NCERT Solutions for Class 11 Biology Chapter 10: Cell Cycle and Cell Division

Concept — cell cycle span. The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises other constituents and divides into two daughter cells. NCERT illustrates a typical eukaryotic cell cycle with human (mammalian) cells in culture, which divide once in approximately every 24 hours.

• Average span = about 24 hours.
• M phase (actual cell division) lasts only about 1 hour of this 24 hours.
• Interphase lasts more than 95% of the cycle (G₁, S and G₂), the cell preparing for division by growth and orderly DNA replication.
• The duration varies from organism to organism and from cell type to cell type; for example yeast completes a cycle in about 90 minutes.

Extra depth. The 24 h value is an average for cells growing in culture; cells in the body may divide much faster or slower, and many adult cells such as heart cells stay in the quiescent G₀ stage and do not divide at all. Individual phase lengths are not given in NCERT, so the arcs of the cycle diagram are only illustrative.

Conclusion: the average cell cycle of a (human) mammalian cell lasts about 24 hours, of which only about one hour is the M phase and more than 95% is interphase.

Concept — two parts of cell division. The M phase starts with nuclear division (karyokinesis), corresponding to the separation of daughter chromosomes, and usually ends with division of the cytoplasm (cytokinesis). Together they complete cell division.

• Mitosis accomplishes the segregation of duplicated chromosomes into daughter nuclei (karyokinesis) and division of the cell itself (cytokinesis).
• In some organisms karyokinesis is not followed by cytokinesis, producing a syncytium — e.g., liquid endosperm in coconut.

Extra depth. Cytokinesis also distributes organelles such as mitochondria and plastids between daughter cells. Karyokinesis is the process that makes the ‘nuclear’ copy equal; cytokinesis makes the ‘cellular’ copy. A multinucleate cell is therefore a case of karyokinesis without cytokinesis.

Conclusion: karyokinesis divides the nucleus into two daughter nuclei, whereas cytokinesis divides the cytoplasm to give two daughter cells.

Concept — interphase. Interphase is the phase between two successive M phases. Although called the resting phase, it is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. It lasts more than 95% of the cell cycle and has three phases — G₁, S and G₂.

1. G₁ phase (Gap 1): interval between mitosis and the initiation of DNA replication. The cell is metabolically active and continuously grows but does not replicate its DNA. (NCERT summary: most organelle duplication also occurs in this phase.)
2. S phase (Synthesis): DNA synthesis or replication takes place. The amount of DNA per cell doubles — if the initial amount is 2C it becomes 4C. There is no increase in chromosome number: a cell with 2n chromosomes at G₁ still has 2n after S phase. In animal cells the centriole duplicates in the cytoplasm while DNA replication begins in the nucleus.
3. G₂ phase (Gap 2): proteins are synthesised in preparation for mitosis while cell growth continues.

Extra depth. Cell growth in terms of cytoplasmic increase is continuous throughout, but DNA synthesis occurs only in the S phase. Cells that stop dividing leave G₁ and enter the quiescent stage G₀ (e.g., heart cells), remaining metabolically active. The replicated DNA of S and G₂ is still intertwined until chromosome condensation begins in prophase.

Conclusion: during interphase the cell grows (G₁), replicates its DNA (S, 2C → 4C, chromosome number unchanged) and synthesises proteins for mitosis (G₂), thus preparing for division.

Concept — quiescent stage. Some cells in adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells lost because of injury or cell death. These cells that do not divide further exit the G₁ phase to enter an inactive stage called the quiescent stage (G₀) of the cell cycle.

• Where it fits: G₀ is entered from G₁ — the cell leaves the active cycle before DNA replication (S phase).
• Metabolism: cells in G₀ remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
• Examples: heart cells (do not appear to divide); cells that divide only occasionally to replace cells lost by injury or death.
• Return to cycle: when needed (e.g., injury), such cells can re-enter the cycle and divide.

Extra depth. G₀ is not a ‘dead’ state — the cell is alive and functional, only its proliferation is switched off. It explains how an organism keeps most of its adult cells specialised yet can still heal wounds.

Conclusion: G₀ is the inactive quiescent stage entered from G₁ by cells that do not divide further; they stay metabolically active and divide only when required.

