NCERT Solutions for Class 11 Biology Chapter 10: Cell Cycle and Cell Division – Free PDF Download

Chapter 10, Cell Cycle and Cell Division, explains how a cell grows, copies its genetic material and splits into daughter cells through an orderly sequence of events called the cell cycle. It covers interphase (the G1, S and G2 sub-phases), the four stages of mitosis — prophase, metaphase, anaphase and telophase — and the two successive divisions of meiosis (meiosis I and meiosis II) that produce haploid gametes. The chapter closes with the biological significance of mitosis (growth, repair, asexual reproduction) and meiosis (genetic variation, maintenance of chromosome number across generations). These Class 11 Biology Chapter 10 solutions are also useful as quick revision notes before exams.

Exercises

Q1. What is the average cell cycle span for a mammalian cell?

The average cell cycle span for a mammalian cell is approximately 24 hours. Of this, the M phase (mitosis) lasts only about an hour, while interphase — G1, S and G2 combined — occupies roughly 95% of the total cycle time, which is why interphase is often (misleadingly) called the “resting phase”.

Q2. Distinguish cytokinesis from karyokinesis.

Karyokinesis and cytokinesis are the two sequential events of the M phase — karyokinesis (division of the nucleus) always precedes cytokinesis (division of the cytoplasm).

Karyokinesis	Cytokinesis
Division of the nucleus, in which replicated chromosomes are equally distributed to two daughter nuclei.	Division of the cytoplasm following karyokinesis, producing two separate daughter cells.
Involves prophase, metaphase, anaphase and telophase.	Occurs by furrowing (cleavage) in animal cells and by cell-plate formation in plant cells.
Ensures each daughter nucleus gets an identical set of chromosomes.	Ensures each daughter cell gets its own share of cytoplasm and organelles.

Q3. Describe the events taking place during interphase.

Interphase is the interval between two successive M phases and is metabolically the most active part of the cell cycle, as the cell prepares itself for division. It is divided into three sub-phases:

• G1 phase (Gap 1): The interval between the end of the previous mitosis and the start of DNA synthesis. The cell grows continuously, synthesises RNA and proteins, and increases in size, but DNA is not yet replicated.
• S phase (Synthesis): DNA replication occurs, so the DNA content of the cell doubles. If the cell had a chromosome number 2N and DNA content 2C at the start, it still has 2N chromosomes but 4C DNA content at the end of S phase (since each chromosome now consists of two sister chromatids). In animal cells, the centriole also duplicates during this phase.
• G2 phase (Gap 2): Protein synthesis continues, and the cell keeps growing in preparation for mitosis. Organelles required for cell division, and other proteins, are synthesised here.

Q4. What is G0 (quiescent phase) of cell cycle?

Not all cells that enter G1 go on to divide. Cells that are not actively dividing — for example, most cells in an adult mammal, or nerve and heart-muscle cells — exit the G1 phase and enter an inactive stage called the G0 or quiescent phase. Cells in G0 remain metabolically active and continue to perform their normal functions, but they do not proliferate unless the organism specifically calls upon them to do so (for instance, to replace cells lost through injury or cell death).

Q5. Why is mitosis called equational division?

During mitosis, a single replicated chromosome divides such that its two sister chromatids separate and move into two different daughter nuclei. As a result, the chromosome number of each daughter cell remains exactly the same as that of the parent cell (2N → 2N in diploid cells). Because the chromosome number is conserved rather than reduced, mitosis is described as an equational division.

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.

(i) Chromosomes align at the spindle equator (forming the metaphase plate) during metaphase of mitosis (or metaphase I/II of meiosis).
(ii) The centromere splits and sister chromatids separate towards opposite poles during anaphase of mitosis (this happens in anaphase II, not anaphase I, of meiosis).
(iii) Homologous chromosomes pair up (synapsis) during the zygotene sub-stage of prophase I of meiosis.
(iv) Crossing over between non-sister chromatids of homologous chromosomes occurs during the pachytene sub-stage of prophase I of meiosis.

Q7. Describe the following: (a) synapsis (b) bivalent (c) chiasmata. Draw a diagram to illustrate your answer.

(a) Synapsis: During the zygotene sub-stage of prophase I of meiosis, homologous chromosomes (one maternal, one paternal) come together and pair lengthwise, point by point. This precise pairing is called synapsis, and it is held together by a protein-rich structure called the synaptonemal complex.

(b) Bivalent (tetrad): The complex formed by a pair of synapsed homologous chromosomes is called a bivalent. Since each chromosome in the pair already consists of two sister chromatids (having replicated during the premeiotic S phase), a bivalent contains four chromatids and is therefore also called a tetrad.

(c) Chiasmata: At the end of pachytene, crossing over takes place between non-sister chromatids of the homologous chromosomes, exchanging genetic material. During diplotene, the synaptonemal complex dissolves and the paired homologous chromosomes start separating from each other, except at the points where crossing over occurred. These X-shaped points of attachment are called chiasmata.

