⨵ Cell Division

Learn about Cell Division.

Cell Division

  • Cell is the functional and structural unit of living organism.
  • Cell is the basic unit of life.
  • All the cell originates by cell division in existing cell. The division of chromosomes and cytoplasm of a cell into daughter cells is called cell division.
    • Karyokinesis: Means Division of Nucleus
    • Cytokinesis: Means Division of cytoplasm

Cell division cycle

  • S-phase: Synthesis phase
    • Synthesis of DNA/ Replication of DNA/Doubling of DNA occurs.
    • DNA doubles at Interphase of mitosis & Pre-meiotic interphase.
  • G : Means gap
    • G1 : Synthesis of RNA & Protein. It is pre DNA phase. It is resting phase.
    • G2 : Post DNA synthesis phase. Here planning of cell division occurs.
  • Metabolically the most active stage is Inter phase.
  • Karyokinesis takes less than one hour in most of the cases.
  • There are five types of cell division:
    • Amitosis (no - Thread or Chromosome) means chromosome does not appear and cell divides directly i.e. direct nuclear division e.g. Fission (bacteria), budding (yeast)
    • Endomitosis or C-mitosis (Colchicine).
      • Chromosome doubling is not followed by cell plate formation.
      • Colchicine is a chemical which stops the spindle fiber formation & induces polyploidy.
      • Colchicine is obtained from Colchicum luteum (family liliaceae)
    • Brachymeiosis: Nucleus divides thrice. First and third division are reductional and 2nd is mitosis. Discovered by Claussen e.g. Ascospores formation in Ascomycetes.

Mitosis

  • Takes place in both vegetative and reproductive cells.
  • Nucleus and cytoplasm divide once.
  • Two genetically identical cells are formed.
  • “Mitosis” was used firstly by Walter Flemming in 1882.

Meiosis

  • Takes place in reproductive cells.
  • Nucleus and Cytoplasm: divide twice.
  • Four genetically but not identical cells are formed.
  • Meiosis means separation of chromosomes in sex cells.
  • Chromosomes split longitudinally only once.
  • Chromosome number is reduced to half.
  • Discovered by Strasburger.
  • The term ‘meiosis’ (reductional division) was given by Farmer & Moore.
  • Neurospora: Used in genetic studies because
    • All the products of meiosis are within the same sac.
    • Neurospora has a short life cycle.
  • Asynapsis: Not allow to pair with homologous chromosomes and remain unpaired due to presence of gene.
  • Desynapsis: Chromosomes actually paired during diakinesis, chiasma falling apart due to presence of genes.

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