Concept — equational division. Mitosis is the M phase of the cell cycle in which the nucleus divides and the chromosomes duplicated in the S phase are shared between two daughter cells. Because the number of chromosomes in the parent and in the progeny cells is the same, mitosis is also called equational division.

• S phase: each chromosome replicates into two sister chromatids (DNA 2C → 4C); chromosome number remains 2n.
• Anaphase: centromeres split and sister chromatids separate equally to opposite poles.
• Telophase and cytokinesis: two daughter nuclei, each receiving a complete set (2n chromosomes, 2C DNA), identical to the parent.
• So the parent diploid cell (2n) gives two diploid daughter cells (2n) with identical genetic complement — the division is equal in chromosome number (and in genetic content).

Extra depth. NCERT also calls meiosis the reduction division, since it halves the chromosome number; mitosis is its contrast — ‘equational’ because it only conserves it. Mitosis is usually restricted to diploid cells but in some lower plants and social insects haploid cells also divide by mitosis, producing haploid daughter cells — still with the same number as the parent.

Conclusion: mitosis is called equational division because the chromosome number (and genetic content) of the daughter cells remains equal to that of the parent cell.

Concept — matching events to stages. The events occur in mitosis (i, ii) and in meiosis I (iii, iv). Recall: mitosis = prophase, metaphase, anaphase, telophase; prophase I of meiosis = leptotene, zygotene, pachytene, diplotene, diakinesis.

• Remember the order of the five substages of prophase I: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis (mnemonic: Lazy Zebras Play Dead Dogs).
• Chiasmata become visible in diplotene, after crossing over — crossing over itself happens in pachytene.

Extra depth. In metaphase of mitosis and metaphase II the chromosomes (with two chromatids) are on the plate; in metaphase I bivalents are on the plate. Likewise in anaphase of mitosis and anaphase II sister chromatids separate, while in anaphase I homologous chromosomes separate and sister chromatids stay together.

Conclusion: (i) metaphase, (ii) anaphase, (iii) zygotene of prophase I, (iv) pachytene of prophase I.

Concept — events of prophase I of meiosis. Prophase I is long and complex and has five substages. Synapsis, bivalent formation and chiasmata are three linked events seen in zygotene, pachytene and diplotene.

• (a) Synapsis: during zygotene the chromosomes start pairing together; this process of association of homologous chromosomes is called synapsis. It is accompanied by the formation of the complex structure called the synaptonemal complex (seen in electron micrographs). The paired chromosomes are called homologous chromosomes.
• (b) Bivalent: the complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad (it contains four chromatids). They are more clearly visible at pachytene, when the four chromatids become distinct.
• (c) Chiasmata: at the beginning of diplotene the synaptonemal complex dissolves and the recombined homologous chromosomes of the bivalents tend to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. They move towards the ends (terminalisation) in diakinesis. In oocytes of some vertebrates diplotene may last for months or years.

Extra depth. Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes at recombination nodules, mediated by the enzyme recombinase; chiasmata are the visible consequence and keep homologues linked until anaphase I, which helps their correct separation. Recombination between homologous chromosomes is completed by the end of pachytene.

Conclusion: synapsis is the pairing of homologous chromosomes, a bivalent (tetrad) is the synapsed pair of four chromatids, and chiasmata are the X-shaped sites where non-sister chromatids have crossed over.

Concept — cytokinesis. After karyokinesis, the cell itself divides into two daughter cells by separation of the cytoplasm — cytokinesis. Animal and plant cells use different mechanisms because plant cells are enclosed by a relatively inextensible cell wall.

Extra depth. In some organisms karyokinesis is not followed by cytokinesis and the multinucleate condition called syncytium arises (e.g., liquid endosperm of coconut). The cell plate that forms in plants later gets walls deposited on both sides, forming the new cell walls of the two daughter cells; the plate itself remains as the middle lamella.

Conclusion: in animal cells cytokinesis occurs by a furrow that deepens inward and divides the cell, whereas in plant cells a cell plate forms in the centre and grows outward to meet the lateral walls.

Concept — size of meiotic products. Meiosis of a diploid cell gives four haploid cells. Whether the four cells are equal depends on how the cytoplasm is divided during the two cytokineses (NCERT: meiosis ends with telophase II and cytokinesis, forming a tetrad of cells). In gamete formation the cytoplasm may be shared equally or very unequally; the examples below are standard biology examples (the question asks you to find them).

