphase of mitosis, chromatids separate and the transient cell is temporarily 4n.

Example: A cell with 2n = 14 chromosomes enters meiosis. How many chromosomes and chromatids are present in each daughter cell?

Solution: After meiosis I, each cell is haploid — n = 7 chromosomes, each with 2 chromatids (14 chromatids per cell). After meiosis II, sister chromatids separate, giving four daughter cells each with n = 7 chromosomes and 7 chromatids (since each chromosome is now a single chromatid). Total DNA content per cell is 1C.

Example: If a diploid organism has 2n = 8 chromosomes, how many bivalents form during meiosis I and how many tetrads are visible at pachytene?

Solution: With 2n = 8, there are 4 homologous pairs. Each pair forms one bivalent, so 4 bivalents form. Each bivalent is a tetrad (4 chromatids), so 4 tetrads are visible at pachytene. Crossing over between non-sister chromatids of each bivalent generates recombinant chromosomes.

Exercises (NCERT Q1–Q16)

• 1) What is the average cell cycle span for a mammalian cell?
• 2) Distinguish cytokinesis from karyokinesis.
• 3) Describe the events taking place during interphase.
• 4) What is Go (quiescent phase) of cell cycle?
• 5) Why is mitosis called equational division?
• 6) Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator. (ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place.
• 7) Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.
• 8) How does cytokinesis in plant cells differ from that in animal cells?
• 9) Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
• 10) Distinguish anaphase of mitosis from anaphase I of meiosis.
• 11) List the main differences between mitosis and meiosis.
• 12) What is the significance of meiosis?
• 13) Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur.
• 14) Can there be mitosis without DNA replication in ‘S’ phase?
• 15) Can there be DNA replication without cell division?
• 16) Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell (ii) amount of DNA content (C) per cell

