Q1: What is the Cell Cycle?

Answer: The cell cycle is the ordered sequence of events through which a cell grows, duplicates its DNA, synthesizes cell components, and finally divides into two daughter cells.

Key Points:

  • DNA replication happens once
  • Growth + division are coordinated
  • Essential for life, growth, and repair

Q2: What are the two basic phases of the Cell Cycle?

Answer: The cell cycle has two main phases:

  1. Interphase – The long preparatory phase where the cell grows and replicates DNA (G₁, S, G₂).
  2. M Phase – The division phase involving karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division).

Key Points

  • Interphase ≈ 95% of cycle
  • M phase ≈ actual division

Q3: What is the G0G_0 (Quiescent) Phase?

Answer: The G0G_0 phase is an inactive stage entered by cells that stop dividing. These cells are metabolically active but do not proliferate unless required for repair or replacement.

Examples: Heart cells, neurons

Key Points

  • Exit from G₁
  • No division, but normal function continues

Q4: Why is Mitosis called Equational Division?

Answer: Mitosis is called equational division because the chromosome number remains unchanged in daughter cells. A diploid parent cell (2n) produces two diploid daughter cells (2n).

Key Points

  • Genetic identity maintained
  • No change in chromosome number

Q5: Match the Cell Cycle Stage with the Event

Answer:

  • Chromosomes align at equator → Metaphase
  • Centromere splits → Anaphase
  • Pairing of homologous chromosomes → Zygotene (Prophase I)
  • Crossing over → Pachytene (Prophase I)

Key Points

  • Pairing & crossing over are meiotic events

Q6: Define Synapsis, Bivalent, and Chiasmata

Answer:

  • Synapsis: Pairing of homologous chromosomes during Zygotene.
  • Bivalent: A paired homologous chromosome unit with four chromatids (tetrad).
  • Chiasmata: X-shaped visible points where crossing over occurred.

Key Points

  • Occur only in Meiosis I
  • Responsible for genetic variation

Q7: Distinguish between Anaphase (Mitosis) and Anaphase I (Meiosis)

Answer:

  • Anaphase (Mitosis): Centromeres split and sister chromatids separate.
  • Anaphase I (Meiosis): Centromeres do not split; homologous chromosomes separate.

Key Points

  • Reduction of chromosome number happens in Anaphase I

Q8: How does Cytokinesis differ in Plant and Animal Cells?

Answer:

  • Animal Cells: Cytokinesis occurs by a cleavage furrow, progressing centripetally.
  • Plant Cells: Cytokinesis occurs by cell plate formation, progressing centrifugally.

Key Points

  • Plant cells have rigid cell walls
  • Mechanism differs due to structure

Q9: Give examples of equal and unequal meiotic divisions

Answer:

  • Equal division: Spermatogenesis → four equal spermatids
  • Unequal division: Oogenesis → one ovum + three polar bodies

Key Points

  • Cytoplasm distribution decides size

Q10: What is the significance of Meiosis?

Answer: Meiosis is significant because:

  1. It maintains chromosome number across generations.
  2. It creates genetic variation through crossing over and independent assortment.

Key Points

  • Essential for sexual reproduction
  • Basis of evolution

Q11: Can Mitosis occur without the S Phase?

Answer: No. Without DNA replication in the S phase, there are no sister chromatids to separate, making mitosis impossible.

Key Points

  • S phase is mandatory
  • Ensures equal DNA distribution

Q12: How do Chromosome Number (N) and DNA Content (C) change?

Answer:

  • G₁: 2N, 2C
  • After S phase: 2N, 4C
  • Anaphase: 4N, 4C (temporary)
  • Daughter cells: 2N, 2C

Key Points

  • DNA doubles before chromosome separation

Q13: How do haploid organisms divide?

Answer: Haploid organisms divide by mitosis for growth. During sexual reproduction, they form gametes by mitosis, which fuse to form a diploid zygote that later undergoes meiosis.

Key Points

  • Mitosis maintains ploidy
  • Meiosis restores haploid phase