• Why unequal in oogenesis? The egg needs a large store of cytoplasm, yolk and nutrients for early development; the polar bodies receive almost none and degenerate, so the chromosomes are reduced without wasting cytoplasm.
• Why equal in spermatogenesis? Sperms need to be small and motile; all four products are functional.
• Genetic content: in every case each of the four cells is haploid (n); only the amount of cytoplasm differs.

Extra depth. The terms ‘spermatid’, ‘polar body’ and ‘megaspore’ belong to later chapters (reproduction in organisms and flowering plants); here it is enough to know the contrast. Because oogenesis involves unequal cytokinesis it is an example of how the same nuclear process (meiosis) can have different cytoplasmic outcomes.

Conclusion: four equal cells arise in spermatogenesis and microsporogenesis, while four unequal cells (one large and three small) arise in oogenesis and megasporogenesis.

Concept — two anaphases. In mitotic anaphase the sister chromatids separate; in anaphase I of meiosis the homologous chromosomes separate while sister chromatids stay together.

• In both, spindle microtubules attached to kinetochores pull chromosomes toward the poles, with the centromere leading and arms trailing.
• NCERT key phrases: mitotic anaphase — “centromeres split and chromatids separate; chromatids move to opposite poles”; anaphase I — “homologous chromosomes separate, while sister chromatids remain associated at their centromeres”.

Extra depth. In meiosis II the anaphase (anaphase II) is like mitotic anaphase — the centromere splits and sister chromatids separate — which is why meiosis II is described as resembling a normal mitosis.

Conclusion: in mitotic anaphase the centromeres split and sister chromatids go to opposite poles, whereas in anaphase I the centromeres stay intact and homologous chromosomes separate, halving the chromosome number at each pole.

Concept — two kinds of division. Mitosis is the equational division of somatic cells (growth and repair); meiosis is the reduction division of specialised diploid cells that form gametes. Mitosis occurs once per cell cycle; meiosis involves two successive divisions with only one DNA replication.

Extra depth. The fusion of two haploid gametes at fertilisation restores the diploid number; without meiosis the chromosome number would double every generation. Meiosis II resembles mitosis but starts with haploid cells and no S phase, so the products are haploid.

Conclusion: mitosis is a single equational division giving two identical diploid cells, whereas meiosis consists of two divisions with synapsis and crossing over, giving four genetically different haploid cells.

Concept — role of meiosis. Sexual reproduction needs fusion of two gametes, each with a complete haploid set of chromosomes. Meiosis forms these haploid gametes from diploid cells, and fertilisation restores the diploid phase.

1. Conservation of chromosome number: meiosis is the mechanism by which the specific chromosome number of each species is conserved across generations in sexually reproducing organisms — even though, paradoxically, the process itself reduces the chromosome number by half. Gametes are haploid (n); at fertilisation n + n = 2n restores the diploid number.
2. Genetic variability: meiosis increases genetic variability in the population from one generation to the next. Crossing over between non-sister chromatids of homologues (pachytene) produces recombinant chromosomes, and the separation of homologues in anaphase I gives new combinations of chromosomes in gametes.
3. Evolution: variations are very important for the process of evolution; natural selection acts on them.
4. Haploid phase of the life cycle: meiosis ensures production of the haploid phase in sexually reproducing organisms.
5. Gamete formation: meiosis occurs during gametogenesis in plants and animals.

Extra depth. If gametes were diploid, each fertilisation would double the chromosome number and the genome would become unmanageable in a few generations. Because every gamete is a unique recombinant, siblings (except identical twins) differ genetically.

Conclusion: meiosis conserves the species chromosome number across generations by forming haploid gametes and increases genetic variability, which is essential for evolution.

Concept — mitosis in haploid cells. In animals mitosis is generally seen in diploid somatic cells, with a few exceptions where haploid cells divide by mitosis. NCERT states that plants show mitotic divisions in both haploid and diploid cells, and that in some lower plants and social insects haploid cells also divide by mitosis (section 10.1.1 and 10.3). The question is meant for discussion; the points below can be taken to the teacher.

• (i) Haploid insects and lower plants: in male honey bees all body cells are haploid, yet they divide mitotically for growth; in algae, mosses and ferns the dominant or independent gametophyte is haploid and grows through mitosis (see alternation of generations in Chapter 3).
• (ii) Haploid cells in higher plants that do not divide: terminally differentiated haploid cells of the female gametophyte such as the egg cell and synergids, and the male gametes, normally do not divide further; the egg completes its role by fusing with a male gamete.
• Point to discuss: division is a property of the cell state (meristematic or growing) and not of ploidy alone.