Answers at a glance

1. 1
   Q1 → The average cell cycle span of a mammalian cell is about 24 hours. The 24 hours is distributed as about 12 hours in the G1 phase, about 6 to 8 hours in the S phase in which the DNA is replicated, about 3 to 4 hours in the G2 phase, and about 1 hour in the M phase, so that mitosis and cytokinesis together form a very small part of the cycle. The figure is an average and not a constant, and it must be qualified in three ways. The duration differs greatly between cell types: it may be as short as an hour or two in a rapidly dividing cell, is typically several hours in a cultured mammalian cell, and is of several days, or the cell does not divide at all, in the case of the human liver and nerve cells, which normally leave the cycle and lie in the G0 stage. The rate also depends on the temperature, rising to an optimum and falling again, and the figure quoted is for the human body temperature of 37 degrees Celsius. It further depends on the availability of the nutrients and of the growth factors, since G1 is the decision point at which the cell either enters the S phase or leaves the cycle for the G0. The importance of knowing the span is practical, since a cancer cell is characterised precisely by its loss of the control of the G1, so that it divides far more rapidly than a normal cell, and the timing of chemotherapy depends on the length of the phases, a drug being administered to strike the cells while they are in the S phase and are synthesising DNA and are therefore most vulnerable to a DNA-damaging agent.
2. 2
   Q2 → Karyokinesis and cytokinesis are the two divisions of the M phase and they are the nuclear and the cytoplasmic divisions respectively. Karyokinesis is the division of the nucleus, and it comprises the whole of prophase, metaphase, anaphase and telophase; in it the nuclear envelope breaks down, the chromosomes align on the spindle, the centromeres split so that the sister chromatids separate and are drawn to the opposite poles, and two daughter nuclei are re-formed at the poles, and the number of chromosomes in each daughter nucleus being set by the behaviour of the centromere. Cytokinesis is the division of the cytoplasm, and it begins in the late anaphase or in the telophase and is completed only after the nuclei have divided, so that two separate daughter cells are produced, each with its own nucleus, its own cytoplasm and its own share of the organelles, so the daughter cells are only fully formed when the cytokinesis is complete and the karyokinesis alone leaves a cell with two nuclei inside one continuous mass of cytoplasm. The two divisions differ also in their mechanism of cytokinesis, and the difference is dictated by whether a cell wall is present. In an animal cell, which has no cell wall, the cytoplasm is divided by the furrow or cleavage method, in which a constriction appears at the middle of the cell and deepens as a ring of actin and myosin microfilaments contracts, until the cytoplasm is pinched into two, so the division proceeds from the outside inwards. In a plant cell, which has a rigid wall that cannot constrict, the cytoplasm is divided by the cell plate method, in which the Golgi vesicles fuse at the centre of the cell to form a cell plate, new membranes are formed at its margins and the plate grows outwards until it reaches the existing wall, and the plate is then converted into a new and complete cell wall between the two daughter cells. Thus the division of the nucleus is essentially the same in both, and the division of the cytoplasm differs, by a furrow in the animal cell and by a cell plate in the plant cell, precisely because the plant cell wall prevents any constriction.
3. 3
   Q3 → The interphase is the period between the end of one division of the cell and the beginning of the next. It is the longest phase of the cell cycle, occupying about 90 to 95 per cent of its total duration, and it is the phase in which the cell is metabolically very active, is growing, and is replicating its DNA; the chromatin is in the extended and decondensed form, so the chromosomes are not visible as such, and the nuclear envelope is intact. The interphase is divided into the G1 phase, the S or synthesis phase, the G2 phase, and the G0 quiescent phase. In the G1, or first gap phase, which is the longest sub-phase and occupies about half of the interphase, the cell grows continuously in size and mass, synthesises the RNA, the proteins and the enzymes that the division will need, and produces the ATP and the cell components for the division. The G1 is also the decision phase, being the checkpoint at which the cell assesses its size, its nutrient and growth-factor supply and its energy state, and it either commits to the S phase or leaves the cycle to enter the G0. In the S, or synthesis phase, the defining event of the interphase, the DNA is replicated, so that every chromosome, which previously consisted of a single chromatid, is converted into a chromosome of two identical sister chromatids joined at the centromere; the DNA content of the cell is thereby doubled while the number of chromosomes is not, since the chromosomes are counted at the centromere and the two chromatids still share the one centromere. The replication is semiconservative and proceeds from a replication fork, and the histones are synthesised and the new DNA is packaged around them during this phase, so a cell in the S phase is the most vulnerable to a DNA-damaging drug and to radiation, which is the basis of the timing of chemotherapy. In the G2, or second gap phase, lasting about three to four hours in a mammalian cell, the cell continues to prepare for the division and synthesises the proteins the division itself requires, in particular the tubulin of the spindle, so that the machinery of the mitosis is ready when the M phase begins; the G2 is a second checkpoint, at which incomplete or damaged replication arrests the cycle. Cells that leave the cycle, usually from the G1, pass into the G0 quiescent phase, in which they are metabolically active but neither divide nor prepare to divide; the neurons, the muscle cells and the liver cells of the adult human are examples, and the fact that a differentiated cell can be induced to divide again, as in the formation of a callus in plant tissue culture and in the formation of a cancer cell, shows that the exit from the cycle is a reversible regulation. The significance of the interphase is therefore that the cell doubles its DNA and doubles its mass, so that each daughter cell can receive a complete set, that it checks itself at the G1, at the G2 and at the metaphase, which makes the division orderly and prevents the transmission of damaged DNA, and that since the interphase occupies about 95 per cent of the cycle, a growing and dividing cell spends almost all of its time not dividing at all.
4. 4
