How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Cell Cycle covers interphase, mitosis, meiosis, and their significance — focus on stage-wise events and comparing the two types of division.
Key Concepts (Quick Recap)
Cell Cycle Overview
The cell cycle is the sequence of events from one cell division to the next. It consists of:
Interphase (longest phase, ~95% of cycle):
- G1 Phase (Gap 1): Cell growth, normal metabolism, preparation for DNA replication
- S Phase (Synthesis): DNA replication; each chromosome duplicates into two sister chromatids
- G2 Phase (Gap 2): Protein synthesis, preparation for mitosis, organelle duplication
M Phase (Mitotic Phase) (~5% of cycle):
- Mitosis (nuclear division) + Cytokinesis (cytoplasmic division)
G0 Phase (Quiescent stage): Cells that exit the cell cycle and stop dividing.
Interphase — Detailed Breakdown
| Phase | Duration | Key Events |
|---|---|---|
| G1 Phase | 8–10 hours | Cell growth; RNA and protein synthesis; histone synthesis; cell size increases |
| S Phase | 6–8 hours | DNA replication; each chromosome duplicates to form sister chromatids; centriole duplication begins |
| G2 Phase | 4–6 hours | Protein synthesis for mitosis; organelle duplication; ATP accumulation; preparation for M phase |
Mitosis — Stages and Events
| Stage | Key Events |
|---|---|
| Prophase | Chromatin condenses into chromosomes; centrioles move to poles; spindle fibres form; nuclear membrane and nucleolus disappear |
| Metaphase | Chromosomes align at the equatorial plate (metaphase plate); centromeres attach to spindle fibres; best stage for chromosome counting |
| Anaphase | Centromeres split; sister chromatids separate and move to opposite poles; chromatids become independent chromosomes |
| Telophase | Chromosomes decondense at poles; nuclear membrane reappears; nucleolus reappears; spindle fibres disappear |
Cytokinesis: Division of cytoplasm — in plant cells, a cell plate forms from the centre outward; in animal cells, a cleavage furrow forms from the outside inward.
Meiosis — Overview
Meiosis involves two successive divisions without DNA replication between them:
Meiosis I (Reductional division) — reduces chromosome number from 2n to n:
| Sub-stage | Key Events |
|---|---|
| Prophase I | Leptotene (chromosomes become visible) → Zygotene (synapsis/forming bivalents) → Pachytene (crossing over) → Diplotene (chiasmata visible) → Diakinesis (terminalisation of chiasmata) |
| Metaphase I | Bivalents align at equatorial plate |
| Anaphase I | Homologous chromosomes separate; chromatids do NOT split |
| Telophase I | Two haploid nuclei form |
Meiosis II (Equational division) — similar to mitosis but with haploid chromosome number:
| Stage | Key Events |
|---|---|
| Prophase II | Chromosomes condense again |
| Metaphase II | Chromosomes align at equatorial plate |
| Anaphase II | Centromeres split; sister chromatids separate |
| Telophase II | Four haploid daughter cells formed |
Mitosis vs Meiosis — Comparison
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | One | Two (Meiosis I and II) |
| Daughter cells | Two | Four |
| Chromosome number | 2n (diploid) | n (haploid) |
| Genetic variation | No variation (identical cells) | Variation (crossing over, independent assortment) |
| Occurs in | Somatic cells | Germ cells (gamete formation) |
| Prophase | Simple | Complex (with synapsis and crossing over) |
| Significance | Growth, repair, asexual reproduction | Gamete formation, maintains chromosome number |
| Duration | Short | Longer |
I. Choose the Best Answer (MCQs)
Q1. The longest phase of the cell cycle is (a) M phase (b) G1 phase (c) S phase (d) Interphase
Q2. DNA replication occurs during (a) G1 phase (b) S phase (c) G2 phase (d) M phase
Q3. Crossing over occurs during which stage of meiosis? (a) Leptotene (b) Zygotene (c) Pachytene (d) Diakinesis
Q4. The best stage to count the number of chromosomes is (a) Prophase (b) Metaphase (c) Anaphase (d) Telophase
