Cell Cycle and Cell Division Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1The question asks for the span, not for the phases, so the answer is a single figure, and it should be given as a range with the qualification that it varies with the cell type. The qualification is the mark, because the average hides a spread of severalfold.
2. 2The figure to write is 24 hours, and this is the value NCERT quotes for a typical mammalian cell in G1, that is in the G1 phase, as the figure of about 12 hours for the G1 and about 6 hours for the S, about 3 to 4 hours for the G2 and about 1 hour for the M phase. So the answer is that the average cell cycle span of a mammalian cell is about 24 hours, and it should be qualified in three ways.
3. 3The qualifications to add. First, the duration is not the same for every cell, since the 24 hours is an average taken over the mammalian cell types that are commonly studied, and the range is wide, from as little as an hour or two in the case of the bacterium-like and the rapidly dividing cells, through the several hours typical of a cultured mammalian cell in laboratory conditions, up to a few days in the case of the human liver cell and of the human nerve cell, which normally do not divide at all in the adult and are said to be in the G0 stage. Second, the temperature matters, since the rate of division rises with the temperature up to an optimum and falls again, and the whole of the mammalian figure is quoted for the human body temperature of 37 degrees Celsius.
4. 4Third, the duration also depends on the availability of the nutrients and of the growth factors, since the G1 is the point at which the cell decides whether to enter the S phase, and a cell that is short of nutrients or of a growth factor will leave the cycle and enter the G0 rather than proceed. And it is worth closing with the point that this is the practical reason for the study of the cycle, because the duration of the cycle and of its phases is what a tumour cell alters, since a cancer cell divides far more rapidly than a normal cell and it is the loss of the control of the G1 that characterises a malignancy, and the same figure is the basis of the chemotherapy schedules in which a drug is timed to the S phase, when the cell is synthesising DNA and is therefore most sensitive to a drug that damages the DNA.

Step-by-step solution

1. 1The distinction is of the letter prefix, since cyto is cytoplasm and karyo is nucleus, so the answer is simply that one divides the cytoplasm and the other the nucleus, and everything else follows from that. Give the mechanism of each, the timing of each, and then the two ways in which cytokinesis is achieved.
2. 2Karyokinesis, the nuclear division, is the stage in which the nucleus divides. It comprises the whole of the M phase before the cytoplasm is divided, that is prophase, metaphase, anaphase and telophase, and in it the nuclear envelope breaks down, the chromosomes align, the sister chromatids separate and are drawn to the poles, and two daughter nuclei are re-formed. Its essential feature for the purpose of this question is that the chromosome number of the daughter nuclei is determined by whether the centromeres split, and that in karyokinesis the nuclear material is partitioned but the cell as a whole has not yet divided, so at the end of karyokinesis a cell may contain two nuclei and one continuous mass of cytoplasm.
3. 3Cytokinesis, the cytoplasmic division, is the division of the cytoplasm, and it begins in late anaphase or in telophase and completes only after the nuclei have divided, so that two separate daughter cells with their own nuclei and their own cytoplasm are produced. Its essential feature is that it partitions not merely the nucleus but the whole cell content, including the organelles and the cytoplasm, so that each daughter cell receives a complete and roughly equal set of the cell contents. The term cytokinesis is thus the one that corresponds to the cell division as the ordinary word is used, since the two daughter cells are only formed when the cytoplasm is divided.
4. 4The two kinds of cytokinesis complete the answer, and the question expects both, since the mechanism differs with the type of cell. In an animal cell, where there is no cell wall, cytokinesis is achieved by the furrow or the cleavage method: a constriction or furrow appears in the middle of the cell, deepening as a ring of microfilaments of actin and myosin contracts, and the constriction continues until the cytoplasm is pinched into two, so the division is a constriction from the outside inwards. In a plant cell, where the rigid cell wall prevents any such constriction, cytokinesis is achieved by the cell plate method: a new wall forms in the middle of the cell by the fusion of the Golgi vesicles at the centre of the cell to make a cell plate, the plate grows outwards until it reaches the existing wall, and new membranes are made at its edges and new material is deposited, so that a new and complete cell wall is constructed between the two daughter cells. And the two methods are mutually exclusive as alternatives dictated by the presence or the absence of a cell wall, which is the cleanest way to state the difference in the mechanism.