   Q4 → The G0 or quiescent phase is a state of the cell in which the cell has left the cell cycle and is metabolically active but is neither dividing nor preparing to divide; the cells in the G0 do not divide and do not prepare to divide, and it is called the zero stage because the cell is not progressing through the cycle at all. It is entered, in the usual case, from the G1 phase, and unlike the other phases it is not a normal part of the cycle but a resting state. Its characteristics are that the cells are metabolically active rather than dead, so that they are performing their normal physiological work, that they are generally fully differentiated and specialised, as a neuron is specialised for the transmission of impulses, and that their DNA, their nucleus and their full complement of chromosomes are intact and functional, so that they are not degenerate in any respect. The examples in animals are the neurons, the muscle cells, the cells of the lens of the eye, the heart muscle cells and the cells of the adult liver, all of which are terminally differentiated and normally never divide again, though some can be induced to re-enter the cycle, as the remaining liver cells divide and regenerate the lost tissue after an injury; in plants the differentiated parenchyma can likewise be induced to divide and form a callus and regenerate a whole plant, and the meristematic cells that are not actively dividing represent the plant condition. It should be noted that the cell cycle of the bacteria is not of the G1, S, G2 and M type, so the G0 is a feature of the eukaryotic cell cycle. The importance of the G0 lies in two directions. It is the mechanism of differentiation, since it is by leaving the cycle and becoming quiescent that a cell commits itself to a single specialised function, so that the division of labour in a multicellular body is achieved by cells that stop dividing rather than by cells that keep dividing. It is also the point at which the control of the cycle fails in cancer, since a cancer cell is precisely a cell in which the checkpoint that should send it into the G0 has failed, so that it proceeds round the cycle again and again and forms a tumour, having defeated the control that operates at the metaphase to detect a misattached chromosome and to delay the anaphase. The two are the same fact from opposite sides, since the G0 is the mechanism by which an organism restrains cell division and the loss of the G0 is the mechanism by which a cancer cell escapes that restraint.
5. 5
   Q5 → Mitosis is called equational division because the two daughter cells it produces are equal to the parent cell and equal to each other in respect of the number of chromosomes and the amount of DNA, so that the chromosome number of each daughter is the same as that of the parent, that is 2n gives 2n and 2n, and this equality of the chromosome number and of the DNA content is what the word equational denotes. The mechanism that produces the equality has three steps. In the S phase the DNA is replicated, so that every chromosome of the parent comes to consist of two identical sister chromatids joined at the single centromere; the DNA content is thereby doubled while the chromosome number is unchanged, since chromosomes are counted at the centromere and the two chromatids still share it. In the metaphase the chromosomes align singly on the spindle at the metaphase plate, one centromere at a single point, and not in pairs as they align in the first division of meiosis. In the anaphase the centromeres split and the sister chromatids, being identical to one another, separate and are drawn to the opposite poles, so that each pole receives one complete set of chromosomes and each new nucleus receives exactly the complement that the parent nucleus had. The two daughter cells therefore have the same number of chromosomes and the same amount of DNA as the parent, and being derived from identical sister chromatids they are also genetically identical to the parent and to each other, and nothing has been halved. The contrast is with the first division of meiosis, in which the homologous chromosomes and not the sister chromatids separate, so that the chromosome number is halved and the division is reductional; the second division of meiosis is equational but operates on already haploid cells. Mitosis is therefore equational because it preserves the chromosome number exactly, and this is what makes it the division by which a single zygote is able to build a whole multicellular organism in which every cell of the body carries the same genome.
6. 6
   Q6 → (i) The chromosomes are moved to the spindle equator at the metaphase. It is at the metaphase that the spindle fibres from the two opposite poles attach to the kinetochores of the centromeres and the chromosomes are drawn to and arranged on the equatorial plate at the middle of the spindle, where they are in the most compact and most clearly visible condition; it is therefore the stage at which the chromosome number can be counted and a karyotype prepared, and it is also the stage of the checkpoint at which the cell confirms that every chromosome is attached from both poles before it allows the anaphase to begin. (ii) The centromere splits and the chromatids separate at the anaphase. It is at the anaphase that the centromeres divide, the two sister chromatids of each chromosome are thereby separated, and each chromatid is drawn to the opposite pole by the shortening of the spindle fibres; each chromatid is now an independent daughter chromosome, so that the two groups move apart and each of the two forming nuclei receives an identical set. (iii) The pairing between homologous chromosomes takes place in the prophase of the first meiotic division, and the pairing itself is at the zygotene. The synapsis, which is this association of the homologues, occurs at the zygotene, when the chromosomes start pairing together, and it is accompanied by the formation of the synaptonemal complex, the pair of synapsed homologues forming the bivalent or the tetrad, which is more clearly visible only at the next stage, the pachytene, where the crossing over occurs; in the corresponding stage of mitosis no pairing of homologues occurs at all. (iv) The crossing over between homologous chromosomes also takes place in the prophase I, again at the pachytene. The exchange of segments between the non-sister chromatids of the paired homologues, which is the crossing over or the recombination, is brought about during the pachytene, after the synapsis has been completed in the zygotene; the chiasmata become visible in the following sub-stage, the diplotene, when the homologues begin to separate and the points of the exchange are seen, so that the chiasmata are the visible evidence of a crossing over that has already occurred, and the exchange is completed by the end of the prophase I. Thus the four events are, in order, the metaphase, the anaphase, the prophase I at the zygotene, and the prophase I at the pachytene, the first two belonging to mitosis, in which single chromosomes align on the equator and the centromeres split, and the last two to the first meiotic division, in which the homologues pair and recombine, which is also why no crossing over occurs in mitosis, since there is no pairing of the homologues for it to occur in.
7. 7
