NCERT Solutions Class 11 Biology Chapter 10 - Cell Cycle and Cell Division

Q1: What is the average cell cycle span for a mammalian cell?
Ans: The average cell cycle span for a mammalian cell is approximately 24 hours. Typically, this time is distributed among the phases as follows: most cells spend about 10 hours in G₁ phase, the S phase (DNA synthesis) takes several hours and its duration can vary with cell type and DNA content, G₂ is usually shorter (a few hours) and M phase (mitosis) lasts about an hour in many mammalian cells. These values are typical averages and vary between cell types and physiological conditions.

Mitotic

Q2: Distinguish cytokinesis from karyokinesis.
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Q3: Describe the events taking place during interphase.
Ans: Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication in an orderly manner. Interphase is divided into three phases.

Interphase

• G₁ phase: The cell grows in size, synthesises RNA and proteins and prepares the cellular machinery needed for DNA replication. It is metabolically very active.
• S phase: DNA synthesis occurs and the entire genome is replicated. The amount of DNA per cell doubles, although the chromosome number (N) remains the same because each chromosome now has two sister chromatids.
• G₂ phase: Further growth occurs and the cell synthesises proteins (including those needed for mitosis), checks DNA integrity and readies itself for entry into M phase.

G₂ Phase

Q4: What is G₀ (quiescent phase) of cell cycle?
Ans: In adult animals, some cells stop dividing and enter a non‑dividing, specialised state called the G₀ phase. Cells in G₀ are metabolically active but do not proliferate unless stimulated. Examples include many neurons and certain muscle cells, which remain in G₀ for long periods. Other cells can re‑enter the cycle from G₀ when required for tissue repair or replacement.

Q5: Why is mitosis called equational division?
Ans: Mitosis is called equational division because it produces two daughter nuclei that have the same number of chromosomes as the parent nucleus. During mitosis, sister chromatids separate and are distributed equally so that each daughter cell receives an identical set of chromosomes; thus the chromosome number remains unchanged (for example, diploid → diploid).

Q6: 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.
Ans:
(i) In Metaphase, chromosomes are aligned at the spindle equator.
(ii) In Anaphase, the centromere splits and sister chromatids separate and move to opposite poles.
(iii) In meiosis, pairing between homologous chromosomes (synapsis) occurs during the zygotene stage of prophase I.
(iv) In meiosis, crossing over between homologous chromosomes occurs during the pachytene stage of prophase I.

Q7: Describe the following:
(a) Synapsis 
(b) Bivalent 
(c) Chiasmata
Draw a diagram to illustrate your answer.
Ans:
(a) Synapsis: The pairing of homologous chromosomes is called synapsis. This occurs during the zygotene stage of prophase I in meiosis. During synapsis, homologues come together lengthwise so that corresponding regions are aligned and prepared for recombination.

(b) Bivalent: A bivalent (also called a tetrad) is the structure formed when two homologous chromosomes pair during synapsis. It consists of four chromatids (two from each homologue) and is visible during prophase I of meiosis.

(c) Chiasmata: Chiasmata are the visible points of contact where non-sister chromatids have exchanged segments during crossing over. They become apparent in the diplotene stage of prophase I and mark the sites of genetic recombination, contributing to variation.

Q8: How does cytokinesis in plant cells differ from that in animal cells?
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Q9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. 
Ans: During spermatogenesis (formation of male gametes)  in humans and many other organisms, the four daughter cells produced by meiosis develop into four equal-sized sperm cells. In contrast, during oogenesis (formation of the female gamete), meiosis produces one large mature ovum and three much smaller polar bodies; thus the four products are unequal in size. The unequal division in oogenesis ensures that the ovum receives most of the cytoplasm and stored nutrients.

Q10: Distinguish anaphase of mitosis from anaphase I of meiosis.
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Q11: List the main differences between mitosis and meiosis.
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Q12: What is the significance of meiosis?
Ans: Meiosis is the process involving the reduction in the amount of genetic material. It comprises two successive nuclear and cell divisions, with a single cycle of DNA replication. As a result, at the end of meiosis II, four haploid cells are formed.

Meiosis

Significance of meiosis

• Maintenance of chromosome number: Meiosis reduces chromosome number by half in gametes so that fertilisation restores the species' characteristic number in the zygote.
• Genetic variation: Crossing over and independent assortment during meiosis create new combinations of alleles, which are essential for evolution and adaptation.
• Source of new traits: Chromosomal mutations and recombination events during meiosis can introduce heritable variation that may be advantageous.
• Formation of gametes: Meiosis is the specialised division that produces haploid gametes required for sexual reproduction.

Q13: 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.
Ans: (i) In some lower plants and certain organisms the dominant life stage may be haploid; in such life cycles, fertilisation is often immediately followed by zygotic meiosis, producing haploid individuals that grow and undergo mitotic divisions in the haploid condition. This pattern is called a haplontic life cycle.
(ii)  In some higher plants, certain cells may undergo repeated rounds of DNA replication without mitosis (a process called endoreduplication), or otherwise fail to divide; this can lead to polyploidy in those cells. Polyploidy can also be artificially induced in plant cells (for example, by treatment with colchicine), and such changes are used in plant breeding to create new varieties.

Q14: Can there be mitosis without DNA replication in ‘S’ phase?
Ans. No. DNA replication in S phase is essential before mitosis because each daughter cell must receive a complete set of genetic information. Without DNA duplication, daughter nuclei could not obtain the full complement of chromosomes and normal cell function would be compromised. Therefore, mitosis cannot correctly occur without prior DNA replication.

Q15: Can there be DNA replication without cell division? 
Ans: Yes. DNA replication can occur without immediate cell division in several situations. Examples include the replication of DNA in organelles such as mitochondria and chloroplasts, and endoreduplication in some plant and animal cells where the genome is replicated repeatedly without mitosis, leading to increased nuclear DNA content (polyploidy) and often larger cell size. If DNA accumulates without division, the nucleus and cell may enlarge.

Q16: 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
Ans: Changes in chromosome number (N) and DNA content (C) differ between mitosis and meiosis. The important points are:

• In mitosis: the number of chromosomes (N) in daughter cells remains the same as that of the parent cell. The DNA content (C) doubles during S phase (replication) and is then equally divided during mitosis so that each daughter cell regains the original DNA amount.
• In meiosis: the reduction in chromosome number happens during anaphase I, when homologous chromosomes are separated - this is a reductional division and results in daughter cells with half the chromosome number. The amount of DNA per cell becomes haploid after anaphase II, when sister chromatids finally separate; thus meiosis I reduces chromosome number (N) while meiosis II reduces DNA content (C) per nucleus to the haploid state.

Number of chromosomes