Step-by-step solution

1. 1This is the largest sub-question of the chapter, and the answer must be structured, since the interphase is the longest part of the cycle and it is divided into three phases, G1, S and G2, with the G0 as a possible fourth. Give the three phases in order, state what happens in each, and then close with the significance.
2. 2Open by defining the interphase as the period between the end of one division and the beginning of the next, and by stating the two facts about it that are always examined: 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 and is growing, and it is during the interphase that the DNA is replicated, which is the essential event. Also note at the outset that the chromatin is in the decondensed or extended form during the interphase, so the chromosomes are not visible as such, and that the nuclear membrane is intact. The interphase is conventionally divided into the G1 phase, the S or synthesis phase, the G2 phase and the G0 phase.
3. 3The G1 phase, the first gap phase. This is the period between the end of the M phase of the previous division and the beginning of the S phase, and it is the longest of the sub-phases, occupying about half of the whole interphase, which is why most of the 24 hours of a mammalian cell cycle is spent here. During the G1 the cell is metabolically active and is growing continuously, and it is increasing in size and in mass, and it is synthesising the RNA, the proteins and the enzymes that it will need, and it is manufacturing the ATP and the cell components that the division will require. The key event of the G1 is the decision to replicate, since this is the checkpoint at which the cell assesses its size, its nutrient and growth-factor supply and its energy state, and if the conditions are favourable it commits to the S phase and if they are not it leaves the cycle and enters the G0. So the G1 is the decision phase, and this is the point the question is testing.
4. 4The S phase, the synthesis phase, and the defining event of the whole interphase. During the S phase the DNA is replicated, so that each chromosome, which up to this point consisted of a single chromatid, is converted into a chromosome consisting of two identical sister chromatids joined at the centromere, and the DNA content of the cell is doubled while the number of chromosomes is not, since the chromosome number is counted at the centromere and the two chromatids still share the one centromere. DNA synthesis is semiconservative, and it occurs at a replication fork that moves along the DNA, and it is also during the S phase that the histone proteins are synthesised and the new DNA is packaged around them to make the chromatin. The cell does grow and continue its metabolism during this phase, but the DNA replication is the event for which it is named, and it is the reason the S phase is so important for the genetics and for the chemotherapy, since a cell in the S phase is the most vulnerable to a drug that damages the DNA, and also to a high dose of radiation.
5. 5The G2 phase, the second gap phase. This is the period between the end of the S phase and the beginning of the M phase, and it lasts about 3 to 4 hours in a mammalian cell. During it the cell continues to grow and to prepare itself for the division, and it synthesises the proteins that the division itself will need, in particular the tubulin of the spindle fibres and the proteins of the nuclear membrane and of the cytoplasm, so that the machinery of the mitosis is in place when the M phase begins. The G2 is also a checkpoint, since if the DNA has been replicated properly and completely, the cell passes into the M phase, and if the DNA is damaged or the replication is incomplete, the cell is held in the G2 and the cycle is arrested, and it is this checkpoint that is one of the main safeguards against the transmission of a mutated genome.
6. 6The G0 or quiescent phase, to mention as the fourth state. Cells that leave the cycle, usually from the G1, enter the G0, in which they are metabolically active but are not dividing and are not preparing to divide, and they may remain there for a long time, or may re-enter the cycle if they are stimulated. The G0 cells are differentiated, that is they are specialised, and the neurons and 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 is seen in the formation of a callus in plant tissue culture and in the transformation of a normal cell into a cancer cell, shows that the exit from the cycle is reversible and is a matter of regulation rather than of permanent loss of the capacity to divide.
7. 7Close with the significance, because the question is worth the description. The interphase is when the cell doubles its DNA and doubles its mass, and the two daughter cells produced at the end of the division each receive half of it, and this is why the division is faithful. It is also the phase in which the cell checks itself, at the G1, at the G2 and, in eukaryotes, at the metaphase of the M phase, and these checkpoints make the division orderly and prevent the transmission of damaged DNA. And it is worth noting that because the interphase occupies about 95 per cent of the cycle, a cell that is growing and dividing is for almost all of its time not actually dividing, so the visible stages of mitosis are a very brief part of the life of a cell.

Step-by-step solution

1. 1Define it, then give its two properties of not dividing and not preparing to divide, and then its importance in the two directions, for the differentiation of specialised cells and for the unregulated proliferation of cancer cells.
2. 2Definition: the G0 or quiescent stage 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, and NCERT is explicit that the cells in the G0 do not divide and do not prepare to divide. It is designated G zero because it is a stage of the cell cycle that has zero duration in the sense that the cell is not progressing through the cycle, and unlike the other phases it is not a part of the normal cycle at all, but a resting state entered from the G1.
3. 3The characteristics to state. The cells of the G0 are metabolically active, so they are not dead and they are not dormant in the physiological sense, but they are carrying out their normal functions, and in a tissue they are performing the work of the organ. They are often fully differentiated, that is specialised, so that a neuron has lost the ability to divide and has become a specialised cell for the transmission of impulses, and the same is true of the muscle cell and of the cell of the lens of the eye. The DNA of a G0 cell is still intact and the nucleus is still functional, and the cell retains the full complement of chromosomes, so it is not in any way degenerate, and it is capable of returning to the cycle if it receives the appropriate stimulus.
4. 4The examples to give, and NCERT suggests them. In animals the G0 cells are the neurons, the muscle cells, the cells of the lens of the eye, the heart muscle cells and the cells of the adult human liver, all of which are terminally differentiated and normally never divide again, so the liver cell is the standard example of a cell that can be induced to re-enter the cycle, since after an injury to the liver the remaining cells are stimulated to divide and to regenerate the lost tissue, which is exactly the regeneration of a callus in tissue culture. In plants the G0 is represented by the cells of the meristem that are not actively dividing, and by the differentiated parenchyma of a tissue that can be induced to divide, as in the callus formation and in the regeneration of a whole plant from a piece of tissue. The most instructive example is the bacterium, since NCERT notes that in the bacteria the cell cycle is not of the G1, S, G2 and M type, so the G0 is a feature of the eukaryotic cell cycle.
5. 5The importance, in two directions, and this is the end to aim at. First, the G0 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 the division of labour in a multicellular organism, one of the facts the chapter returns to, is achieved by cells leaving the cycle rather than by cells continuing to divide. Second, the G0 is the point at which the control of the cycle is lost in cancer, since a cancer cell is a cell in which the checkpoint that should send it into the G0 has failed, so that instead of becoming quiescent the cell proceeds round the cycle again and again and forms a tumour, and the mitotic spindle and the checkpoint machinery that normally operate at the metaphase to detect a misattached chromosome and to delay the anaphase are what a cancer cell has defeated. And the two facts are the same fact seen from two sides, since the G0 is a mechanism of the organism for restraining cell division and the loss of the G0 is a mechanism of the cancer cell for escaping it.

