Cell Cycle and Cell Division - Competency-Based Question Bank (with Answers)

Section A: Case-Based Questions

Case Study 1: A researcher is observing a tissue sample under a microscope and notices cells in different stages of division. She observes a cell where the nuclear envelope has completely disappeared, and chromosomes are aligned at the equatorial plate. Another cell nearby shows sister chromatids being pulled apart towards opposite poles.

1. Identify the two stages of mitosis described in the observation.
   • Answer: Metaphase (alignment at equator) and Anaphase (sister chromatids pulling apart).
2. If the researcher adds Colchicine to the sample, which of the two observed stages will be affected and why?
   • Answer: Metaphase. Colchicine inhibits spindle fiber formation, preventing the chromosomes from being pulled apart in the subsequent stage (Anaphase).
3. Designing an Experiment: Propose a method to determine the duration of the stage where chromosomes align at the equator using time-lapse microscopy.
   • Answer: Capture images at regular 1-minute intervals. Identify the frame where the nuclear envelope disappears (start of Prophase/Prometaphase) and when alignment is complete (Metaphase). Note the time when chromatids start to separate (start of Anaphase). The difference is the duration.
4. What is the significance of the alignment at the equatorial plate for the genetic consistency of the daughter cells?
   • Answer: It ensures that when chromatids separate, each daughter cell receives exactly one copy of every chromosome, maintaining the diploid number.
5. If the tissue sample is from a plant root tip, how would the cytokinesis phase differ from that of an animal tissue sample?
   • Answer: In plant cells, a cell plate forms from the center outwards. In animal cells, a cleavage furrow forms by the constriction of the cell membrane from the outside inwards.

Case Study 2: In a cancer research lab, scientists are studying a group of cells that divide uncontrollably. They find that a specific checkpoint protein, p53, is non-functional in these cells. 6. Explain the role of the p53 protein in the regulation of the cell cycle.

• Answer: p53 acts as a "guardian of the genome." It detects DNA damage at the G1/S checkpoint and can halt the cell cycle for repair or trigger apoptosis if the damage is irreparable.

7. Predict the consequence of a non-functional G1 checkpoint in these cancer cells.
   • Answer: Cells with damaged DNA will enter the S-phase, replicate faulty DNA, and pass mutations to daughter cells, leading to tumor growth.
8. How does the failure of cell cycle checkpoints lead to the accumulation of mutations?
   • Answer: It allows the cell to bypass the repair phase, meaning errors during replication or damage from external factors are never corrected and are instead permanently integrated into the DNA.
9. Create a hypothesis: If a drug is introduced to restore p53 function, what immediate change would you expect in the cell cycle distribution of this population?
   • Answer: A large proportion of the cell population would accumulate/arrest in the G1 phase as the restored p53 halts the cycle to address DNA damage.
10. Differentiate between the behavior of these cancer cells and normal cells exhibiting 'contact inhibition'.
    • Answer: Normal cells stop dividing when they touch neighboring cells (contact inhibition). Cancer cells lack this, continuing to divide and piling up to form a mass (tumor).

Section B: Assertion-Reasoning Questions

Directions: Choose (a) if both A and R are true and R explains A; (b) if both are true but R doesn't explain A; (c) if A is true but R is false; (d) if A is false but R is true.

11. Assertion (A): Meiosis is essential for sexual reproduction. Reason (R): It restores the specific chromosome number of a species by reducing the chromosome number by half in gametes.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.
12. Assertion (A): The G0 phase is a state of dormancy where cells are metabolically inactive. Reason (R): Cells in G0 do not proliferate but remain viable and metabolically active.
    • Answer: (d) A is false but R is true. (G0 cells are metabolically active).
13. Assertion (A): DNA replication occurs twice in Meiosis, once before Meiosis I and once before Meiosis II. Reason (R): Interkinesis is a short phase between Meiosis I and II where no DNA replication occurs.
    • Answer: (d) A is false but R is true. (DNA replication occurs only once, before Meiosis I).
14. Assertion (A): Crossing over leads to genetic variation. Reason (R): It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.
15. Assertion (A): Mitosis is often called equational division. Reason (R): The number of chromosomes in the parent and daughter cells remains the same.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.

Section C: Creating and Designing (Application & Analysis)

16. Designing: Design a flowchart that illustrates the changes in DNA content (C) and Chromosome number (N) for a human cell (2n=46).
    • Answer: G1 (2n, 2C) -> S (2n, 4C) -> G2 (2n, 4C) -> M (2n, 4C) -> Cytokinesis (2n, 2C each).
17. Creating: Create a hypothetical scenario where a cell lacks the enzyme causing the breakdown of the centromere. Describe the resulting anomaly.
    • Answer: During anaphase, sister chromatids would fail to separate. Both chromatids (a whole chromosome) would go to one pole, or neither would, resulting in aneuploidy (one daughter cell having an extra chromosome and the other missing one).
18. Analysis: Identify which phases constitute the cells in the '2C' and '4C' peaks in a DNA content graph.
    • Answer: '2C' peak corresponds to G1 phase (and G0). '4C' peak corresponds to G2 and M phases. Cells between the peaks are in S phase.
19. Designing: Construct a table comparing Mitosis and Meiosis.
    • Answer:

      Feature	Mitosis	Meiosis
      Site	Somatic cells	Germ cells
      Pairing	No	Yes (Synapsis)
      Daughter Cells	2 (Identical)	4 (Varied)
      Divisions	One	Two