   Q7 → Synapsis is the pairing of the two homologous chromosomes during the prophase I of the meiotic division, in which the homologues lie side by side in pairs along their whole length and are in exact register, with gene locus corresponding to gene locus, and the pairing is held together by a proteinaceous structure called the synaptonemal complex; it occurs at the zygotene, when the chromosomes start pairing together, and the pair so formed, the bivalent or the tetrad, becomes clearly visible at the next stage, the pachytene, and it is this pairing that makes the recombination possible, since the exchange of segments occurs between the non-sister chromatids of the homologues so held. A bivalent is the structure formed by a pair of homologous chromosomes that are synapsed during the prophase I, and it is also called a tetrad, because each homologue consists of two sister chromatids after the S phase, so that the pair contains two chromosomes and four chromatids; the number of bivalents in a diploid cell is therefore half the diploid number, that is n, since the 2n homologues form n pairs, and the bivalent persists through the pachytene, in which the crossing over occurs within it, and through the diplotene, and it begins to resolve into two single chromosomes at the end of the prophase I. A chiasma, plural chiasmata, is the point at which the two homologous chromosomes of a bivalent remain attached to each other after the homologues begin to separate in the diplotene, and it was the appearance of these crossing points that gave the stage its name; each chiasma marks the site of a previous exchange of segments between non-sister chromatids, and so its number and position reflect the number and position of the recombinational events that have already occurred at the pachytene; the chiasmata are subsequently terminalised, that is moved towards the ends, and are finally resolved at the end of the prophase I, so that by the metaphase I the homologues are held together only at the chiasmata, which is what allows them to be aligned on the equator and then separated in the first division. The three terms are therefore one idea seen in three ways: synapsis is the process, the bivalent is the structure that results from it, and the chiasma is the point that records the exchange that has occurred within it, and the sequence in time is that the homologues synapse at the zygotene, the bivalent is thereby formed and becomes clearly visible at the pachytene, the crossing over occurs within it at the pachytene, the chiasmata appear at the diplotene, they are terminalised, and they are resolved so that the homologues can separate at the anaphase I. The diagram required is a bivalent drawn in the pachytene or the diplotene, showing two homologous chromosomes lying side by side, each drawn as two sister chromatids joined at its centromere, so that two centromeres and four chromatids are shown; the labels to be written in are the two homologous chromosomes, the centromere and the two sister chromatids of each, the synaptonemal complex running along the length of the paired region, and one chiasma, drawn as the point where two chromatids cross, with a note that the two chromatids involved there are non-sister, that is one from each homolog, and that the chiasma marks a previous exchange of segments.
8. 8
   Q8 → The cytokinesis of a plant cell differs from that of an animal cell because of one fact, the presence in the plant of a rigid, non-contractile cell wall, which makes a constriction of the cell impossible and forces the plant cell to construct a new partition across the middle of the cell instead. In the animal cell, which is bounded only by a flexible plasma membrane, the cytokinesis is by the furrow or cleavage method: the plasma membrane at the middle of the cell is drawn inwards to form a constriction, which deepens as a ring of actin and myosin microfilaments beneath the membrane contracts, and the division proceeds inwards from the periphery until the cytoplasm is pinched completely into two cells of equal size. In the plant cell the cytokinesis is by the cell plate method: the vesicles of the Golgi apparatus move to the middle of the cell and fuse there to form a membrane-bound structure called the cell plate, which arises in the centre of the cell in the equatorial plane; the plate then grows outwards, its margins extending towards the existing wall, and while it grows new membranes are made at its edges and new wall material is deposited, so that it is converted into a new and complete cell wall which is then laid over by the existing wall on either side. The division therefore proceeds outwards from the centre, in the opposite direction to the animal cell, and the two daughter cells are again equal in size but are separated by a straight new wall. Two further differences complete the comparison. The plane of division is fixed in advance in the plant cell, being the plane of the future wall, determined by the position of the pre-prophase band of microtubules, so that the new wall is a flat, straight partition, whereas in the animal cell the plane is not fixed in advance and the furrow may be oblique, and in an irregular cell it may occur at any angle. And the site of the new partition also differs, since in the plant cell the cell plate is formed strictly in the equatorial region of the parent cell, whereas in the animal cell the partition is a cleavage of the membrane and the cytoplasm and no new wall is constructed at all. The two methods are therefore not variations of a single process but two distinct processes dictated by the presence or the absence of a cell wall, and the dependence of the plant cell on its own endomembrane system for the task is why the Golgi apparatus is so abundant in the dividing plant cells and why the cell plate is membrane-bound.
9. 9
   Q9 → The four daughter cells produced by meiosis are equal in size in those organisms in which the meiosis I and the meiosis II are essentially alike, that is of the successive or the simultaneous type, and they are unequal in size in those in which the meiosis I is a reductional division and the meiosis II is equational, since the two products of the first division then differ in size from one another. NCERT states the rule that the four daughter cells are equal in size in a plant or an animal in which the meiosis is of the successive or the simultaneous type, and are unequal where the meiosis I is reductional and the meiosis II is equational. The classic example of the equal case is the insect, such as the grasshopper or the locust, in the spermatogenesis of which the meiosis I is equational rather than reductional, so that the first division gives two daughter cells of the same size and the second divides each of them equally, producing four spermatids of equal size; the same is true of the honey bee and of the silkworm, and of the angiosperms, in which the microspore mother cell of the anther and the megaspore mother cell of the ovule both produce four products of equal size, which is what allows a single microspore and a single megaspore to be functional while the other three products of each tetrad degenerate, and the moss and the fern may be added, the spores produced by their meiosis being equal. The classic example of the unequal case is the angiosperm megaspore mother cell, in which the meiosis I is reductional and produces a dyad consisting of a large functional megaspore and a small degenerate cell, after which the equational meiosis II divides the large cell into four and the small cell into four very small cells, so that of the products one is a large functional megaspore and the rest are small and degenerate. The second example of the unequal case is the liverwort of the genus Marchantia, in which the meiotic division of the spore mother cell gives one cell much larger than the other three, the larger becoming the spore and the smaller ones degenerating. The difference between the two cases is therefore not a difference in the mechanism of the divisions, which are the same in both, but simply whether the first division is equational, as it is in the insect, or reductional, as it is in the angiosperm and in the liverwort.
