Question 1

What is the average cell cycle span for a mammalian cell?

Accepted Answer

- About 24 hours. - This is the span for a typical eukaryotic cell cycle as illustrated by human cells in culture, which divide once in approximately every 24 hours. - Of these 24 hours, cell division proper (M phase) lasts only about an hour; interphase occupies more than 95% of the cycle. - The duration is not fixed — it varies from organism to organism and from cell type to cell type (yeast completes a cell cycle in about 90 minutes).

Question 2

Distinguish cytokinesis from karyokinesis.

Accepted Answer

- Karyokinesis is the division of the nucleus; cytokinesis is the division of the cytoplasm. - Order in M phase: M phase starts with karyokinesis, corresponding to the separation of daughter chromosomes, and usually ends with cytokinesis. - Stages: karyokinesis is divided into four stages — prophase, metaphase, anaphase and telophase; cytokinesis has no such stages. - Outcome: karyokinesis distributes the replicated chromosomes into two daughter nuclei; cytokinesis separates the cell body into two daughter cells and distributes organelles such as mitochondria and plastids between them. - Mechanism: cytokinesis differs between cell types — a furrow in the plasma membrane in animal cells, a cell-plate growing outward in plant cells; karyokinesis proceeds alike in both. - Dependence: karyokinesis is not always followed by cytokinesis; when it is not, a multinucleate syncytium results (e.g., liquid endosperm in coconut).

Question 3

Describe the events taking place during interphase.

Accepted Answer

- Interphase is the phase between two successive M phases; though called the resting phase, it is the time during which the cell prepares for division by cell growth and DNA replication in an orderly manner, and it lasts more than 95% of the cell cycle. - It has three sub-phases — G 1, S and G 2. G 1 phase (Gap 1) - The interval between mitosis and the initiation of DNA replication. - The cell is metabolically active and grows continuously, but does not replicate its DNA. S phase (Synthesis) - The period during which DNA synthesis or replication takes place; the amount of DNA per cell doubles, from 2C to 4C. - There is no increase in chromosome number — a cell with 2n chromosomes at G 1 still has 2n after S phase. - In animal cells, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm. G 2 phase (Gap 2) - Proteins are synthesised in preparation for mitosis, while cell growth continues. - Cells that will not divide further leave G 1 and enter the inactive quiescent stage, G 0.

Question 4

What is G o (quiescent phase) of cell cycle?

Accepted Answer

- G 0 is the quiescent stage of the cell cycle — an inactive stage entered by cells that do not divide further, which exit G 1 instead of proceeding to the S phase. - Cells in G 0 remain metabolically active but no longer proliferate, unless called on to do so depending on the requirement of the organism. - Examples: heart cells of adult animals, which do not appear to divide; and cells that divide only occasionally, as needed to replace cells lost because of injury or cell death.

Question 5

Why is mitosis called equational division?

Accepted Answer

- Because the number of chromosomes in the parent cell and in the progeny cells is the same — the chromosome number is not reduced, so the division is "equational". - The chromosomes duplicate once in S phase, and at anaphase the centromere splits so that one chromatid of each chromosome goes to each pole; each daughter nucleus therefore receives a full set. - The result is two daughter cells with an identical genetic complement, usually diploid, like the parent. - This contrasts with meiosis, called the reduction division, which halves the chromosome number.

Question 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.

Accepted Answer

- (i) Chromosomes are moved to spindle equator — Metaphase (of mitosis; the same happens at metaphase I and metaphase II of meiosis). - (ii) Centromere splits and chromatids separate — Anaphase (of mitosis; and anaphase II of meiosis). - (iii) Pairing between homologous chromosomes takes place — Zygotene stage of Prophase I of meiosis I; the pairing is called synapsis. - (iv) Crossing over between homologous chromosomes takes place — Pachytene stage of Prophase I of meiosis I, at the recombination nodules.

Question 7

Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.

Accepted Answer

(a) Synapsis - The pairing together of homologous chromosomes, which begins at the zygotene stage of prophase I. - The association is accompanied by the formation of a complex structure called the synaptonemal complex, seen in electron micrographs of this stage. (b) Bivalent - The complex formed by a pair of synapsed homologous chromosomes, also called a tetrad. - Its four chromatids become distinct and clearly appear as a tetrad at pachytene, the stage at which recombination nodules appear and crossing over occurs between non-sister chromatids. (c) Chiasmata - The X-shaped structures at which recombined homologous chromosomes remain linked after crossing over. - They are recognised at diplotene, when the synaptonemal complex dissolves and the homologues of a bivalent tend to separate from each other except at the sites of crossovers. - At diakinesis the chiasmata undergo terminalisation (they move towards the chromosome ends). Diagram to draw — three small panels of one pair of homologous chromosomes, left to right: - Zygotene: two homologous chromosomes drawn side by side and touching along their length, one shaded and one unshaded so they can be told apart, each with a small circle marking its centromere; label the line between them synaptonemal complex and the pairing itself synapsis. - Pachytene: the same pair now drawn with each homologue split lengthwise into two chromatids, so four parallel chromatids in all, still joined at the two centromeres; label the whole structure bivalent (tetrad), and mark a small knob where two non-sister chromatids cross as the recombination nodule (site of crossing over). - Diplotene: the two homologues drawn pulling apart, remaining attached at one or two points where a shaded chromatid crosses over an unshaded one, making an X; label each such X a chiasma and show the exchanged (recombined) segments by the change of shading along the chromatid arms. - Keep the shading consistent across all three panels so that the exchange of segments is visible in the last one.