20. Creating: Predict the ploidy levels of the cells after 3 generations starting from a diploid cell if it skips S-phase every alternate cycle.
    • Answer: Gen 0: 2n. Gen 1 (normal): 2n. Gen 2 (skip S): n (haploid). Gen 3 (normal): n (stays haploid as S-phase doubles DNA but ploidy is based on chromosome sets).
21. Visualisation: Key difference in chromosome separation in Anaphase I vs Anaphase.
    • Answer: In Anaphase I of Meiosis, homologous chromosomes separate (centromeres don't split). In Anaphase of Mitosis, sister chromatids separate (centromeres split).
22. Application: A gardener wants to propagate a plant rapidly without changing its genetic makeup. Inducing Meiosis or Mitosis?
    • Answer: Mitosis. It produces genetically identical clones (vegetative propagation). Meiosis introduces variation.
23. Analysis: Why is S-phase called "Synthesis" if chromosome number doesn't increase?
    • Answer: Because DNA is synthesized (duplicated). Chromosome number depends on centromeres; since sister chromatids stay attached, the count remains the same.
24. Designing: Mnemonic for substages of Prophase I.
    • Answer: Lazy Zebra Plays Dirty Dice (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis).
25. Creating: Diary entry from a chromosome.
    • Answer: "Today I hugged my homologous partner so tight that we swapped pieces of our sweaters (genes). I feel like a whole new version of myself now!"

Section D: Competency & Critical Thinking

26. Why do chemotherapy drugs target spindle fibers?
    • Answer: Spindle fibers are needed to pull chromosomes apart. Without them, cells cannot complete mitosis, stopping the rapid division of cancer cells.
27. A cell has 20 chromosomes at G1. How many chromatids at the end of S phase?
    • Answer: 40 chromatids. Each of the 20 chromosomes replicates to form two sister chromatids.
28. "All cells in the human body divide." Critique this.
    • Answer: False. Nerve cells and skeletal muscle cells do not divide after maturity. RBCs lack a nucleus and cannot divide.
29. If a cell fails to undergo cytokinesis after karyokinesis, what is the result?
    • Answer: Multinucleated condition (Syncytium or Coenocytic). Example: Coconut water (liquid endosperm).
30. Significance of the Synaptonemal Complex.
    • Answer: It mediates synapsis (pairing) of homologous chromosomes in meiosis, which is required for crossing over. Absent in mitosis because pairing isn't needed.
31. Why is the onion root tip chosen?
    • Answer: It is a meristematic region where cells are actively and rapidly dividing, making it easy to find different stages of mitosis.
32. Is Interphase a "resting phase"?
    • Answer: No. It is metabolically the most active phase as the cell is growing, synthesizing proteins, and replicating DNA.
33. Chiasmata shifted towards the ends. Identify phase.
    • Answer: Diakinesis. The process is called Terminalisation.
34. What if Meiosis I was equational?
    • Answer: Gametes would remain diploid (2n). Fertilization would result in polyploidy (4n), doubling the chromosome number every generation.
35. Calculate mule's chromosome number.
    • Answer: Horse (64/2=32) + Donkey (62/2=31) = 63. Mule is sterile because 63 is an odd number; homologous pairing cannot occur during Meiosis.
36. Bivalent formation at equator. Phase?
    • Answer: Metaphase I of Meiosis. Bivalents (pairs of homologs) only form in Meiosis.
37. SA/V ratio and cell division.
    • Answer: As a cell grows, its volume increases faster than surface area. This reduces the efficiency of transport. Division restores a high SA/V ratio.
38. Model for "Contact Inhibition".
    • Answer: Like a crowded bus where no more people can enter once seats are full. Surface receptors sense neighbors and send "stop division" signals.
39. Criteria for Late Prophase vs Early Metaphase.
    • Answer: Late Prophase: Nuclear envelope fragmenting, chromosomes scattered. Early Metaphase: Spindle attached, chromosomes moving towards (but not yet at) the center.
40. How does recombination contribute to evolution?
    • Answer: It creates new combinations of alleles, increasing genetic diversity in a population, which is the raw material for natural selection.

Section E: Advanced Competency

41. Non-disjunction of Trisomy 21. When?
    • Answer: Anaphase I (homologs fail to separate) or Anaphase II (sister chromatids fail to separate).
42. Cell Cycle analogy.
    • Answer: G1/S is like a passport check; G2/M is like a final security scan before boarding (M phase).
43. mRNA longevity in G1 vs Prophase.
    • Answer: G1 mRNA is stable for protein synthesis. In Prophase, transcription stops as chromatin condenses, so mRNA production ceases.
44. Impact of "cohesin-ase" in Metaphase I.
    • Answer: Sister chromatids would separate early. In Meiosis I, homologs should separate. Premature chromatid separation leads to incorrect chromosome counts.
45. Toxin inhibits cell plate in plants.
    • Answer: The cell would become multinucleated but would not divide into two separate cells, leading to a failure in tissue growth.
46. Shoe analogy for Homologous Chromosomes.
    • Answer: They are like a pair of shoes—same size and style (genes for same traits) but one comes from the left (mom) and one from the right (dad).
47. Why is Meiosis II "Mitotic"?
    • Answer: Because it separates sister chromatids, just like mitosis, and the chromosome number remains the same (n -> n).
48. If spore is 'C', what was Mother Cell in G2?
    • Answer: '4C'. Meiosis reduces 4C (G2) -> 2C (after Meiosis I) -> 1C (after Meiosis II).
49. Graph of Chromosomes in Meiosis.
    • Answer: Starts at 2n, stays 2n until end of Meiosis I (where it drops to n), stays n through Meiosis II.
50. Implications of Amitosis in humans.
    • Answer: Genetic chaos. Without the precise distribution of chromosomes, daughter cells would get random amounts of DNA, leading to immediate cell death or severe malfunction.