આ MCQ મોડ્યુલ આના પર આધારિત છે: NCERT Exercises and Solutions: Cell Cycle and Cell Division
NCERT Exercises and Solutions: Cell Cycle and Cell Division
આ મૂલ્યાંકન આના પર આધારિત હશે: NCERT Exercises and Solutions: Cell Cycle and Cell Division
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
NCERT Exercises and Solutions: Cell Cycle and Cell Division
Chapter 10 — Summary
The cell cycle and cell division provide the molecular basis for growth, repair, and reproduction. The essentials:
- Cell cycle = Interphase (G₁ + S + G₂) + M phase. In a 24-h cycle, interphase ~23 h, M phase ~1 h.
- G₀ = quiescent (non-dividing) state. Neurons, mature liver cells live here.
- Checkpoints (G₁/S, G₂/M, spindle) ensure quality control. Failure → cancer.
- Mitosis = equational division in somatic cells. Phases: Prophase, Metaphase, Anaphase, Telophase (PMAT) + cytokinesis. Produces 2 genetically identical daughters with same chromosome number (2n).
- Cytokinesis = cleavage furrow (animals) or cell plate (plants).
- Meiosis = special reductional division in germ cells. Two divisions: Meiosis I (reductional) + Meiosis II (equational on haploid cells). Produces 4 genetically distinct haploid gametes.
- Prophase I has five sub-stages: Leptotene → Zygotene (synapsis) → Pachytene (crossing over) → Diplotene (chiasmata visible) → Diakinesis.
- Genetic variation generated by: (1) Crossing over, (2) Random assortment at metaphase I, (3) Random fertilization. Essential for evolution.
Key Terms — Click Each
- Interphase: Growth + DNA synthesis phase.
- Centromere: Where sister chromatids join.
- Kinetochore: Microtubule-attachment site.
- Spindle: Chromosome-segregating machinery.
- Bivalent: Paired homologous chromosomes (tetrad).
- Chiasma: Visible crossover point.
- Crossing Over: Source of recombinant chromatids.
- Reductional Division: Halving (meiosis I).
- Equational Division: Number-preserving (mitosis, meiosis II).
Chapter 10 — NCERT Exercises (Worked Solutions)
Q1. What is the average cell cycle span for a mammalian cell?
- Interphase ≈ 23 hours (G₁ ~ 9 h, S ~ 6 h, G₂ ~ 4–5 h; varies)
- M phase ≈ 1 hour (mitosis + cytokinesis)
- Yeast — ~90 minutes
- Drosophila embryo cleavage — ~8 minutes
- Mammalian gut lining — ~24 hours
- Mammalian liver in vivo — ~1 year
- Most adult human cells are in G₀ (don't divide).
Q2. Distinguish cytokinesis from karyokinesis.
| Feature | Karyokinesis | Cytokinesis |
|---|---|---|
| What divides | Nucleus | Cytoplasm |
| Phases involved | Prophase, Metaphase, Anaphase, Telophase | Starts in late anaphase/telophase, completes after |
| Outcome | Two daughter nuclei in ONE cell | Two separate daughter cells |
| Mechanism | Chromosome condensation, spindle, kinetochore attachment, separation | Cleavage furrow (animals) or cell plate (plants) |
| Exceptions | Rare without cytokinesis | May fail → multinucleate cells (e.g., muscle, fungi) |
Q3. Describe the events taking place during interphase.
- G₁ phase (Gap 1):
- Cell grows and increases in size
- Active synthesis of RNAs and proteins
- Cellular organelles (mitochondria, Golgi, ER) increase in number
- Cell prepares to enter S phase
- The G₁/S checkpoint decides whether to commit to division
- S phase (Synthesis):
- DNA is replicated — each chromosome is duplicated into two sister chromatids
- DNA content doubles (2C → 4C); chromosome number unchanged (2n)
- In animal cells, centrioles also duplicate in S phase
- Histones (proteins of nucleosomes) are also synthesised to package the new DNA
- G₂ phase (Gap 2):
- Cell continues to grow
- Synthesis of proteins needed for mitosis (tubulin, condensins, cohesins, motor proteins)
- G₂/M checkpoint verifies DNA integrity before allowing mitosis
Q4. What is G₀ (quiescent phase) of cell cycle?
- Are metabolically ACTIVE — they live, perform their functions, respond to environment
- Do NOT divide — they don't replicate DNA or undergo mitosis
- May stay in G₀ temporarily or permanently
- Temporary G₀: Cell can re-enter G₁ when needed. e.g., liver cells (return to division after liver injury); lymphocytes (proliferate when an infection occurs).