Step-by-step solution

1. 1The term equational means equal, and it means equal in respect of the chromosome number and the amount of DNA, so the whole answer is that the parent cell and each of the two daughter cells receive the same chromosome number and the same amount of DNA, and this is what the word equational denotes. Then give the mechanism that produces the equality, and then the contrast with meiosis.
2. 2The definition of equational. Mitosis is called equational division because the two daughter cells produced 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 the daughter cells, the N, is the same as that of the parent, which is why the notation is 2n giving 2n and 2n. This is the meaning of the word equational, an equation being written with an equality sign, and it distinguishes mitosis from the other type of division, which is reductional.
3. 3The mechanism that produces the equality, in three steps. First, the DNA is replicated in the S phase, so that every chromosome of the parent comes to consist of two identical sister chromatids joined at the one centromere, so the DNA content is doubled but the chromosome number is not, the chromosomes being counted at the centromere. Second, 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 do in the first division of meiosis. Third, in the anaphase the centromeres split and the sister chromatids, which are identical, separate and are drawn to the opposite poles, so that each pole receives one complete set of the chromosomes and each new nucleus receives exactly the same complement as the parent nucleus had, and therefore the daughter cells have the same chromosome number and the same DNA content as the parent.
4. 4The contrast, which completes the reason. Because the two sister chromatids of each chromosome are identical to one another, and because only one of each pair goes to each pole, the division gives two cells that are genetically identical to the parent and to each other, and nothing has been reduced, so the division is equational. In the first division of meiosis, in contrast, the homologous chromosomes separate, so that the number of chromosomes is halved and the division is reductional, and the second division of meiosis is equational but operates on already haploid cells. So mitosis is equational because it preserves the chromosome number exactly, and this is what makes it the division by which a zygote builds a multicellular organism in which every cell carries the same genome.

Step-by-step solution

1. 1Identify each event and give the stage in one line each, and then add a sentence of justification to each, since a bare list earns little. Two of the four are mitotic stages and two are the first meiotic division, and that contrast is worth making explicit at the end.
2. 2Event (i), the chromosomes moved to the spindle equator, is metaphase. The stage is the metaphase, and the justification is that it is the stage at which the spindle fibres from the two opposite poles attach to the kinetochores of the centromeres and the chromosomes are brought to and arranged on the equatorial plate, the metaphase plate, at the middle of the spindle, in the most compact and the most clearly visible condition. It is the stage at which the chromosome number can be counted, which is why the metaphase is the stage at which a karyotype is prepared, and it is also the stage of the checkpoint at which the cell verifies that every chromosome is attached to the spindle from both poles before it permits the anaphase.
3. 3Event (ii), the centromere splits and the chromatids separate, is anaphase. The stage is the anaphase, and the justification is that it is the stage at which the centromeres divide, the two sister chromatids of each chromosome are thereby separated from one another, and each chromatid is drawn to the opposite pole by the shortening of the spindle fibres. In mitosis, anaphase begins with the splitting of the centromere and the separation of the sister chromatids, and each chromatid is now an independent daughter chromosome, so the two groups move apart and the two daughter nuclei each receive an identical set.
4. 4Event (iii), the pairing between homologous chromosomes, is prophase I of meiosis, specifically the pachytene of the prophase I. The stage is the prophase of the first meiotic division, and the pair formation itself is at the pachytene, the third of the five sub-stages of the prophase I, the zygotene, the pachytene, the diplotene and the diakinesis following the leptotene. The justification is that the synapsis, which is the pairing of the homologues, begins at the leptotene, when the homologues come together in pairs, and the pairing is complete at the pachytene, when the synaptonemal complex is fully formed, and it is at the pachytene that the non-sister chromatids of the paired homologues begin to exchange their segments. In the equivalent stage of mitosis there is no pairing at all, and this is a point to note.
5. 5Event (iv), the crossing over between homologous chromosomes, is again prophase I, and again specifically the pachytene. The stage is the pachytene of the prophase I, and the justification is that the exchange of segments between the non-sister chromatids of the paired homologous chromosomes, 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 diplotene, when the homologues begin to separate and the points of the exchange are seen as the chiasmata, and this is the distinction to state, since the crossing over itself occurs at the pachytene and the chiasmata are only its visible evidence, appearing in the diplotene, and the exchange itself is completed by the end of the prophase I.
6. 6Close by collecting the four, since the collection is the answer: (i) metaphase; (ii) anaphase; (iii) prophase I, at the pachytene for the completed synapsis; and (iv) prophase I, at the pachytene. And the general observation worth the last line is that the first two events belong to mitosis, in which single chromosomes align on the equator and the centromeres split, whereas the last two belong to the first meiotic division, in which the homologues pair and recombine, and that no crossing over occurs in mitosis because there is no pairing of the homologues for it to occur in.