Extra depth. NCERT asks you to identify, from the alternation of generations of Chapter 3, plant species and stages at which mitosis occurs in haploid cells: for example the gametophyte phase of bryophytes (the dominant stage) and pteridophytes. The statements about specific cells of the embryo sac are standard flowering-plant embryology; confirm details with your teacher as the question suggests.

Conclusion: mitosis occurs in haploid cells of male honey bees and of the gametophytes of lower plants, while differentiated haploid gametic cells of higher plants such as the egg and synergids do not divide.

Concept — role of S phase. In the S phase DNA is replicated, so each chromosome is made of two sister chromatids and the DNA content doubles (2C → 4C). Mitosis separates these chromatids equally. Hence normal mitosis cannot occur without prior DNA replication.

• Answer: No (in a normal cell cycle).
• Reason 1: without S phase each chromosome has only one chromatid; there are no two sister chromatids to separate at anaphase.
• Reason 2: if division happened without replication the daughter cells would receive half the DNA (C instead of 2C) and would not have intact genomes; NCERT stresses that division, DNA replication and growth must be coordinated to ensure progeny cells with intact genomes.
• Reason 3: the mitotic chromosome in prophase/metaphase is described as composed of two chromatids attached at the centromere — formed only after S phase.

Extra depth. Cells that skip S phase and divide would lose DNA with each division; living cells therefore have control mechanisms ensuring DNA synthesis is complete before M phase (the events are ‘under genetic control’ as NCERT states).

Conclusion: no — normal mitosis requires DNA replication in S phase, because mitosis separates the sister chromatids formed by that replication and gives daughter cells with intact genomes.

Concept — replication and division are separable. DNA replication occurs in the S phase and cell division in the M phase; these are different events of the cell cycle, so replication can occur without a following division.

• Answer: Yes. If the M phase (or cytokinesis) does not follow the S phase, the DNA content increases without increase in cell number.
• Karyokinesis without cytokinesis: in some organisms nuclear division is not followed by division of cytoplasm, giving a multinucleate syncytium — e.g., liquid endosperm of coconut (the nuclei replicate DNA and divide, but the cell does not divide).
• Replication without even nuclear division leads to polyploid / endoreduplicated cells (cells with several copies of the genome), a known phenomenon in some tissues of plants and animals (e.g., giant polytene chromosomes in salivary glands of some insect larvae) — additional examples beyond NCERT.
• Quiescent cells are the opposite case (no replication, no division).

Extra depth. The cell cycle is a controlled sequence; the S and M phases are coupled in most cells, but they can be uncoupled in specialised tissues, which is why the question of ‘replication without division’ is meaningful. Note that polytene chromosomes are given as a commonly cited example; check with your teacher.

Conclusion: yes — DNA replication can occur without cell division, as seen in multinucleate (syncytial) cells such as the liquid endosperm of coconut and in polyploid cells.

Concept — tracking N and C. Take a diploid cell with 2N chromosomes and 2C DNA in G₁. Chromosome number is counted by centromeres; DNA content by the amount of DNA in the cell. S phase doubles C but not N; anaphase separates chromatids, temporarily doubling the count in the cell; cytokinesis halves both.

• Mitosis: N stays 2N in each cell; C doubles in S (2C → 4C) and is halved at cytokinesis (4C → 2C in each daughter).
• Meiosis: N is halved at the end of meiosis I (2N → N) and C falls 4C → 2C → C by the end of meiosis II — hence four haploid cells, each with C.
• Worked example from the text: onion root tip, 2N = 16 — G₁: 16 chromosomes, 2C; after S: 16 chromosomes, 4C; after M: 16 chromosomes, 2C in each daughter cell.

Extra depth. When counting chromosomes at anaphase, if each separated chromatid is counted as a chromosome the cell temporarily appears to have 4N; this is why it is useful to state the counting convention. The C values (2C, 4C, C) are relative DNA amounts; they are not given as absolute weights in NCERT.

Conclusion: in mitosis N remains 2N while C goes 2C → 4C → 2C per daughter cell; in meiosis N is halved (2N → N) and C falls 4C → 2C → C, producing four haploid cells.