- Permanent G₀ (terminal differentiation): Cell will never divide again. e.g., neurons, cardiac muscle cells, mature skeletal muscle cells.
- Senescent G₀: Cell has lost the ability to divide due to telomere shortening or stress.
Q5. Why is mitosis called equational division?
- Parent cell: 2n chromosomes
- Each daughter cell after mitosis: 2n chromosomes (identical number)
- DNA replication in S phase: Each chromosome becomes 2 sister chromatids. (Chromosome number remains 2n; DNA content doubles to 4C.)
- Anaphase: Centromeres split. The two sister chromatids separate and move to opposite poles. Each pole receives ONE chromatid from each chromosome — which then counts as one chromosome again.
- Telophase + Cytokinesis: Each daughter cell ends up with 2n chromosomes — same as parent.
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.
- Chromosomes moved to spindle equator: Metaphase (of mitosis); or Metaphase I (of meiosis, where bivalents align); Metaphase II (in meiosis II, individual chromosomes align).
- Centromere splits and chromatids separate: Anaphase (of mitosis); also Anaphase II of meiosis. Note: in Anaphase I, homologues separate but centromeres do NOT split (sister chromatids remain together).
- Pairing between homologous chromosomes: Zygotene stage of Prophase I of Meiosis. This pairing is called synapsis; it is mediated by the synaptonemal complex.
- Crossing over between homologous chromosomes: Pachytene stage of Prophase I of Meiosis. Non-sister chromatids exchange segments at this stage.
Q7. Describe the following: (a) Synapsis (b) Bivalent (c) Chiasmata. Draw a diagram to illustrate your answer.
(b) Bivalent (also called Tetrad): The structure formed by the pairing of two homologous chromosomes during synapsis. Since each homologous chromosome already has 2 sister chromatids, a bivalent has a total of 4 chromatids = 2 maternal + 2 paternal chromatids.
(c) Chiasmata (singular: chiasma): The visible X-shaped point or cross-over where two non-sister chromatids of homologous chromosomes have exchanged segments. They become visible during diplotene stage of Prophase I, after the synaptonemal complex breaks down. Chiasmata physically hold homologues together until they separate in anaphase I.
Q8. How does cytokinesis in plant cells differ from that in animal cells?
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow | Cell plate |
| Direction of formation | Outside → Inside (centripetal) | Inside → Outside (centrifugal) |
| Force | Contraction of actin-myosin ring | Fusion of Golgi-derived vesicles |
| End result | Plasma membranes pinch together; two cells separate | New cell wall + plasma membrane forms in middle |
| Reason for difference | Flexible plasma membrane allows pinching | Rigid cell wall prevents pinching; new wall must be built between daughter nuclei |
- Vesicles from Golgi (containing cell-wall material like polysaccharides) align at the equator.
- Vesicles fuse together → form a small disc called phragmoplast.
- The phragmoplast extends outward to fuse with the existing cell wall.
- The new cell plate becomes the middle lamella; primary cell walls form on each side.
Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
- Spermatogenesis in animals — produces 4 equal-sized, motile sperm cells. All four are functional gametes.
- Pollen grain formation in plants — microsporogenesis produces 4 equal-sized microspores (each becomes a pollen grain).
- Many fungi — meiosis produces 4 equal-sized spores (e.g., in Neurospora — though they later divide once more to give 8 ascospores).
- Oogenesis in animals — produces 1 large egg + 3 tiny polar bodies. The egg keeps almost all the cytoplasm; the polar bodies are essentially genetic "waste" that degenerate.
- Reason: The egg needs cytoplasm/yolk/organelles for embryo development.
- Polar body 1 may or may not divide; polar body 2 from MII has minimal cytoplasm.
- Megasporogenesis in plants (flowering plant ovule) — meiosis produces 4 megaspores, of which 3 typically degenerate and only 1 survives to form the embryo sac.
Q10. Distinguish anaphase of mitosis from anaphase I of meiosis.
| Feature | Anaphase of Mitosis | Anaphase I of Meiosis |
|---|---|---|
| What separates | Sister chromatids (centromeres split) | Homologous chromosomes (sister chromatids stay together) |
| Centromere status | Splits at anaphase | Does NOT split; chromatids remain joined |
| Each daughter receives | One sister chromatid from each chromosome | One entire homologue (with 2 sister chromatids) from each pair |
| Chromosome number change | From 2n to 2n (same) | From 2n to n (halved) |
| Genetic constitution | Both daughter cells genetically identical | Two cells differ — different mix of maternal/paternal homologues |
| Crossing over effect | Not relevant — no prior crossover | Recombinant chromatids carry mixed alleles |
Q11. List the main differences between mitosis and meiosis.