Step-by-step solution

1. 1Define each term precisely, then give the stage and the mechanism, then explain how the three are related to each other, since the question is really about one event, the pairing and the crossing over of the homologues, seen in three ways. A diagram of a bivalent with its chiasmata must be described, and a text answer should state exactly which structures to label.
2. 2(a) Synapsis. 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, from one end to the other, and so that the two chromosomes come to be in exact register, with gene locus corresponding to gene locus, and the pairing is held by a proteinaceous structure called the synaptonemal complex. The synapsis begins at the leptotene, the first of the five sub-stages of the prophase I, and is complete by the pachytene, the third. So synapsis means the coming together and the side-by-side apposition of the homologues, and it is what makes the recombination possible, since the exchange of segments occurs between the non-sister chromatids of the homologues that are held in this pairing.
3. 3(b) Bivalent. A bivalent is the structure formed by a pair of homologous chromosomes that are synapsed during the prophase I of meiosis, that is, the paired unit, and it is also called a tetrad, since each of the two homologues consists of two sister chromatids after the S phase, so that the pair contains two chromosomes and four chromatids altogether, hence tetrad. The number of bivalents in a cell is therefore half the haploid number of chromosomes, and the number of tetrads is the haploid number, since in a diploid cell of 2n chromosomes the homologues form n pairs. The bivalent persists through the pachytene, in which the crossing over occurs within it, and through the diplotene, in which the chiasmata become visible, and it is at the end of the prophase I that it begins to resolve into the two single chromosomes of the bivalent as the homologues prepare to separate in the anaphase I.
4. 4(c) Chiasmata. 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 of the prophase I, and the appearance of such crossing points gave the name to the stage. Each chiasma marks the site of a previous exchange of segments between non-sister chromatids, that is of a crossing over, and its number and position reflect the number and position of the recombinational events that have already taken place at the pachytene. As the diplotene progresses the chiasmata are terminalised, that is they move towards the ends of the chromosomes, and they 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 permits them to be aligned and then separated in the first division.
5. 5The relationship, which is what makes the three terms one idea. Synapsis is the process, that is the act of pairing; the bivalent is the structure, that is the result of the pairing, the paired homologues; and the chiasma is the point, that is the visible evidence of the exchange that has occurred within the pair. So the sequence in time is: the homologues synapse at the leptotene, the bivalent is thus formed and is complete at the pachytene, the crossing over occurs within the bivalent at the pachytene, the chiasmata appear at the diplotene as the homologues begin to separate, they are terminalised, and they are finally resolved so that the homologues can separate at the anaphase I.
6. 6The diagram to draw and to label. Draw a bivalent in the pachytene or the diplotene, that is two homologous chromosomes lying side by side, each drawn as two sister chromatids joined at the centromere, so that the pair contains two centromeres and four chromatids. The labels to write in are, for the left chromosome, its centromere and its two sister chromatids, and for the right chromosome the same, then the two homologous chromosomes as a pair, then the synaptonemal complex running along the length of the paired region if the stage is the pachytene, and then the chiasma, drawn as a single crossing point where two of the non-sister chromatids cross each other, with a label to show that this is the point of a previous exchange of segments. The diagram should also carry the sub-stage name and the note that the two chromatids involved at the chiasma are non-sister, that is one from each homolog, and not the two sisters of the same chromosome. And if the question allows a second figure, a small second diagram of the bivalent at the anaphase I, with the two homologues moving apart and still joined at a chiasma, completes the illustration of the resolution.

Step-by-step solution

1. 1Answer in the order of the cause, the method and the result, since the whole difference follows from the presence of a cell wall in the plant. State the cause first, because it explains everything else, then the two methods, then the plane of division and the orientation of the cell plate, and finally the outcome.
2. 2The cause of the difference is the cell wall. A plant cell is surrounded by a rigid, non-contractile cell wall, and a constriction of the cell, which is the method used by an animal cell, is therefore impossible, so the plant cell must divide its cytoplasm by constructing a new partition across the middle of the cell instead. The animal cell has no such wall and is bounded only by a flexible cell membrane, and this membrane can be pulled inwards, so the animal cell can and does divide by constriction. Everything else in the answer follows from this one fact, and it is the fact to lead with.
3. 3The method in the animal cell, the furrow or cleavage method. In the animal cell the plasma membrane at the middle of the cell is drawn inwards to form a constriction or a furrow, which deepens as a ring of actin and myosin microfilaments lying beneath the membrane contracts, and the furrow becomes progressively deeper until the cytoplasm is pinched completely into two and the two daughter cells separate. The division therefore proceeds inwards from the periphery, and the constriction is at the middle of the cell, so the two daughter cells are of equal size.
4. 4The method in the plant cell, the cell plate method. In the plant cell, because the wall will not constrict, the vesicles of the Golgi apparatus move to the middle of the cell, where they fuse to form a new membrane-bound structure called the cell plate, which therefore arises in the centre of the cell in the equatorial plane. The cell 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 there, so that the plate is converted into a new and complete cell wall, and the new wall 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 of equal size, separated by a straight new wall.
5. 5Two further differences complete the answer. The first is the plane of division. In a plant cell the plane of division is determined before the division begins and is in the plane of the future new wall, and it is the position of the pre-prophase band of microtubules that fixes the plane, so the new cell wall is a flat, straight partition; in an animal cell the plane of the constriction is not fixed in advance, and the furrow can be oblique or even occur in more than one plane, and in the irregular cells of a tissue it can be at any angle. The second is the site of the new partition relative to the nucleus. In the plant cell the cell plate is formed strictly in the equatorial plane and in the equatorial region of the parent cell, so it lies in the plane of the equator, and this is why the plant daughter cells are always of the same size; in the animal cell the furrow is a constriction of the membrane and of the cytoplasm, and the new daughter cells are separated by a cleavage, so there is no new wall constructed at all. The consequence to close on is that the two methods are not variations of one process but two different processes dictated by the presence or the absence of a cell wall, and that the plant cell therefore uses the vesicles of its own endomembrane system to build the new partition, which is why a cell plate is a membrane-bound structure and why the Golgi apparatus is so abundant in the dividing plant cells.