10. 10
    Q10 → The anaphase of mitosis and the anaphase I of meiosis differ as follows. In the anaphase of mitosis it is the centromeres that split, and the sister chromatids of each chromosome separate from one another and are drawn to the opposite poles; in the anaphase I of meiosis the centromeres do not split, and it is the homologous chromosomes that separate from one another and move to the opposite poles, each homologue still consisting of its two sister chromatids joined at the centromere, which is why the anaphase I is also called the reductional anaphase. In the mitotic anaphase each structure that reaches a pole is a single chromatid, now an independent chromosome, so each pole receives 2n single-chromatid chromosomes in a 2n cell; in the anaphase I each pole receives n whole chromosomes, each still of two chromatids, and each pole therefore has n chromosomes and 2C of DNA, so that although the total for the cell is still 2n chromosomes and 4C of DNA, they are now grouped as n and n instead of 2n in one set, and the halving of the anaphase I is thus in the grouping and not in the total, a subtlety that is often missed. The two poles of the mitotic anaphase are genetically identical, since each pole receives one of the two identical sister chromatids of every chromosome, so the two daughter cells are identical to each other and to the parent; the two poles of the anaphase I are genetically different, since the homologues that separate carry different alleles of the genes and have also been altered by the crossing over at the pachytene, so that the two cells that result are not identical, and this is the genetic basis of the variation that sexual reproduction produces, the subsequent meiosis II separating sister chromatids that were never identical. And the mitotic anaphase completes the only division of the process, the chromosomes decondensing, the nuclear envelope being re-formed and the cytokinesis producing two diploid daughter cells, whereas the anaphase I completes only the first of the two divisions, with no cytokinesis and in many organisms no re-formed nuclear envelope, the cell proceeding directly to the meiosis II in which the sister chromatids finally separate, so that the reduction achieved in the first division is maintained and the four haploid products are completed. In short, the mitotic anaphase separates sister chromatids and is equational and genetically conservative, and the anaphase I separates homologous chromosomes and is reductional and the source of variation.
11. 11
    Q11 → Mitosis and meiosis differ in the following main respects. The site: mitosis occurs in the somatic and the meristematic cells, that is the body cells, of plants and of animals, whereas meiosis occurs in the germ cells, that is the cells of the gonads, the testis and the ovary, and is restricted to the sexually reproducing organisms. The number of divisions: mitosis is a single division with one round of DNA replication, giving two products, whereas meiosis is two successive divisions, the meiosis I and the meiosis II, with only one round of DNA replication before them, since there is no replication between the two, and it gives four products. The phase of replication: in mitosis the DNA replicates in the S phase of the interphase before the division, and in meiosis also in the S phase of the interphase, but once only, before the meiosis I, so the two meiotic divisions are both performed on chromosomes that have already been replicated. The synapsis and the crossing over: mitosis has neither, the homologues never pair and the chromosomes align singly, whereas meiosis pairs the homologues in the prophase I and undergoes a crossing over between the non-sister chromatids at the pachytene, producing recombinational variation. The condition at the metaphase: in the metaphase of mitosis the chromosomes, each of two sister chromatids, align singly on the equatorial plate, whereas in the metaphase I of meiosis the homologues are already paired as bivalents, and the bivalents align with the two centromeres of each pair lying on either side of the plate so that the homologues can be segregated in the anaphase I, and in the metaphase II the chromosomes align singly as in mitosis but the cell is already haploid. The behaviour of the centromere: in mitosis the centromere splits at the anaphase, so that the sister chromatids separate and each becomes an independent chromosome, whereas in the meiosis I the centromere does not split and the homologues separate with the sisters still joined, and the centromere splits only in the anaphase II. The number, the size and the equality of the products: mitosis gives two equal daughter cells, whereas meiosis gives four which may be equal in size, as in the insect where the meiosis I is equational, or unequal, as in the angiosperm megaspore mother cell and in the liverwort where the meiosis I is reductional. The nature of the products: mitosis gives two daughter cells that are diploid and genetically identical to the parent and to each other, 2n giving 2n and 2n, whereas meiosis gives four haploid cells, n, which are not identical to one another, since the homologues and the recombined chromatids have been distributed among them, and this dissimilarity is the raw material of sexual reproduction. And the significance: mitosis is the means by which the number of cells is increased in growth and in the repair of tissue, and it is how a single zygote builds a whole multicellular organism with the same genome in every cell and how asexual reproduction is achieved, whereas meiosis halves the chromosome number, so that it is halved at the formation of the gametes and restored by the fertilisation, keeping the chromosome number of the species constant from generation to generation, and it supplies the genetic variation on which evolution depends.
12. 12