| Feature | Mitosis | Meiosis |
|---|---|---|
| 1. Site | Somatic cells (body) | Germ cells (in gonads) |
| 2. Number of divisions | One | Two (MI + MII) |
| 3. DNA replication | Once before each division | Once before two divisions |
| 4. Pairing of homologues | Absent | Present (zygotene of MI) |
| 5. Crossing over | Absent | Present (pachytene of MI) |
| 6. Chiasmata | Absent | Present |
| 7. Number of daughter cells | 2 | 4 |
| 8. Chromosome number in daughters | Same as parent (2n) | Half of parent (n) |
| 9. Genetic identity of daughters | Identical to parent (clones) | Genetically different from parent and each other |
| 10. Duration of Prophase | Short (~ minutes) | Very long (hours to years; 5 sub-stages) |
| 11. Purpose | Growth, repair, asexual reproduction | Gamete formation for sexual reproduction |
| 12. Type | Equational | MI reductional, MII equational |
Q12. What is the significance of meiosis?
- Maintenance of constant chromosome number:
- Meiosis halves the chromosome number in gametes (2n → n).
- Fertilization restores the diploid number (n + n = 2n).
- Without meiosis, chromosome number would double every generation — biologically impossible.
- Genetic variation:
- Crossing over: Recombination of maternal and paternal alleles in pachytene.
- Random assortment: Each bivalent independently orients at metaphase I (2²³ = 8.4 million possibilities for humans).
- Random fertilization: Any sperm × any egg combination.
- Combined: each child of two parents is genetically unique among ~70 trillion possibilities.
- Basis of evolution:
- The variation generated by meiosis is the raw material for natural selection.
- Different individuals respond differently to environmental challenges.
- Favourable variants survive and reproduce → species adapts over generations.
- Sexual reproduction (and meiosis) is favoured by evolution because it accelerates adaptation, especially against fast-evolving threats like parasites and pathogens.
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.
- Male honey bees (drones): Develop from unfertilized eggs and are haploid throughout life. They grow by mitosis of haploid cells.
- Many fungi: Spend much of life in the haploid state. Bread mould, yeast (haploid phase) grow by mitosis.
- Mosses (bryophyte gametophyte): The dominant generation is haploid; grows by mitosis.
- Algae like Chlamydomonas, Spirogyra: Haploid filaments grow by mitosis.
- Fern prothallus: The gametophyte stage is haploid; expands by mitotic division.
(ii) Haploid cells in higher plants where NO further cell division occurs:
- Male gametes (sperm cells in pollen): After being released into the embryo sac, they fuse with the egg → fertilization. They do not divide further.
- Female gametes (egg cells in embryo sac): Once formed, the egg cell does not divide until fertilization triggers embryogenesis.
- Synergids and antipodal cells in the embryo sac: These haploid cells degenerate after fertilization without further division.
Q14. Can there be mitosis without DNA replication in 'S' phase?
What happens if S phase is skipped?
- Chromosomes have only 1 chromatid (not 2).
- At metaphase, each chromosome would attach to only one spindle pole.
- At anaphase, the chromosome cannot split — it would simply move to one pole, while the other pole gets nothing.
- Result: One daughter cell gets the chromosome; other gets no chromosome → both abnormal, likely die.
Special exception in some organisms — endoreduplication / endomitosis:
- Some cells undergo multiple rounds of S phase WITHOUT mitosis → polyploid cells (e.g., liver cells, plant endosperm).
- This is the REVERSE — DNA replication without mitosis. Doesn't violate the rule that mitosis needs replicated DNA.
Q15. Can there be DNA replication without cell division?
DNA is replicated but the cell doesn't divide → ploidy increases. Examples:
- Liver cells (hepatocytes): Many human liver cells are 4n, 8n, or even 16n after repeated rounds of replication without division. This increases cellular protein synthesis capacity — useful for detoxification.
- Plant endosperm: In many flowering plants, endosperm cells undergo endoreduplication, becoming highly polyploid → boosts nutrient storage for the seed.
- Salivary gland cells of Drosophila: Famous "polytene chromosomes" form by ~1000 rounds of replication without separation — chromosomes appear as giant banded structures, used by geneticists to map genes.
- Megakaryocytes (bone marrow cells that produce platelets): Become highly polyploid before fragmenting into platelets.
- Plant pith cells, trichomes: Often polyploid via endoreduplication.
Biological logic: Endoreduplication is a way to increase cellular biosynthetic capacity (more gene copies = more transcription) without the energy cost of cell division.