Step-by-step solution

1. 1Answer the two halves, and the key to the whole question is the difference between an equational second division and a reductional first division, since that is what decides whether the four cells are equal. So write the principle first, and then the examples under each head.
2. 2The principle, and this is the whole answer in one sentence. The four daughter cells of meiosis are equal in size in those organisms in which the meiosis is of the successive or the simultaneous type, that is where the meiosis I and the meiosis II are of the same kind and the divisions are essentially alike, and they are unequal in size in those organisms in which the meiosis I is a reductional division and the meiosis II is equational, so that the two sets of products of the first division, each being dyad, differ from one another. So the four cells are equal when both divisions are alike, and the cells are unequal when the first division is reductional and the second equational. NCERT states this directly, 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.
3. 3Where the four cells are equal in size. The standard example is the insect, that is the grasshopper or the locust, in which the four products of one meiotic division are all of approximately the same size, and this is so because in the spermatogenesis of the insect the meiosis I is equational and not reductional, so that the first division produces two daughter cells of the same size, and the second division then divides each of them equally, giving four spermatids of the same size. The same is true of the honey bee, the silkworm and of the rice and of the maize of the angiosperms, since the megaspore mother cell of the ovule and the microspore mother cell of the anther in the angiosperm both undergo a meiosis in which the four products are of the same size, and this is what allows a single megaspore and a single microspore to be functional while the other three are degenerate in each case, and it is the reason that only one of the four microspores normally forms a viable pollen grain. The moss and the fern are also cited, since the spores of a moss produced by meiosis are equal.
4. 4Where the four cells are unequal in size. The standard example is the angiosperm megaspore mother cell, in which the meiosis I is reductional and produces a dyad of two cells that are not of the same size, a large functional megaspore and a small degenerate cell, and the meiosis II, which is equational, then divides the large cell into four and the small cell into four very small cells, so that of the eight cells that result, one is a large functional megaspore and the others are small and degenerate. This is the unequal case and the angiosperm is the illustration of it. The second example is the liverwort of the genus Marchantia, in which the cells formed by the meiosis in the spore mother cell are unequal, one being much larger than the other three, since the larger one becomes the spore and the smaller ones degenerate. And it is worth closing with the general point, that the difference between the equal and the unequal cases is not a difference in the mechanism of the two divisions, which are the same everywhere, but a difference in whether the first division is equational, as in the insect where all four products are equal, or reductional, as in the angiosperm and the liverwort where they are unequal.

Step-by-step solution

1. 1This is a five-mark table question, so give the points of comparison as rows and fill the two columns. The contrast has one organising principle, namely what separates, the centromere and the sister chromatids in mitosis against the homologous chromosomes in anaphase I, and everything else follows from that.
2. 2Point one, what separates. 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. This is the fundamental contrast and should be stated first, and it is why the anaphase I is also called the reductional anaphase while the anaphase of mitosis is not.
3. 3Point two, the state of the chromosomes at the pole. In the anaphase of mitosis each structure that arrives at a pole is a single chromatid, and it is now an independent chromosome, so that the two groups at the two poles contain 2n single-chromatid chromosomes in a 2n cell. In the anaphase I of meiosis each structure that arrives at a pole is a whole chromosome, still consisting of two sister chromatids joined at the centromere, so that each pole receives n duplicated chromosomes, and since the homologues have separated the two poles are genetically different from one another, whereas the two poles of the mitotic anaphase are genetically identical because the chromatids that reach them were sisters.
4. 4Point three, the chromosome number and the DNA content. In the anaphase of mitosis the two groups move apart but the whole cell still contains 2n chromosomes and 4C of DNA in total, the two C per pole having come from the 2C of the parent cell, so the division is equational and the chromosome number is unchanged. In the anaphase I of meiosis the two groups are haploid, each with n chromosomes and 2C of DNA, and the whole cell therefore still has 2n chromosomes and 4C of DNA, but they are now grouped as n and n instead of 2n in one set, so the division is reductional and the halving is in the grouping rather than in the total, which is the subtlety that is often missed and is worth stating.
5. 5Point four, whether the two poles are genetically identical. This is the consequence of the first point and should be stated as a separate row. In the anaphase of mitosis the two poles receive identical sets of daughter chromosomes, since each pole receives one of the two identical sister chromatids of every chromosome, so the two daughter cells are genetically identical to each other and to the parent. In the anaphase I of meiosis the two poles receive different sets, since the homologues are separated and the homologous chromosomes carry different alleles of the genes, so the two cells that result are not genetically identical, and this is the genetic basis of the variation that sexual reproduction produces, for the subsequent meiosis II separates the sister chromatids that were never identical, having been altered by the crossing over at the pachytene.
6. 6Point five, the fate of the two groups and the number of the divisions. In the anaphase of mitosis the single division is complete at the telophase that follows, the chromosomes decondense, the nuclear envelope is re-formed, and cytokinesis then gives two diploid daughter cells, and the process is over. In the anaphase I of meiosis the first of the two divisions is complete, but there is no cytokinesis and no nuclear envelope is re-formed at the end of the first division in many organisms, the cell proceeding directly to the meiosis II, in which the sister chromatids finally separate, so that the reduction achieved in the anaphase I is maintained and the four haploid products are completed.