Diagram description: Picture two homologous chromosomes, each already split into two sister chromatids (four chromatids total), lying side by side. Where the two non-sister chromatids overlap and exchange segments, an X-shaped junction is drawn and labelled “chiasma” — this junction is the visible sign of crossing over holding the bivalent (tetrad) together as the chromosomes begin to separate.

Q8. How does cytokinesis in plant cells differ from that in animal cells?

Cytokinesis in plant cells	Cytokinesis in animal cells
Occurs by formation of a cell plate that grows outward from the centre of the cell to the periphery.	Occurs by cleavage, i.e., a furrow that begins at the periphery and progresses inward.
The cell plate is formed by the fusion of Golgi-derived vesicles containing cell-wall material.	Cleavage begins with the contraction of a peripheral ring of actin microfilaments (the contractile ring) at the equator.
A rigid cell wall is present, so the cell cannot simply pinch inward.	The plasma membrane pinches inward until the cell is cleaved into two.
No midbody is formed.	A dense midbody often remains briefly at the site of division.

Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Equal-sized daughter cells: During spermatogenesis (formation of sperm) in human beings, meiosis of a primary spermatocyte produces four spermatids of equal size.

Unequal-sized daughter cells: During oogenesis (formation of the ovum) in human beings, meiosis of a primary oocyte is asymmetric — the cytoplasm is not divided equally at each division. This produces one large, functional ovum and three small, non-functional polar bodies that eventually degenerate.

Q10. Distinguish anaphase of mitosis from anaphase I of meiosis.

Anaphase of mitosis	Anaphase I of meiosis
The centromere of each chromosome splits, and the two sister chromatids separate and move to opposite poles as independent daughter chromosomes.	The centromere does not split; instead, the two homologous chromosomes of each bivalent separate from each other, with their sister chromatids still joined at the centromere.
Results in two genetically identical sets of chromosomes moving to each pole.	Results in the reduction of chromosome number by half (2N → N) at each pole; each chromosome moving to a pole still consists of two chromatids (a dyad).

Q11. List the main differences between mitosis and meiosis.

Mitosis	Meiosis
Occurs in somatic (body) cells.	Occurs in germ cells/reproductive cells during gametogenesis.
One division per cell cycle, producing two daughter cells.	Two successive divisions (meiosis I and meiosis II) per cell cycle, producing four daughter cells.
Chromosome number in daughter cells is the same as the parent cell (2N → 2N).	Chromosome number is halved (2N → N); daughter cells are haploid.
Daughter cells are genetically identical to the parent cell.	Daughter cells are genetically different from each other and from the parent cell due to crossing over and independent assortment.
Homologous chromosomes do not pair (no synapsis); no crossing over.	Homologous chromosomes pair (synapsis) and undergo crossing over during prophase I.
Necessary for growth, repair and asexual reproduction.	Necessary for the production of gametes in sexually reproducing organisms.

Q12. What is the significance of meiosis?

Meiosis is significant for several reasons:

• Gamete formation: It produces the haploid gametes required for sexual reproduction.
• Maintenance of chromosome number: By halving the chromosome number in gametes, meiosis ensures that fertilisation restores the diploid chromosome number of the species in the zygote, keeping the chromosome number constant across generations.
• Genetic variation: Crossing over during prophase I and the independent assortment of homologous chromosomes at metaphase I generate new combinations of parental traits, increasing genetic variability within a population.
• Raw material for evolution: The variations produced during meiosis provide the raw material on which natural selection acts, making meiosis important for evolution and for plant/animal breeding programmes.
• Source of mutation: Occasional errors during meiosis can produce chromosomal variations, some of which may prove advantageous to the organism.

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.

(i) In several lower plants (such as algae like Chlamydomonas and Spirogyra) and in certain insects (such as the drones of the honeybee, which develop from unfertilised eggs), the organism itself is haploid, and its cells still undergo mitotic division for growth. Since these cells are already haploid, the mitotic divisions that occur in them are ordinary mitosis, not meiosis — meiosis in such organisms occurs only at the time of gamete or spore formation (or, in some life cycles, immediately after zygote formation, a pattern called zygotic meiosis).

(ii) In higher plants, certain haploid cells never divide by mitosis after they are formed. For example, the microspores (pollen grains) and the synergid and antipodal cells of the embryo sac are haploid cells that, once formed, generally do not undergo further cell division; they are terminally differentiated and function without dividing again.

Q14. Can there be mitosis without DNA replication in the ‘S’ phase?

No. Mitosis cannot occur without prior DNA replication in the S phase. DNA replication ensures that each chromosome has two identical sister chromatids before the M phase begins. Since mitosis is an equational division that distributes one complete chromatid to each daughter cell, skipping S-phase replication would leave insufficient genetic material to give both daughter cells a full, identical set of chromosomes. Hence, DNA replication in S phase is an essential prerequisite for mitosis.