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.
| Stage | Chromosome Number (N) | DNA Content (C) |
|---|---|---|
| G₁ phase | 2n | 2C |
| S phase (end) | 2n | 4C (doubled) |
| G₂ phase | 2n | 4C |
| Prophase (mitotic) | 2n | 4C |
| Metaphase | 2n | 4C |
| Anaphase (just after split) | 4n (sister chromatids now count as chromosomes; only momentarily) | 4C |
| Telophase (each new nucleus) | 2n | 2C per cell |
| After cytokinesis (each daughter cell) | 2n | 2C |
| Stage | N | C |
|---|---|---|
| G₁ (germ cell) | 2n | 2C |
| S phase end | 2n | 4C |
| Prophase I, Metaphase I | 2n | 4C |
| After Meiosis I (each cell) | n | 2C |
| Metaphase II | n | 2C |
| After Meiosis II (each gamete) | n | C |
🎯 Mixed Competency-Based Questions
Q1. The stage in which DNA is synthesised is: L1 Remember
Q2. In which substage of meiosis are chromosomes most condensed? L2 Understand
Q3. Apply: If a cell with 16 chromosomes (2n=16) undergoes (a) one mitosis vs (b) one meiosis, how many cells of what chromosome number result? L3 Apply
(b) Meiosis: 1 cell (2n=16) → 4 daughter cells (gametes), each with n = 8 chromosomes. All four are genetically distinct from each other and from parent (due to crossing over + random assortment).
Numerical summary:
- Mitosis output: 2 cells × 16 chromosomes = 32 chromosomes total (from 32 chromatids after S phase)
- Meiosis output: 4 cells × 8 chromosomes = 32 chromosomes total (same total, distributed differently)
Q4. Evaluate: "Cancer is a disease of failed cell-cycle regulation." Justify. L5 Evaluate
- Normal cell-cycle control: Cells divide only when growth signals are received; checkpoints stop division if DNA is damaged; senescence/apoptosis kicks in if too many problems accumulate.
- Cancer cells subvert these controls:
- Loss of p53 (tumour suppressor / "guardian of the genome") — present in over 50% of cancers. Without p53, damaged cells continue dividing.
- Loss of Rb (retinoblastoma protein) — releases the G₁/S checkpoint; cell divides without growth signals.
- Activated oncogenes (Ras, Myc) push cells into S phase without need.
- Telomerase reactivation — cancer cells regain ability to extend telomeres → immortal.
- Spindle checkpoint failure → chromosome instability, aneuploidy.
- Therapeutic implication: Many anti-cancer drugs target cell cycle — methotrexate, hydroxyurea (S phase), vincristine, paclitaxel (M phase). Modern targeted therapies (e.g., CDK4/6 inhibitors like palbociclib) directly inhibit cell-cycle progression.
Q5. HOT (Create): Design an experiment to determine if a chemical drug specifically blocks mitosis (not interphase). L6 Create
- Materials: Cultured dividing cells (e.g., HeLa cells); drug at various doses; control medium; flow cytometer or fluorescent DNA stain (e.g., DAPI); fluorescent BrdU (for S-phase labelling).
- Hypothesis: If the drug specifically blocks mitosis, cells will accumulate in M phase (4C DNA content) and have visible mitotic figures.
- Procedure:
- Plate 4 cultures: (A) control no drug, (B) low-dose drug, (C) high-dose drug, (D) positive control (colchicine — known M-phase blocker).
- Treat for 24 hours.
- Half of each culture → harvest for flow cytometry (DNA content analysis with DAPI).
- Other half → fix on slides, stain, count mitotic figures (chromosomes visible) under microscope.
- Add BrdU for the last 1 hour of treatment → label cells in S phase. If drug doesn't block S, BrdU will still be incorporated.
- Expected results if drug blocks mitosis:
- Flow cytometry: large peak at 4C DNA content (G₂/M), small at 2C (G₁).
- Mitotic index (% cells with visible chromosomes) — sharply elevated (similar to colchicine control).
- BrdU positive cells — present (S phase not blocked).
- Expected if drug blocks S phase instead:
- Cells accumulate at 2C–4C intermediate; mitotic index normal or low; BrdU positive (cells stuck DURING S).
- Controls:
- Untreated control — normal distribution.
- Vehicle (DMSO without drug) — confirm drug effect not from solvent.
- Dose-response — confirm specificity.
- Reversibility — wash out drug, see if cells resume cycling.
🧠 Mixed Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Mitosis produces two genetically identical daughter cells.
R: DNA replication is semi-conservative, and sister chromatids are precisely separated during anaphase.
A: Cytokinesis in plant cells occurs by cell-plate formation.
R: Plant cells have rigid walls that prevent constriction; a new wall must be built between daughter nuclei.
A: Chiasmata are visible in diplotene of Prophase I.
R: Sister chromatids separate during diplotene.