SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Cell Cycle and Cell Division

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Exercises 10.11–10.16 (part 2 of 2)

  1. Exercise 10.11

    List the main differences between mitosis and meiosis.

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    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.

  2. Exercise 10.12

    What is the significance of meiosis?

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    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.

  3. Exercise 10.13

    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.

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    This is a discussion activity — the chapter asks you to work it out with your teacher and to recall alternation of generations from Chapter $\displaystyle 3$, 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 \(\displaystyle G_{0}\) 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.

  4. Exercise 10.14

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

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    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 \(\displaystyle 2C\) to \(\displaystyle 4C\) precisely so that each daughter cell can end up with \(\displaystyle 2C\). Divide a \(\displaystyle 2C\) cell without replication and each daughter gets \(\displaystyle 1C\).
    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.

  5. Exercise 10.15

    Can there be DNA replication without cell division?

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    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 \(\displaystyle G_{1}\) for \(\displaystyle G_{0}\) neither replicate their DNA nor divide.

  6. Exercise 10.16

    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

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    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 \(\displaystyle 2n\), \(\displaystyle 2C\) at \(\displaystyle G_{1}\).
    Interphase
    \(\displaystyle G_{1}\): N = \(\displaystyle 2n\); C = \(\displaystyle 2C\).
    S: N stays \(\displaystyle 2n\) (no increase in chromosome number); C rises from \(\displaystyle 2C\) to \(\displaystyle 4C\) as each chromosome gains a second chromatid.
    \(\displaystyle G_{2}\): N = \(\displaystyle 2n\); C = \(\displaystyle 4C\).
    Mitosis
    Prophase and metaphase: N = \(\displaystyle 2n\) (each chromosome of two chromatids); C = \(\displaystyle 4C\).
    Anaphase: centromeres split, so the cell momentarily contains \(\displaystyle 4n\) chromosomes, \(\displaystyle 2n\) moving to each pole; C is still \(\displaystyle 4C\) in the undivided cell.
    Telophase and after cytokinesis: each daughter cell is \(\displaystyle 2n\), \(\displaystyle 2C\) — identical to the parent at \(\displaystyle G_{1}\), which is why mitosis is equational.
    Meiosis I
    Prophase I and metaphase I: N = \(\displaystyle 2n\) (as \(\displaystyle n\) bivalents); C = \(\displaystyle 4C\).
    Anaphase I: homologues separate with centromeres intact, so \(\displaystyle n\) chromosomes (each still of two chromatids) move to each pole; no momentary doubling here.
    Telophase I / dyad of cells: each cell is \(\displaystyle n\), \(\displaystyle 2C\).
    Interkinesis: no DNA replication, so the values stay at \(\displaystyle n\), \(\displaystyle 2C\).
    Meiosis II
    Prophase II and metaphase II: N = \(\displaystyle n\); C = \(\displaystyle 2C\).
    Anaphase II: centromeres split, so the cell momentarily holds \(\displaystyle 2n\) chromosomes, \(\displaystyle n\) going to each pole.
    Telophase II / tetrad of cells: each of the four daughter cells is \(\displaystyle n\), \(\displaystyle C\).
    Overall: mitosis returns \(\displaystyle 2n\), \(\displaystyle 2C\) → \(\displaystyle 2n\), \(\displaystyle 2C\); meiosis takes \(\displaystyle 2n\), \(\displaystyle 2C\) → four cells of \(\displaystyle n\), \(\displaystyle C\), the halving of chromosome number happening at anaphase I and the halving of DNA content once at each of the two cytokineses.
    NCERT_Solution_Class11_Biology_Ch10_Q10-16