Class 11 Biology: Cell Cycle and Cell Division

• Introduction to Cell Division

  • Definition of Cell Cycle
  • Significance: Growth, Repair, and Reproduction
  • Discovered by Prévost and Dumas (1824) on zygote cleavage

• Phase 1: Interphase (Resting Phase / Preparation Phase)

  • Duration: >95% of the cell cycle duration
  • G1 Phase (Gap 1 / Pre-DNA Synthesis)
    • Cell metabolically active
    • Continuous growth and organelle duplication
  • S Phase (Synthesis)
    • DNA replication (amount per cell doubles, chromosome number remains same)
    • Centriole duplication in cytoplasm (animal cells)
  • G2 Phase (Gap 2 / Post-DNA Synthesis)
    • Protein synthesis
    • Tubulin synthesis for spindle apparatus
    • Cell growth continues

• Phase 2: M Phase (Mitosis / Meiosis)

  • Quiescent Stage (G0 Phase)
    • Exit from G1 for non-dividing cells (e.g., heart cells)
    • Metabolically active without proliferation
  • Karyokinesis (Nuclear Division)
    • Prophase
      • Chromatin condenses into chromosomes
      • Nucleolus and nuclear membrane disappear
      • Spindle fibers initiate
    • Metaphase
      • Chromosomes at equatorial plate (Metaphase plate)
      • Kinetochores attach to spindle fibers
      • Best phase to study chromosome morphology
    • Anaphase
      • Centromeres split
      • Sister chromatids move to opposite poles
    • Telophase
      • Chromosomes reach poles and decondense
      • Nuclear envelope reforms
      • Nucleolus reappears
  • Cytokinesis (Cytoplasmic Division)
    • Animal Cells: Formation of cleavage furrow (centripetal)
    • Plant Cells: Formation of cell plate via phragmoplast (centrifugal)

• Significance of Mitosis

  • Produces diploid daughter cells with identical genetic makeup
  • Essential for multicellular organism growth
  • Cell repair and regeneration (e.g., upper layer of epidermis, gut lining)
  • Asexual reproduction in unicellular organisms

• Meiosis

  • Definition: Reductional division producing haploid gametes
  • Meiosis I (Reductional Division)
    • Prophase I (Longest and most complex)
      • Leptotene: Chromosomes visible
      • Zygotene: Synapsis (pairing of homologous chromosomes) + Synaptonemal complex
      • Pachytene: Crossing over between non-sister chromatids via recombinase + Recombination nodules
      • Diplotene: Dissolution of synaptonemal complex + Chiasmata formation (X-shaped structures)
      • Diakinesis: Terminalization of chiasmata + Nucleolus/nuclear membrane disappear
    • Metaphase I: Homologous pairs align at equator
    • Anaphase I: Separation of homologous chromosomes (sister chromatids remain attached)
    • Telophase I: Nuclear membrane reforms, may interphase briefly (interkinesis)
  • Meiosis II (Equational Division)
    • Similar to mitosis
    • Prophase II: Nuclear envelope breaks down
    • Metaphase II: Chromosomes align at equator
    • Anaphase II: Splitting of centromeres and separation of sister chromatids
    • Telophase II: Four haploid daughter nuclei formed

• Significance of Meiosis

  • Maintains chromosome number constant generation after generation
  • Generates genetic variations via crossing over (crucial for evolution)