    Q12 → The meiosis is significant in two principal respects, and the first is that it maintains the constancy of the chromosome number of the species from generation to generation. The body cells of a sexually reproducing organism are diploid, with 2n chromosomes, but the gametes formed in reproduction are haploid, with n, and this halving is achieved by the meiosis, so that the chromosome number is reduced from 2n to n at the formation of the gametes and is restored to 2n at the fertilisation, when the two haploid gametes fuse to form the zygote. In each generation therefore the number is halved once and doubled once, and without the meiosis it would increase at every generation through the fertilisation, so it is precisely the alternation of the two processes that keeps the number of the species constant, and this is the central significance of the division. The second is that the meiosis is the source of the genetic variation on which evolution depends, and it produces it at two levels. In the anaphase I the homologous chromosomes separate, and the distribution of the two members of each pair to the two poles is independent of the distribution of every other pair, so that any of the possible combinations of the maternal and the paternal homologues may go to a given pole, and in man, with 23 pairs of chromosomes, the number of possible combinations is 2^23, that is 8388608. Note that this is the number of combinations possible over the population as a whole and not the number of gametes made at one time, since a single meiosis yields only four gametes, but it does mean that the four gametes of any one meiosis are genetically different from one another. In the prophase I, further, the crossing over between the non-sister chromatids of the paired homologues at the pachytene, and the fact that the chromatids of a chromosome are no longer identical as a result of it, produces still further combinations, so that the variation is not merely a reshuffling of whole chromosomes but a genuine exchange of segments giving a new arrangement of genes. Two further consequences follow from these. The meiosis, by producing haploid cells, makes sexual reproduction possible, since the fusion of two haploid gametes to form a diploid zygote is possible only if the gametes are haploid, so that the meiosis and the fertilisation are the two complementary halves of one cycle. And because the gametes it produces are genetically different from one another, every individual of a sexually reproducing population is genetically unique, so that the population rather than the individual is the unit of evolution, since a population in which every individual differs is a population in which natural selection has something to select upon. Thus the meiosis not only keeps the chromosome number constant across the generations but supplies the raw material of variation, and it is in this double sense that it is central to the continuity and to the evolution of the species.
13. 13
    Q13 → The two cases set against each other show that haploidy and the capacity for cell division are independent properties, and that neither determines the other. In the haploid insects and the lower plants, in which cell division occurs, the insects, such as the grasshopper and the locust, are the standard example, and in the male the spermatogenesis begins with a diploid spermatogonium which becomes a primary spermatocyte in which the meiosis I is equational, so that the two products of the first division are of equal size, after which the equational meiosis II divides each of them equally to give four spermatids of equal size which grow into four spermatozoa; the cells that are actually dividing here, the spermatogonia, the primary and the secondary spermatocytes and the spermatids, are diploid, and it is the products of that division that are haploid. In the lower plants, that is the algae, the bryophytes and the pteridophytes, the plant body that bears the gametes is the haploid gametophyte, and here the cells that divide are haploid spores and the cells of the gametophyte, a haploid spore germinating and dividing by mitosis to produce a multicellular gametophyte, so that a haploid cell divides by mitosis to produce a haploid cell. In the haploid cells of the higher plants in which division does not occur, the haploid generation of the angiosperm is confined to the flower, the male gametophyte being the pollen grain with its vegetative and generative cells and the female gametophyte the seven-celled and eight-nucleate embryo sac, and these cells are highly differentiated and do not divide. The reason is not that they lack the machinery of the mitosis, since they are living cells with functional nuclei and the whole mitotic apparatus is present, but that their programme is set for a specific terminal function and not for growth: the vegetative cell of the pollen is specialised for the nutrition of the tube that grows from it, the generative cell for the single division that forms the two male gametes, and the egg cell for the fusion with a male gamete, so that where a division does take place it is a single directed division for the formation of the gametes and not the vegetative cell division of growth. The conclusion is therefore that a haploid cell can and does divide, as the haploid spores and gametophytes of the lower plants show, and a haploid cell can also fail to divide, as the differentiated haploid cells of the flower show, and that what decides the matter is the state of differentiation of the cell and the developmental programme it is committed to and not its chromosome number. This follows from the principle of the chapter, for a mitosis requires only that the DNA be replicated in the S phase and that the centromeres be able to split at the anaphase, and both of these work equally well in a haploid cell as in a diploid one, since the purpose of the mitosis, the production of two cells each with a complete and identical set of chromosomes, is achieved in a haploid cell as perfectly as in a diploid one.
14. 14
    Q14 → No. A mitosis cannot take place without a preceding DNA replication in the S phase, and the reason is that the whole mechanism of the mitosis is constructed to operate on chromosomes that have been duplicated, for the S phase exists for precisely that purpose, to convert every chromosome, which before it consisted of a single chromatid, into a chromosome of two identical sister chromatids joined at the centromere. Each stage requires this condition. The prophase requires the duplicated chromosomes so that they may condense into the visible paired chromatids. The metaphase requires a duplicated chromosome, for it is only on a chromosome that has two sister kinetochores, one at each centromere, that the spindle fibres from the two poles can attach in the bipolar manner and bring the chromosome to the equatorial plate, and a single chromatid carries only one kinetochore and cannot be attached and pulled as a mitotic chromosome. The anaphase requires the centromere to split, and it can split only after replication, because what splits is the pair of sister chromatids and each of them then becomes an independent daughter chromosome. And the telophase requires two complete groups of chromosomes to reconstitute the two daughter nuclei. A mitosis without a replication is therefore not merely abnormal but mechanically impossible, for there would be no sister chromatids to separate and only half the chromosomes to distribute. If such a division were nevertheless induced, the centromeres could not split normally in the absence of the sisters, the single chromatids would be drawn to the poles, and each daughter cell would