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.

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 …

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