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These are complete NCERT Solutions for CBSE Class 11 Biology Chapter 10 — Cell Cycle and Cell Division. Attempt each question before checking. Use precise terminology — marks are given for terms like 'synaptonemal complex', 'chiasmata', 'bivalent', 'crossing over', etc.
Key Concepts (Quick Recap)
- Cell cycle: Interphase (G₁ + S + G₂) + M phase (Mitosis + Cytokinesis).
- G₁ phase: Cell growth, protein synthesis, preparation for S phase.
- S phase: DNA replication — each chromosome becomes two sister chromatids; DNA doubles from 2C to 4C.
- G₂ phase: Protein synthesis for mitosis; preparation for cell division.
- M phase: Mitosis (PMAT — Prophase, Metaphase, Anaphase, Telophase) + Cytokinesis.
- Mitosis: One division, produces 2 identical diploid cells; somatic cell division; growth, repair, asexual reproduction.
- Meiosis: Two divisions, produces 4 non-identical haploid cells; germ cell division; gamete formation.
- Meiosis I: Reductional division — homologous chromosomes separate; crossing over, synapsis, bivalent formation.
- Meiosis II: Equational division — sister chromatids separate (similar to mitosis).
- Significance of meiosis: Maintains chromosome number across generations; creates genetic variation through crossing over and independent assortment.
Intext Questions
Page 157
Q1. What is the average cell cycle span for a mammalian cell?
Answer
The average cell cycle span for a mammalian cell is approximately 24 hours. The typical distribution is: G₁ phase (~10-12 hours), S phase (~6-8 hours), G₂ phase (~4-5 hours), and M phase (~1 hour).
Q2. Distinguish cytokinesis from karyokinesis.
Answer
| Feature | Karyokinesis | Cytokinesis |
|---|
| Definition | Division of the nucleus | Division of the cytoplasm |
| Process | Chromosomes separate into two daughter nuclei | Cytoplasm divides into two daughter cells |
| Sequence | Occurs first | Occurs after karyokinesis |
| In animal cells | By mitotic/meiotic spindle | By cleavage furrow (contractile ring of microfilaments) |
| In plant cells | By mitotic/meiotic spindle | By cell plate formation (Golgi-derived vesicles) |
Q3. Describe the events taking place in interphase.
Answer
Interphase is the longest phase of the cell cycle and consists of three sub-phases:
1. G₁ phase (Gap 1): Cell grows in size, synthesises proteins and RNA, and accumulates building blocks for DNA replication. The cell checks internal and external conditions at the G₁ checkpoint before proceeding.
2. S phase (Synthesis phase): DNA replication occurs. Each chromosome duplicates to form two sister chromatids joined at the centromere. The DNA content doubles (2C → 4C). Histone proteins are also synthesised.
3. G₂ phase (Gap 2): The cell continues to grow, synthesises proteins needed for mitosis (like tubulin for spindle fibres), and prepares for M phase. A G₂ checkpoint ensures DNA is fully replicated and undamaged before division.
Q4. What is G₀ (quiescent) phase of the cell cycle?
Answer
The G₀ phase is a resting or quiescent state that some cells enter from the G₁ phase instead of continuing through the cell cycle. Cells in G₀:
- Are not actively dividing but are metabolically active.
- May remain in G₀ for days, weeks, or even years.
- May re-enter the cell cycle if stimulated (e.g., liver cells re-enter upon injury).
- Some cells permanently stay in G₀ (e.g., mature neurons and cardiac muscle cells that never divide again).
Chapter-End Exercise
Q1. What is the average cell cycle span for a mammalian cell?
Answer
(See Intext Page 157 Q1 — answered above.)
Q2. Distinguish cytokinesis from karyokinesis.
Answer
(See Intext Page 157 Q2 — answered above.)
Q3. Describe the events taking place in interphase.
Answer
(See Intext Page 157 Q3 — answered above.)
Q4. What is G₀ (quiescent) phase of the cell cycle?
Answer
(See Intext Page 157 Q4 — answered above.)
Q5. Why is mitosis called equational division?
Answer
Mitosis is called equational division because the number of chromosomes in the daughter cells is equal to that of the parent cell. A diploid (2n) parent cell produces two diploid (2n) daughter cells — the chromosome number is maintained. The genetic material is equally distributed: each daughter cell receives an identical copy of every chromosome.
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
Answer
(i) Metaphase — Chromosomes align at the metaphase plate (spindle equator).
(ii) Anaphase — Centromere splits, sister chromatids separate and move to opposite poles.
(iii) Zygotene stage of Prophase I (Meiosis I) — Homologous chromosomes pair (synapsis) to form bivalents.
(iv) Pachytene stage of Prophase I (Meiosis I) — Crossing over occurs between non-sister chromatids of homologous chromosomes at chiasmata.
Q7. Describe the following: (a) Synapsis (b) Bivalent (c) Chiasmata
Answer
(a) Synapsis: The process of pairing of homologous chromosomes during the Zygotene stage of Prophase I in meiosis. Each pair of homologous chromosomes is held together by a protein structure called the synaptonemal complex. Synapsis is essential for crossing over and proper chromosome segregation.