receive only half the chromosome complement of the parent, so that the outcome would be a reduction of the kind performed by the anaphase I of meiosis and not the equational division that the mitosis is, and the daughter cells would be effectively haploid although the parent was diploid; alternatively the metaphase checkpoint, which verifies that every chromosome is attached to the spindle from both poles, would be unsatisfied, so that in a cell with intact checkpoints the cycle would be arrested and the division blocked, while in a cell in which the checkpoint has failed, as in a cancer cell, the abnormal division would indeed occur and would produce aneuploid daughter cells. The apparent exceptions are not exceptions. A haploid cell with a nucleus divides by mitosis quite readily, and in the organisms with a haplontic life cycle, the algae, the bryophytes and the pteridophytes, the zygote is the only diploid stage and its first division is a mitosis, but that division too is preceded by its own S phase, and the haploid daughters then divide mitotically, each after its own replication, since a complement of n chromosomes has to be replicated just as a complement of 2n does. The bacterial cell is a different matter altogether, for it has no S phase and no mitosis and no spindle, its single circular chromosome being replicated and the cell then simply divided by a fission; and the true converse of this question, a replication of the DNA with no division at all, is the endoreduplication.
15. 15
    Q15 → Yes. The replication of the DNA and the division of the cell are two separate events and they need not coincide, so that a cell may replicate its DNA and then not divide. The ordinary case is that every cell of a normal body that is not dividing has nevertheless replicated its DNA, since it did so in the S phase of the interphase preceding its last division and has either divided or is still carrying the chromosomes it was given in that phase. The clearest and commonest example is a cell in the G0 quiescent stage, such as a neuron or a liver cell of the adult, which has completed an S phase and has not divided and so rests with its DNA already replicated and its chromosomes in the replicated condition of two sister chromatids, so the replication has occurred and the division has not. The more instructive case is the endoreduplication, in which a cell replicates its DNA repeatedly without any division at all, and this occurs in several normal tissues. The best example is the polytene chromosome of the dipterans, such as the fruit fly Drosophila and the mosquito, in which the DNA of a salivary gland cell is replicated again and again with neither division nor segregation, so that the single chromosome becomes a bundle of hundreds or thousands of chromatids aligned side by side, the bands of the polytene chromosome being the enlarged chromomeres. The second example is the endosperm of a seed, the nutritive tissue of the angiosperm, in which the nuclei divide without the cytokinesis, so that a single nucleus becomes a polyploid nucleus with many times the normal chromosome complement and the endosperm cells are polyploid. In plants the endoreduplication also produces the polyploid cells of the leaf and of the succulent tissues, and in the endosperm of the date the nuclear division proceeds without the cellular division. The reason it can happen is that the control of the cycle lies in a set of checkpoints, at the G1, at the G2 and at the metaphase, and it is the checkpoint that requires the completion of the division before a further replication is licensed which normally enforces the coupling of the replication to the division; if that requirement is released, or if the division machinery is not assembled, the cycle can be run repeatedly in the S-phase mode and the DNA is replicated again and again with no division, which is the definition of the endoreduplication. The consequence is the increase in the amount of DNA and in the ploidy of the cell with no change in its type or in its function, so that the cell can produce a large quantity of a particular protein, as the endosperm does in accumulating reserves and as the polytene cell does in making a single transcript in bulk, and the cell grows correspondingly in size, which is why the endoreduplication is a normal feature of these specialised tissues and not a pathological event.
16. 16
    Q16 → The number of chromosomes is counted by the number of centromeres, so a chromosome that consists of two sister chromatids counts as one chromosome and not as two, while the DNA content, the C, is the amount of DNA present, so a cell whose chromosomes are all duplicated contains twice the DNA of a cell whose chromosomes are single even though the chromosome number is the same. A diploid cell in the G1 therefore has 2n chromosomes and 2C of DNA. In the S phase the DNA is replicated, so the DNA content rises from 2C to 4C while the chromosome number stays at 2n throughout, because the two sister chromatids share the one centromere, and this is the essential feature of the replication. At the end of the S phase and through the G2, the prophase and the metaphase, the cell has 2n chromosomes and 4C of DNA, the chromosomes being single in the sense that each is one duplicated chromosome, and in the metaphase they align singly on the equatorial plate. In the anaphase, when the centromeres split and the sister chromatids separate, each of the two poles receives 2n single-chromatid chromosomes and 2C of DNA, so that in the cell as a whole the chromosome number appears to double from 2n to 4n and the DNA per pole appears to halve from 4C to 2C, although this is not a true change in the complement of a nucleus but the consequence of the cell being divided in two at that moment, and each chromatid, now an independent chromosome, is a full chromosome. In the telophase each of the two forming nuclei has 2n chromosomes and 2C of DNA, and after the cytokinesis each of the two daughter cells has 2n and 2C, exactly the condition in which the parent began, so the cycle is closed and the division was equational. It follows that the DNA content changes only once in the cycle, in the S phase, rising from 2C to 4C, and returns to 2C only because the cell is divided into two, while the chromosome number changes only at the anaphase, and then only in the cell as a whole, never within a nucleus, since each daughter nucleus has 2n. The contrast with meiosis completes the analysis. In meiosis the DNA is also replicated only once, in the S phase, from 2C to 4C, but the chromosome number falls at the first division, since the centromeres do not split and the two products of the meiosis I each have n chromosomes and 2C of DNA, and then in the meiosis II, with no intervening replication, the centromeres split and the four products each have n chromosomes and 1C of DNA. The cell has therefore passed from 2n and 2C in the mitosis to 2n and 2C in the two daughter cells, an equational division, and from 2n and 2C to n and 1C in meiosis, a reductional one, and these two columns of figures are the clearest statement of the difference between the two divisions that the chapter has been making.