Q15. Can there be DNA replication without cell division?

Yes. DNA replication (S phase) and cell division (M phase) are separate, sequential events of the cell cycle, and replication does not always have to be followed immediately by division. In fact, cases exist in nature where DNA replicates repeatedly without any accompanying nuclear or cell division — a phenomenon called endoreplication. A well-known example is the formation of giant polytene chromosomes in the salivary gland cells of Drosophila larvae, where the DNA replicates many times over but the chromatids do not separate and the cell does not divide, producing a chromosome with hundreds of DNA strands lying side by side.

Q16. Analyse the events during every stage of the cell cycle and notice how the following two parameters change: (i) the number of chromosomes (N) per cell (ii) the amount of DNA content (C) per cell.

Taking a diploid cell that starts G1 with 2N chromosomes and 2C DNA content:

• G1 phase: Chromosome number = 2N, DNA content = 2C. Each chromosome consists of a single chromatid; no replication has occurred yet.
• S phase: DNA replicates, so DNA content doubles from 2C to 4C. The chromosome number, however, remains 2N, because each chromosome now simply consists of two identical sister chromatids joined at the centromere rather than becoming two separate chromosomes.
• G2 phase: Chromosome number stays at 2N, DNA content remains at 4C (no further replication; the cell prepares for division).
• M phase (mitosis) — anaphase: Sister chromatids separate and move to opposite poles as independent chromosomes. Chromosome number effectively becomes 4N momentarily (2N at each pole, i.e., 2N per future daughter cell), and DNA content per pole is 2C; after cytokinesis, each daughter cell returns to 2N chromosomes and 2C DNA content — identical to the starting G1 cell.
• Meiosis I — anaphase I: Homologous chromosomes (not sister chromatids) separate, so chromosome number is reduced from 2N to N at each pole, while DNA content is reduced from 4C to 2C (each chromosome still has two chromatids).
• Meiosis II — anaphase II: Sister chromatids finally separate, reducing DNA content from 2C to 1C, while chromosome number remains N. The final four haploid daughter cells each have N chromosomes and 1C DNA content.

Class 11 Biology Chapter 10 – Notes and Extra Questions

This chapter’s NCERT exercise has 16 questions, ranging from short one-line recall answers (average cell cycle span, why mitosis is equational) to longer explanatory and comparison-based questions (differences between mitosis and meiosis, anaphase of mitosis vs anaphase I of meiosis, and the chromosome/DNA-content analysis in Q16). While the total number of questions has stayed the same through the 2023 rationalisation, several NCERT-aligned resources reordered the questions in their updated 2023-24 editions — for instance, “Distinguish anaphase of mitosis from anaphase I of meiosis” and “List the main differences between mitosis and meiosis” now appear earlier in the sequence than in older (pre-2023) editions, and the expected answer for “Can there be DNA replication without cell division?” is generally presented in a more concise form today, without the detailed polyteny/polyploidy discussion some older solution sets included. For revision, focus on mastering the terminology — chromatid, spindle fibre, kinetochore, synapsis, bivalent/tetrad, chiasmata and crossing over — since board exams frequently test these as short-answer or diagram-based questions, alongside the mitosis-vs-meiosis and cytokinesis (plant vs animal) comparison tables.

Frequently Asked Questions

How many questions are in the NCERT Class 11 Biology Chapter 10 exercise?

The end-of-chapter exercise for Cell Cycle and Cell Division contains 16 questions, covering the cell cycle, mitosis, meiosis and their significance. This count has remained unchanged through the 2023 syllabus rationalisation, though the order of a few questions differs slightly between older and newer editions of solution guides.

What is the basic difference between mitosis and meiosis?

Mitosis is an equational division occurring in somatic cells that produces two genetically identical daughter cells with the same chromosome number as the parent. Meiosis is a reductional division occurring in germ cells that produces four genetically variable, haploid daughter cells with half the parent’s chromosome number, and it involves synapsis and crossing over, which mitosis does not.

Why is prophase I of meiosis divided into five sub-stages?

Prophase I is much longer and more complex than mitotic prophase because it is when homologous chromosomes pair (synapsis), exchange genetic material (crossing over) and begin to separate. To describe this sequence of distinct events clearly, it is divided into five sub-stages — leptotene, zygotene, pachytene, diplotene and diakinesis — each marked by a specific, identifiable change in chromosome behaviour.

What is the significance of the G0 phase in the cell cycle?

The G0 phase allows a cell to exit the active cell cycle and remain metabolically functional without dividing. This is essential for tissues made of long-lived, non-dividing cells such as neurons and cardiac muscle cells, and it also lets other cells “pause” in a dormant state until the body specifically signals a need for more cells, such as during wound repair.