(b) Bivalent (Tetrad): A pair of homologous chromosomes that have undergone synapsis. Since each chromosome consists of two sister chromatids, a bivalent has 4 chromatids (2 from each homologue) — hence it is also called a tetrad. Each diploid cell has n bivalents during Prophase I.
(c) Chiasmata: The points of contact between non-sister chromatids of homologous chromosomes where crossing over has occurred. Chiasmata become visible during the Diplotene stage of Prophase I, after the synaptonemal complex dissolves. They are the visible manifestations of crossing over and hold homologous chromosomes together until Anaphase I.
Q8. How does cytokinesis in plant cells differ from that in animal cells?
Answer
| Feature | Animal Cells | Plant Cells |
|---|
| Method | Cleavage furrow | Cell plate formation |
| Mechanism | Contractile ring of microfilaments (actin + myosin) constricts from periphery to centre | Golgi-derived vesicles fuse at the centre and grow outward to the walls |
| Direction | Outside → Inside (centripetal) | Centre → Outside (centrifugal) |
| Formation | No cell plate | Cell plate forms at equator |
| Result | Two daughter cells separate | New cell wall and plasma membrane form on both sides of the cell plate |
| Pre-existing structure | No wall to divide | Cell wall must be formed between daughter cells |
Q9. Find out the correct option out of the four options given below: The number of chromosome sets at the end of meiosis-I is: (a) haploid (b) diploid (c) haploid (d) tetraploid
Wait — let me reconsider. At the end of Meiosis I, the chromosome number is reduced from diploid (2n) to haploid (n). However, each chromosome still consists of two sister chromatids (they separate only in Meiosis II). So the number of chromosome sets is haploid, but the DNA content is still doubled.
Answer(c) haploid — at the end of Meiosis I, each daughter cell has n chromosomes (haploid set), though each chromosome has two chromatids.
Q10. Find out the correct option: The number of chromatids at the end of Meiosis-I is: (a) same as that at the beginning (b) half of that at the beginning (c) same as that at the beginning (d) double of that at the beginning
Wait — at the beginning of Meiosis I, each chromosome has 2 chromatids. At the end of Meiosis I, each daughter cell has n chromosomes, each still with 2 chromatids. So the number of chromatids per cell is halved (from 4n to 2n), but per chromosome it's the same.
Let me re-evaluate. If we consider a diploid cell with 2n chromosomes: before Meiosis I, after S phase, there are 2n chromosomes with 4n chromatids. After Meiosis I, each cell has n chromosomes with 2n chromatids. The total chromatids per cell is halved.
Answer(b) half of that at the beginning — each daughter cell has half the number of chromatids compared to the parent cell at the start of Meiosis I.
Q11. Differentiate between: (a) Metaphase-I and Metaphase (b) Anaphase-I and Anaphase (c) Telophase-I and Telophase
Answer
(a) Metaphase I (Meiosis) vs Metaphase (Mitosis):
| Feature | Metaphase I | Metaphase (Mitosis) |
|---|
| Chromosomes at equator | Homologous chromosome pairs (bivalents) align | Individual chromosomes align |
| Pairing | Homologous chromosomes are paired (bivalents) | No pairing — chromosomes are separate |
| Chromatids separation | No — sister chromatids remain together | Sister chromatids separate in Anaphase |
| Number of chromosomes per cell | 2n (but arranged as n bivalents) | 2n |
(b) Anaphase I (Meiosis) vs Anaphase (Mitosis):
| Feature | Anaphase I | Anaphase (Mitosis) |
|---|
| What separates | Homologous chromosomes separate | Sister chromatids separate |
| Centromere split | Does NOT split | Splits |
| Movement | Whole chromosomes (with 2 chromatids) move to poles | Individual chromatids move to poles |
| Result | Reduces chromosome number (2n → n) | Maintains chromosome number (2n → 2n) |
(c) Telophase I (Meiosis) vs Telophase (Mitosis):
| Feature | Telophase I | Telophase (Mitosis) |
|---|
| Chromosome number | n (haploid) | 2n (diploid) |
| Chromatids per chromosome | 2 (each chromosome has sister chromatids) | 1 (chromatids have separated) |
| Followed by | Meiosis II (no S phase) | Cell may enter G₁ |
| Outcome | 2 haploid cells (each with duplicated chromosomes) | 2 diploid cells (each with single chromatids) |
Q12. What is the significance of meiosis?
Answer
The significance of meiosis:
1. Maintains chromosome number: Reduces diploid (2n) to haploid (n), so that when two gametes fuse during fertilisation, the diploid number is restored. Without meiosis, chromosome number would double every generation.
2. Creates genetic variation: Through two mechanisms:
- Crossing over (Prophase I): Exchange of genetic material between non-sister chromatids of homologous chromosomes, creating new allele combinations.
- Independent assortment (Metaphase I): Random orientation of homologous chromosome pairs, producing 2ⁿ possible combinations.
3. Essential for sexual reproduction: Produces gametes (sperm and egg) with half the chromosome number.
4. Evolutionary significance: Genetic variation produced by meiosis provides the raw material for natural selection, enabling species to adapt to changing environments.
Internal Links
- [Chapter 10 Guide — Cell Cycle and Cell Division](/school/cbse/class/11/biology/cbse-cell-cycle-and-cell-division)
- [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
- [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)