MCQ Questions

1 Mark Questions

1. 1
   What is a kinetochore? (Ans: the protein disc at the centromere that attaches to spindle fibres)
2. 2
   Name the stage at which chromosomes align at the metaphase plate. (Ans: metaphase)
3. 3
   Define haploid chromosome number. (Ans: n — one set of chromosomes, as in gametes)
4. 4
   What is cytokinesis? (Ans: division of the cytoplasm after nuclear (karyokinesis) division)

2 Mark Questions

1. 1
   Differentiate between mitosis and meiosis on any two points.
2. 2
   Why is interphase considered a 'preparatory' (resting) phase? (Ans: cell grows, DNA replicates and ATP/proteins are synthesised)
3. 3
   What are synapsis and chiasmata? (Ans: pairing of homologous chromosomes (synapsis) and X-shaped points of crossing over (chiasmata))

3 Mark Questions

1. 1
   Describe the five substages of prophase I (leptotene to diakinesis) with the key events of each.
2. 2
   Explain cytokinesis in plants and animals, noting the differences (cell plate formation vs furrow).
3. 3
   State the significance of mitosis and of meiosis. (Ans: growth/repair/cloning; halving of chromosome number and genetic variation for gametes)

5 Mark Questions

1. 1
   Describe the events of meiosis I in detail, emphasising how the homologous chromosomes separate and what happens during crossing over.
2. 2
   Draw a labelled diagram of the cell cycle showing G1, S, G2 and M phases and explain what happens at each stage, including the checking (checkpoint) mechanisms. (Ans: growth, DNA synthesis, preparation for division; checkpoints at G1/S and G2/M)