Step-by-step solution

1. 1This is the summary question of the chapter, so it must be given as a table of contrasts, and the best answer is to take the rows in NCERT's order: the site, the number of divisions, the phase in which the DNA replicates, the synapsis and the crossing over, the condition of the chromosomes at the metaphase, the behaviour of the centromere, the number and the size of the daughter cells, and the nature of the daughter cells. Nine rows earn full marks, and the last two rows are the ones examiners use for the significance.
2. 2Row one, the site. Mitosis occurs in the somatic or the meristematic cells, that is in the body cells, of both the plants and the animals. Meiosis occurs in the germ cells, that is in the cells of the gonads, so in the testis and in the ovary, and it is restricted to the sexually reproducing organisms. This is the first distinction and the one from which the rest follow, since the division of a body cell makes two identical body cells and the division of a germ cell makes the gametes.
3. 3Row two, the number of divisions. Mitosis is a single division, one nuclear division followed by one cytokinesis, so the products are two. Meiosis is two successive divisions, the meiosis I and the meiosis II, with only one round of DNA replication before them, so the products are four. The point to add is that there is no DNA replication between the meiosis I and the meiosis II, which is why the second division is the equational one and why the four products are haploid.
4. 4Row three, the phase in which the DNA replicates. In mitosis the DNA replicates during the S phase of the interphase that precedes the division. In meiosis too the DNA replicates once, in the S phase of the interphase that precedes the meiosis I, and not again before the meiosis II, so that the two divisions are both performed on the already-replicated chromosomes. This is the point that Q14 and Q15 turn on, and it is worth stating that the meiosis is therefore two divisions after a single replication, and the mitosis is one division after a single replication.
5. 5Row four, the synapsis and the crossing over. In mitosis there is no pairing of the homologous chromosomes, that is no synapsis, and consequently no crossing over, so the chromosomes align singly. In meiosis the homologues pair in the prophase I to form the synapsis, and the crossing over between the non-sister chromatids occurs at the pachytene, producing the recombinational variation.
6. 6Row five, the condition of the chromosomes at the metaphase. In the metaphase of mitosis the chromosomes, each of two sister chromatids, align singly on the equatorial plate. In the metaphase I of meiosis the homologous chromosomes are already paired as bivalents or tetrads, and the bivalents align on the equatorial plate with the two homologues of each pair side by side and their centromeres lying on either side of the plate, so that the two homologues are held for the segregation in the anaphase I. In the metaphase II the chromosomes align singly as in a mitotic metaphase, but the cell is haploid. This is the row that shows most clearly why the first meiotic division is the reductional one.
7. 7Row six, the behaviour of the centromere. In mitosis the centromere splits in the anaphase, so the sister chromatids separate and each becomes an independent chromosome. In the meiosis I the centromere does not split, and the homologues separate while the sister chromatids remain joined; the centromere splits only in the anaphase II, in which the sister chromatids separate. The centromere behaviour is the mechanical basis of the reduction and of the two-stage segregation.
8. 8Row seven, the number, the size and the equality of the daughter cells. Mitosis produces two daughter cells, which are equal in size. Meiosis produces four daughter cells, which may be equal in size, as in the insect where the meiosis I is equational, or may be unequal, as in the angiosperm megaspore mother cell and in the liverwort where the meiosis I is reductional. This is the row Q9 turns on, so it should be written with the qualification rather than as a bare statement that the four are equal.
9. 9Row eight, the nature of the daughter cells. Mitosis produces two daughter cells that are genetically identical to the parent and to each other, and diploid, since the chromosome number is unchanged, 2n giving 2n and 2n. Meiosis produces four haploid daughter cells, that is n, which are not identical to one another, since the homologues and the recombined chromatids have been separated among them, and this genetic dissimilarity of the gametes is the raw material of sexual reproduction. Row nine, the significance. Mitosis is the device by which the number of cells is increased in growth and in the repair of the tissue, and it is the mechanism by which a single zygote builds the whole multicellular organism with an identical genome in every cell, and by which the asexual reproduction is achieved. Meiosis is the device by which the chromosome number is halved, so that it is halved at the formation of the gametes and restored to the diploid state at the fertilisation, so that the chromosome number of the species remains constant from generation to generation, and it is the source of the genetic variation on which evolution depends, and this is Q12 in its essentials.

Step-by-step solution

1. 1Give the significance in two halves, since the question is asking for the two things meiosis does: it maintains the constancy of the chromosome number across the generations, and it generates the genetic variation on which evolution depends. Take them in that order and support each with its mechanism.
2. 2The first significance, the maintenance of the chromosome number. The cells of the body of a sexually reproducing organism are diploid, with 2n chromosomes, but the gametes that are formed by the process of reproduction are haploid, with n, and this halving is achieved by the meiosis. So the meiosis reduces the chromosome number from the diploid to the haploid state, 2n giving n, at the formation of the gametes, and the diploid state is restored at the fertilisation, when the two haploid gametes fuse to form the zygote, 2n. So in each generation the chromosome number is halved once and restored once, and this is why the chromosome number of the species remains constant from generation to generation, and it would drift upwards without the meiosis, since the fertilisation doubles it at every generation, and downwards by the same argument, so it is precisely the alternation of the two processes that keeps the number fixed. This is the central significance and it should be stated first, and the phrase to use is that the meiosis maintains the diploid number of the species constant across the generations.
3. 3The second significance, the generation of variation. The meiosis is the source of the genetic variation on which evolution depends, and it produces it at two levels. At the first level, in the anaphase I, the homologous chromosomes separate and the distribution of the two members of each homologous pair to the two poles is independent of the distribution of every other pair, and so any one of the possible combinations of the maternal and the paternal homologues may go to a given pole, and since the number of chromosomes in a human is 23 pairs, the number of possible combinations is 2^23, that is 8388608, of which only a very small fraction is ever realised, but the number is the point, since it means that the gametes produced by one meiosis are genetically all different. At the second level, in the prophase I, the crossing over between the non-sister chromatids of the paired homologues at the pachytene, together with the chromatids that are not identical, produces still further combinations, so that the variation is not merely the reshuffling of whole chromosomes but a genuine exchange of segments and a new arrangement of genes.
4. 4Then the third and fourth points, which the examiners like and which the question rewards. The third is that the meiosis, by producing haploid cells, makes possible the sexual reproduction, since the fusion of two haploid gametes to form a diploid zygote is only possible if the gametes are haploid, so the meiosis and the fertilisation are the two halves of one cycle. The fourth is that the meiosis, by producing gametes that differ genetically from one another, is what makes every individual of a sexually reproducing population genetically unique, and therefore what makes the population the unit of evolution, since a population in which every individual is different is a population in which natural selection has something to act upon, and this is the link from the cell division to the Darwinian theory, and the whole question closes on that link.