Q5. Mitosis results in (a) Two haploid cells (b) Four haploid cells (c) Two diploid cells (d) Four diploid cells
Q6. The reduction in chromosome number occurs during (a) Mitosis (b) Meiosis I (c) Meiosis II (d) Interphase
Q7. In plant cells, cytokinesis occurs by (a) Cleavage furrow (b) Cell plate formation (c) Budding (d) Fragmentation
II. Fill in the Blanks
1. The sequence of events from one cell division to the next is called the cell cycle. 2. The longest phase of the cell cycle is interphase. 3. DNA replication occurs during the S phase of interphase. 4. The phase in which cells exit the cell cycle and stop dividing is called G0 phase (quiescent stage). 5. During prophase, the nuclear membrane and nucleolus disappear. 6. Chromosomes align at the equatorial plate during metaphase. 7. Sister chromatids separate during anaphase of mitosis. 8. The nuclear membrane reappears during telophase. 9. In plant cells, cytokinesis occurs by cell plate formation. 10. In animal cells, cytokinesis occurs by cleavage furrow. 11. Crossing over occurs during pachytene stage of prophase-I. 12. Synapsis (pairing of homologous chromosomes) occurs during zygotene. 13. Meiosis produces four haploid daughter cells. 14. Meiosis I is called reductional division because it reduces the chromosome number. 15. Meiosis II is called equational division because it separates sister chromatids like mitosis.
III. True or False (with corrections for false statements)
IV. Match the Following
| Column A | Column B | Answer |
|---|---|---|
| 1. Prophase | (a) Chromatids move to poles | (c) Chromosomes condense, nuclear membrane disappears |
| 2. Metaphase | (b) Nuclear membrane reappears | (d) Chromosomes align at equatorial plate |
| 3. Anaphase | (c) Chromosomes condense, nuclear membrane disappears | (a) Chromatids move to poles |
| 4. Telophase | (d) Chromosomes align at equatorial plate | (b) Nuclear membrane reappears |
| 5. S phase | (e) Crossing over | (f) DNA replication |
| 6. Pachytene | (f) DNA replication | (e) Crossing over |
V. Answer Briefly (2-mark questions)
VI. Answer in Detail (5-mark questions)
Common Mistakes to Avoid
- Saying mitosis reduces chromosome number: Mitosis produces two DIPLOID (2n) daughter cells identical to the parent. Only MEIOSIS reduces the chromosome number from 2n to n.
- Confusing chromatids and chromosomes: Before S phase, each chromosome has one chromatid. After S phase, each chromosome has two sister chromatids. After anaphase, each chromatid becomes an independent chromosome.
- Confusing anaphase of mitosis with anaphase-I of meiosis: In mitosis anaphase, sister chromatids separate. In meiosis anaphase-I, homologous chromosomes separate (not sister chromatids).
- Confusing synapsis and crossing over: Synapsis = pairing of homologous chromosomes (zygotene). Crossing over = exchange of genetic material (pachytene). They are different events.
- Forgetting that Meiosis II is equational: Meiosis I is reductional (2n → n). Meiosis II is equational (n → n), similar to mitosis but starting with haploid cells.
- Confusing cytokinesis in plant and animal cells: Plant cells — cell plate from centre outward. Animal cells — cleavage furrow from outside inward.
- Saying DNA replicates before Meiosis II: DNA replication occurs only BEFORE Meiosis I. There is NO DNA replication between Meiosis I and Meiosis II.
Exam Checklist
- [ ] Can you name and describe all phases of the cell cycle (G1, S, G2, M, G0)?
- [ ] Can you describe the four stages of mitosis with key events?
- [ ] Can you describe the five sub-stages of prophase-I of meiosis?
- [ ] Can you compare mitosis and meiosis (at least 8 points)?
- [ ] Can you explain crossing over and its significance?
- [ ] Can you explain what happens during the S phase?
- [ ] Can you differentiate between anaphase of mitosis and anaphase-I of meiosis?
- [ ] Can you explain cytokinesis in plant vs animal cells?
- [ ] Can you state the significance of mitosis and meiosis?
- [ ] Can you explain synapsis, bivalent, and chiasmata?