Question 8

How does cytokinesis in plant cells differ from that in animal cells?

Accepted Answer

- Animal cells divide the cytoplasm by a furrow: a furrow appears in the plasma membrane, gradually deepens, and ultimately joins in the centre, dividing the cell cytoplasm into two. It thus proceeds inward from the surface. - Plant cells cannot do this because they are enclosed by a relatively inextensible cell wall; they undergo cytokinesis by a different mechanism. - In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls — the opposite direction to the animal furrow. - The new wall begins as a simple precursor, the cell-plate, which represents the middle lamella between the walls of the two adjacent daughter cells. - Common to both: at the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

Question 9

Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Accepted Answer

- Four equal-sized daughter cells: meiosis during spermatogenesis in animals (one primary spermatocyte gives four equal spermatids) and during microsporogenesis in flowering plants (one microspore mother cell gives four equal microspores/pollen grains). - Four unequal-sized daughter cells: meiosis during oogenesis in animals, where cytokinesis itself is unequal, giving one large ovum and small, non-functional polar bodies. - What this chapter itself supplies: meiosis is met with during gametogenesis in plants and animals, and telophase II ends in a tetrad of cells, i.e., four haploid daughter cells — the chapter treats all four simply as "four haploid cells". - Chapter 10 names no gamete-forming process and never compares the sizes of the four cells, so the examples above are drawn from beyond this chapter.

Question 10

Distinguish anaphase of mitosis from anaphase I of meiosis.

Accepted Answer

- What separates: in anaphase of mitosis the sister chromatids of each chromosome separate and become daughter chromosomes; in anaphase I the whole homologous chromosomes separate from each other. - Centromere: it splits in anaphase of mitosis, so the chromatids come apart; it does not split in anaphase I — the sister chromatids remain associated at their centromeres and travel to the pole together. - What reaches each pole: one chromatid of every chromosome in mitosis; one complete homologue of every pair, still carrying both its chromatids, in anaphase I. - Chromosome number at each pole: the same as the parent cell ( 2n ) in mitosis; half that of the parent cell ( n ) in anaphase I — this is the step that makes meiosis a reduction division. - What preceded it: in mitosis the chromosomes lay singly on the metaphase plate; anaphase I follows the pairing of homologues into bivalents and crossing over in prophase I, so the chromosomes separating are recombined. - Genetic outcome: mitosis keeps the two daughter nuclei genetically identical; anaphase I contributes to genetic variability.

Question 11

List the main differences between mitosis and meiosis.

Accepted Answer

- Number of divisions: mitosis is a single cycle of nuclear and cell division; meiosis involves two sequential cycles — meiosis I and meiosis II — but only a single cycle of DNA replication. - Number of daughter cells: two in mitosis; four at the end of meiosis II. - Chromosome number: conserved, hence mitosis is the equational division; halved, hence meiosis is the reduction division giving haploid cells. - Where it occurs: mitosis in diploid somatic cells (with exceptions — haploid cells of male honey bees, and haploid as well as diploid cells in plants); meiosis in specialised diploid cells destined to form gametes, i.e., during gametogenesis. - Prophase: short and relatively simple in mitosis; prophase I is typically longer and more complex, subdivided into leptotene, zygotene, pachytene, diplotene and diakinesis. - Pairing and crossing over: absent in mitosis; in meiosis homologous chromosomes pair ( synapsis ) to form bivalents, and crossing over occurs between non-sister chromatids at pachytene. - Metaphase: individual chromosomes align on the metaphase plate with sister kinetochores attached to opposite poles; at metaphase I the bivalents align on the equatorial plate, with microtubules from opposite poles attached to the kinetochores of homologous chromosomes. - Anaphase: centromeres split and chromatids separate in mitosis; at anaphase I the homologous chromosomes separate while sister chromatids stay joined at their centromeres — the chromatids separate only at anaphase II. - Genetic result: daughter cells with an identical genetic complement in mitosis; genetically different daughter cells in meiosis, because of recombination. - Significance: mitosis gives growth of multicellular organisms, cell repair and restoration of the nucleo-cytoplasmic ratio, and continuous growth from meristems in plants; meiosis produces the haploid phase, conserves the species chromosome number across generations and increases genetic variability for evolution.

Question 12

What is the significance of meiosis?

Accepted Answer

- Meiosis is the mechanism by which the specific chromosome number of each species is conserved across generations in sexually reproducing organisms — paradoxically, by reducing the number by half in the gametes, so that fertilisation restores the diploid number. - It produces the haploid phase of the life cycle: four haploid daughter cells from one diploid cell, which become the gametes. - It increases genetic variability in the population of organisms from one generation to the next, chiefly through crossing over and the resulting recombination of genetic material between homologous chromosomes. - These variations are very important for the process of evolution.