Step-by-step solution

1. 1This is a discussion question and the answer should be an explanation, not a definition, so the reasoning must be given in full. The point to establish is that haploidy does not by itself decide whether a cell divides, and that what decides it is the state of the cell and its programme, since the machinery of mitosis is present in a haploid cell exactly as it is in a diploid one.
2. 2Part (i), the haploid insects and the lower plants in which cell division occurs. The insects, that is 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 the two cells that result from the first division are of the same size, and the meiosis II then divides each of them equally, giving four spermatids of equal size which grow into four spermatozoa. The cells of the testis that divide here are the spermatogonia, the primary and the secondary spermatocytes and the spermatids, and the fact to notice is that these dividing cells are diploid, so the cell division in the insect is division of diploid cells, and it is the products of that division that are haploid. The lower plants, the algae, the bryophytes and the pteridophytes, are haploid organisms in the sense that the plant body which bears the gametes is the haploid gametophyte, and here the cells that divide are the haploid spores and the cells of the gametophyte itself, and a haploid spore germinates and divides by mitosis to produce a multicellular gametophyte, so that in the lower plants a haploid cell divides by mitosis to produce a haploid cell, which is possible because the mitotic machinery of a haploid cell is entirely adequate and there is no need for the homologous pairs in order to distribute the chromosomes at the anaphase, since one chromatid of each chromosome can simply go to each pole and the two poles then receive identical and complete sets.
3. 3Part (ii), the haploid cells of the higher plants in which cell division does not occur. In the angiosperms the plant body is diploid, the sporophyte, and the haploid generation is confined to the gametophyte, which is represented only by the haploid cells within the flower, so the male gametophyte is the pollen grain, which has a vegetative cell and a generative cell, and the female gametophyte is the embryo sac, which has seven cells and eight nuclei, of which one is the egg cell. These haploid cells are highly differentiated and they do not divide, and they are unable to divide, and the reason is not that they lack the machinery of the mitosis, since they are living cells with a nucleus and they could divide, but that their programme is set for a specific terminal function and not for growth. So the vegetative cell of the pollen grain is specialized for the nutrition of the tube that grows out of it, the generative cell is specialized to divide once to form the two male gametes, and the egg cell of the embryo sac is specialized to fuse with a male gamete, and in each of these the division that does occur is not a vegetative division for growth but a single division directed to the formation of the gametes, which is a different function from the cell division of the growth of the plant.
4. 4The conclusion to draw from the two parts, and this is the point of the discussion. Haploidy and the capacity for cell division are two independent properties, and neither determines the other, so the answer to the question is that a haploid cell can and does divide, as in the haploid spores and the cells of the gametophyte of the lower plants, and a haploid cell can also fail to divide, as in the highly differentiated haploid cells of the flower of the higher plants, and what decides the matter is the state of differentiation of the cell and the programme of development it is committed to, not its chromosome number. And this is consistent with the general principle of the chapter, that a mitosis requires the DNA of the cell to be replicated in the S phase and the centromeres to be able to split at the anaphase, and both of these processes work equally well whether the cell is haploid or diploid, since the mitosis is the equational division and its purpose, 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.

Step-by-step solution

1. 1The answer is no, and the reasoning has to be given rather than asserted, because the question is really asking for the consequence of the S phase. Then state the consequences of attempting it, and then the exceptions that are worth knowing.
2. 2Why the answer is no. The S phase exists for one purpose, to replicate the DNA so that each chromosome will consist of two sister chromatids joined at the centromere, and the whole of the mechanism of the mitosis is designed to work on chromosomes in that replicated condition. Each of the four stages requires it. The prophase requires the duplicated chromosomes, so that they can condense into the visible paired chromatids. The metaphase requires a chromosome that is duplicated, so that a kinetochore is formed at the centromere and a spindle fibre can attach to it and pull the chromosome, and a single chromatid has only one kinetochore and is unattached to the centromere. The anaphase requires the centromere to split, and the centromere can only divide after the chromosome has been replicated, since it is the two sister chromatids that are being pulled apart and each becomes a daughter chromosome. And the telophase requires the two groups to reconstitute two nuclei each with a complete complement. So a mitosis without a preceding replication is not merely abnormal, it is mechanically impossible, since there would be no sister chromatids to separate and only half the chromosomes to distribute.
3. 3What would actually happen, and this is worth stating. If a cell were induced to enter the mitosis without passing through the S phase, so that its chromosomes consisted of single chromatids, then at the anaphase the centromeres could not split in the normal way, since the two sisters that split are absent, and the single chromatids would be drawn to the poles, and the two daughter cells would each receive only half the chromosome complement of the parent, so the result would not be the equational division that the mitosis is but a reduction of the kind that the meiosis I performs, and the daughter cells would each be haploid in effect though the parent was diploid. In other words, the cell would attempt the anaphase of a meiosis I with the chromosomes of a diploid cell and would produce two cells each with half the required complement, and the cell would then be unable to continue normally. The other outcome is a failure at the metaphase checkpoint, since the checkpoint that verifies that every chromosome is attached to the spindle from both poles would be unsatisfied and the cycle would be arrested, so that in a cell with a functional checkpoint system the division is blocked rather than completed, and in a cell in which the checkpoint has failed, as in a cancer cell, the abnormal division does occur and produces aneuploid daughter cells.
4. 4The exceptions, and they should be named, because the examiner will expect them and because they show the rule is a real rule. There is no mitosis in a haploid cell at all, since the S phase and the DNA replication do not occur before the first division of the zygote in the non-vegetative tissues of the algae, the bryophytes and the pteridophytes, so the zygote divides by mitosis while still haploid, and this is possible because there is nothing to duplicate, the haploid set being complete in itself. And in a cell of a bacterium there is neither an S phase nor a mitosis, since the bacterial chromosome is a single circular molecule and the bacterial division is a simple fission preceded by a replication of the DNA, so no mitotic spindle is required.