Question 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.

Accepted Answer

- This is a discussion activity — the chapter asks you to work it out with your teacher and to recall alternation of generations from Chapter 3, so the full answer is not printed in this chapter. (i) Haploid insects and lower plants where cell division occurs - The chapter's own example is the male honey bee (drone): in animals mitotic division is normally seen only in diploid somatic cells, but a few haploid cells divide by mitosis, and male honey bees are the case named. - In plants, mitotic divisions occur in both haploid and diploid cells; the haploid gametophyte generation of lower plants therefore grows and multiplies by mitosis. The chapter tells you to identify the plant species and the stages from the examples of alternation of generations in Chapter 3. - The chapter also notes that in some lower plants and some social insects, haploid cells divide by mitosis. (ii) Haploid cells in higher plants where cell division does not occur - The chapter does not name any. What it does say is that the haploid product of meiosis in higher plants goes on to form gametes, and that cells which stop dividing pass into the quiescent G 0 stage, staying metabolically active without proliferating. - Cells usually cited here — the synergids and antipodal cells of the embryo sac, and the male gametes — belong to the reproduction chapters, not to this one; check them with your teacher.

Question 14

Can there be mitosis without DNA replication in ‘S’ phase?

Accepted Answer

- No — mitosis cannot occur normally without DNA replication in the S phase. - The S phase is what gives each chromosome its two sister chromatids held together at the centromere; without it, a chromosome enters prophase as a single chromatid. - Mitosis works by splitting the centromere at anaphase and sending one chromatid of each chromosome to each pole. With no replication there is nothing to split, so the chromosomes would have to be shared out — each daughter cell receiving only half the genome. - The DNA content makes this plain: S phase raises the DNA per cell from 2C to 4C precisely so that each daughter cell can end up with 2C. Divide a 2C cell without replication and each daughter gets 1C. - Mitosis would then no longer be an equational division, and the whole point of the cell cycle — that division, DNA replication and growth be coordinated so the progeny cells contain intact genomes — would fail.

Question 15

Can there be DNA replication without cell division?

Accepted Answer

- Yes — DNA replication (and even nuclear division) can occur without the cell dividing into two. - The chapter's own example: in some organisms karyokinesis is not followed by cytokinesis, so a multinucleate condition arises and a syncytium is formed — e.g., the liquid endosperm in coconut. Here the DNA has been replicated and the nuclei have divided repeatedly, but the cytoplasm has never been partitioned into separate daughter cells. - This is possible because cell growth, DNA replication and cell division are separate events of the cell cycle; they are normally coordinated, but the coordination can be broken. - Note the converse is also implied by the chapter: cells that leave G 1 for G 0 neither replicate their DNA nor divide.

Question 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

Accepted Answer

- Two rules govern both parameters: the DNA amount (C) doubles only in the S phase and is halved at each cytokinesis; the chromosome number (N) changes only when centromeres split (it doubles momentarily) or when the cell divides. - Take a diploid parent cell that is 2n, 2C at G 1. Interphase - G 1: N = 2n; C = 2C. - S: N stays 2n (no increase in chromosome number); C rises from 2C to 4C as each chromosome gains a second chromatid. - G 2: N = 2n; C = 4C. Mitosis - Prophase and metaphase: N = 2n (each chromosome of two chromatids); C = 4C. - Anaphase: centromeres split, so the cell momentarily contains 4n chromosomes, 2n moving to each pole; C is still 4C in the undivided cell. - Telophase and after cytokinesis: each daughter cell is 2n, 2C — identical to the parent at G 1, which is why mitosis is equational. Meiosis I - Prophase I and metaphase I: N = 2n (as n bivalents); C = 4C. - Anaphase I: homologues separate with centromeres intact, so n chromosomes (each still of two chromatids) move to each pole; no momentary doubling here. - Telophase I / dyad of cells: each cell is n, 2C. - Interkinesis: no DNA replication, so the values stay at n, 2C. Meiosis II - Prophase II and metaphase II: N = n; C = 2C. - Anaphase II: centromeres split, so the cell momentarily holds 2n chromosomes, n going to each pole. - Telophase II / tetrad of cells: each of the four daughter cells is n, C. - Overall: mitosis returns 2n, 2C → 2n, 2C; meiosis takes 2n, 2C → four cells of n, C, the halving of chromosome number happening at anaphase I and the halving of DNA content once at each of the two cytokineses.

NCERT Solutions · Class 11 Biology Cell Cycle and Cell Division

Exercises 10.1–10.10 (part 1 of 2)

Exercise 10.1

Exercise 10.2

Exercise 10.3

Exercise 10.4

Exercise 10.5

Exercise 10.6

Exercise 10.7

Exercise 10.8

Exercise 10.9

Exercise 10.10

Exercises 10.11–10.16 (part 2 of 2)

Exercise 10.11

Exercise 10.12

Exercise 10.13

Exercise 10.14

Exercise 10.15

Exercise 10.16