Step-by-step solution

1. 1The answer is yes, and this is the more interesting of the two questions, since a cell can replicate its DNA and then not divide, which is the normal condition in the G0 and is the basis of the endoreduplication. So give the ordinary case first, then the special one, and then the consequence.
2. 2The ordinary case, which is the answer to the question. Yes: the replication of the DNA and the division of the cell are two separate events, and they need not coincide. Every cell in a normal body that is not dividing has nevertheless replicated its DNA, since it did so in the S phase of the interphase before it last divided, and it has then either divided or, if it did not, it is still carrying the replicated chromosomes it was given. The clearest and most frequent example is a cell in the G0, the 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 is sitting in the quiescent state with its DNA already replicated and its chromosomes in the replicated condition of two sister chromatids. So the DNA replication has occurred and the cell division has not, and this is the normal state of a great many of the cells of the body at any moment.
3. 3The special and more instructive case, the endoreduplication. A cell can also replicate its DNA repeatedly without any division at all, and this is called the endoreduplication, and it occurs in several normal tissues. The clearest example is the giant molecule of the polytene chromosome of the dipteran, that is of the fruit fly Drosophila and of the mosquito, in which the DNA of the salivary gland cell is replicated again and again with no division and no segregation, so that the single chromosome becomes a many-stranded bundle of hundreds or thousands of chromatids aligned side by side, and the bands of the polytene chromosome are the enlarged chromomeres. The second example is the endosperm of a seed, the nutritive tissue of the angiosperm, in which the nuclei of the endosperm divide without the cytokinesis, so that a single nucleus becomes a polyploid nucleus with many times the normal complement of chromosomes, and the endosperm cells that result are polyploid. The third is the leaf of a plant and the succulent tissues, where the endoreduplication produces the polyploid cells that are characteristic of them, and the fourth is the megaspore mother cell in some plants and the endosperm of the date, in which the nuclear division proceeds without the cellular division.
4. 4Why it can happen, and this is the mechanistic answer. The control of the cycle in a eukaryote is a set of checkpoints, at the G1, at the G2 and at the metaphase, and it is the checkpoint system that normally enforces the coupling of the replication to the division, so that a cell will not begin a second S phase until the previous division has been completed, and the licence for a fresh replication is issued only on the condition that the cell has divided. If the checkpoint that requires the completion of the division before a further replication is released, or if the cell is in a state in which the division machinery is not assembled, then the cycle can be run in the S-phase mode repeatedly, and the DNA is replicated again and again with no division, and that is precisely the definition of the endoreduplication. And the consequence of the endoreduplication, which is worth one closing sentence, is the increase in the amount of DNA and in the ploidy of the cell without any change in its type or in its function, so that the cell can produce a large amount of a particular protein, as the endosperm does when it is accumulating reserves and as the polytene cell does when it is producing a large quantity of a single transcript, and the cell grows in size accordingly, which is why the endoreduplication is a normal feature of certain specialised tissues and not a pathological event.

Step-by-step solution

1. 1This question is best answered as a table, with the stages as the rows and the chromosome number and the DNA content as the two columns, and the table should be given both at the start and the end of the mitosis as well as at the S phase, since the doubling of the DNA and the return of the chromosome number are the two events being examined.
2. 2Fix the notation before the table, because this is where marks are lost. The number of chromosomes is counted by the number of centromeres, so a chromosome with two sister chromatids counts as one chromosome, not two, and this is the single most important convention in the whole question. The DNA content, the C, is measured as the amount of DNA, so a cell whose chromosomes each consist of two sister chromatids contains twice the DNA of a cell whose chromosomes are single, though the chromosome number is the same. So a diploid cell in the G1 has 2n chromosomes and 2C of DNA, and the same cell after the S phase has still 2n chromosomes but 4C of DNA, because the replication doubled the DNA and not the chromosome number, and this is the whole basis of the table.
3. 3The table for the mitosis, in a cell with 2n. In the G1 the cell has 2n chromosomes and 2C of DNA, and this is the condition in which it enters the S phase. In the S phase the DNA is being replicated, so the DNA content increases progressively from 2C to 4C while the chromosome number remains 2n throughout, since the two chromatids share the one centromere, and this is the point of the whole question. At the end of the S phase, and through the G2, the cell has 2n chromosomes and 4C of DNA. In the prophase and the metaphase the cell still has 2n and 4C, since nothing has yet separated, and in the metaphase the chromosomes align singly on the equator. In the anaphase, when the centromeres split and the sister chromatids separate, the chromosome number as counted per daughter nucleus doubles, so each pole now has 2n chromosomes, and the DNA per pole is 2C, and this is worth stating carefully, since it looks like a doubling but is in fact a halving, because the cell is being divided in two at the same moment. 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, which is the same as the parent had in its G1, so the cycle has closed and the division was equational.
4. 4The two changes to state explicitly at the end. The DNA content changes only once in the cycle, in the S phase, when it rises from 2C to 4C, and it falls back from 4C to 2C only because the cell is divided into two at the mitosis, so that each daughter receives half. The chromosome number per cell changes only at the anaphase, and it appears to double from 2n to 4n in the cell as a whole, but this is a consequence of the division, since each nucleus and each daughter cell has 2n, which is why the mitosis is called equational, and the DNA per nucleus correspondingly appears to halve from 4C to 2C at the same moment. So the two parameters behave differently: the DNA is doubled once in the S phase, and the chromosome number is only momentarily doubled in the anaphase of the cell as a whole and is never doubled within a nucleus.
5. 5Close with the contrast with meiosis, since it makes the analysis complete and it is the natural extension the examiner expects. In meiosis the DNA is still replicated only once, in the S phase, from 2C to 4C, and the chromosome number falls at the first division, since the centromeres do not split and the two daughter cells of the meiosis I each have n chromosomes with 2C of DNA, and in the meiosis II, with no intervening replication, the centromeres split and the four products each have n chromosomes and 1C of DNA, so the cell has passed from 2n and 2C to n and 1C, which is the reduction, and it is precisely the contrast with the mitosis, in which the cell has passed from 2n and 2C to 2n and 2C, that makes the two divisions equational and reductional respectively.