Cell Division

• Cell cycle


  The life period of a cell during which a cell synthesizes DNA (replication), grows and divides into two daughter cells
• Cell growth


  • Continuous process
  • DNA synthesis occurs only at a specific stage
• Duration of cell cycle


  • Varies in each organism and each cell type
  • Typical eukaryotic cell cycle (e.g. human cell) is about 24 hrs
  • In Yeasts, it is only about 90 minutes
• Cell cycle phases


  • Interphase
  • M Phase
• Interphase


  • The phase between two successive M phases
  • Includes cell growth & DNA synthesis
  • Lasts more than 95% of the duration of cell cycle
• Interphase phases


  • G1 phase (Gap 1 or Antephase)
  • S (Synthetic) phase
  • G2 (Gap 2) phase
• G1 phase


  • First growth phase
  • Interval between mitosis and DNA replication
  • Cell becomes metabolically active
  • Prepares machinery for DNA replication
  • Synthesizes RNA and proteins
• S phase


  • DNA replication takes place
  • Amount of DNA per cell doubles, but chromosome number is not increased
• G2 phase


  • Second growth phase
  • Cell growth continues
  • Synthesis of RNA and proteins continues
  • Cell is prepared for mitosis
• M Phase


  • Actual cell division (mitosis)
  • Includes Karyokinesis (nuclear division) and Cytokinesis (division of cytoplasm)
  • Lasts for only about an hour in human cell cycle
• • Some cells do not show division (e.g. heart cells)
  • Many other cells divide only occasionally to replace damaged or dead cells
• Quiescent stage (Go)


  • Cells that do not divide further exit G1 phase and enter an inactive stage
  • Remain metabolically active but do not proliferate
• Mitosis


  • Cell division occurring in somatic cells
  • Also called as equational division as the number of chromosomes in the parent and progeny cells is same
  • Generally seen in diploid cells, also occurs in haploid cells of some lower plants and some social insects
  • Involves major reorganization of all cell components
• Stages of mitosis


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase


  • Longest phase in mitosis
  • Chromosomal materials (chromatin fibres) are untangled and condensed to form mitotic chromosomes
  • Centrosomes begin to move towards opposite poles of the cell
  • Cells at the end of prophase do not show Golgi complexes, endoplasmic reticulum, nucleolus & nuclear envelope
• Metaphase


  • Nuclear envelope completely disintegrates
  • Chromosome condensation is completed
  • Chromosomes come to lie at the equator (metaphase plate)
  • Spindle fibres from both poles are connected to chromatids by their kinetochores in the centromere
• Anaphase


  • Centromere of each chromosome divides longitudinally resulting in the formation of two daughter chromatids
  • As the spindle fibres contract, the chromatids move from the equator to the opposite poles
• Telophase


  • Chromosomes cluster at opposite poles and uncoil into chromatin fibres
  • Nuclear envelope assembles around the chromatin fibres
  • Nucleolus, Golgi complex and ER reappear
  • The spindle fibres disappear
• Cytokinesis


  • Division of cytoplasm resulting in the formation of 2 daughter cells
  • In animal cells, a cleavage furrow appears in plasma membrane and gradually deepens and joins in the centre dividing the cytoplasm into two
  • In plant cells, vesicles from Golgi bodies accumulate at the equator, grow outward and meet the lateral walls to form the cell-plate which separates the 2 daughter cells
• • During cytokinesis, organelles like mitochondria and plastids get distributed between the daughter cells
  • In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition (syncytium) (e.g. liquid endosperm in coconut)
• Significance of mitosis


  • Produces diploid daughter cells with identical genome
  • Helps to retain the same chromosome number in all somatic cells
  • Helps in the body growth of multicellular organisms
  • Restores the nucleo-cytoplasmic ratio that disturbed due to cell growth
  • Helps in cell repair & replacement
• Mitosis


  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase

  6. Cytokinesis
• Mitosis


  • Produces diploid daughter cells with identical genome
  • Retains the same chromosome number in all somatic cells
  • Helps in body growth of multicellular organisms
  • Restores the nucleo-cytoplasmic ratio
  • Helps in cell repair and replacement
• Meiosis


  1. Meiosis I

  2. Meiosis II
• Meiosis


  • Involves two cycles but only a single cycle of DNA replication
  • Involves pairing of homologous chromosomes and recombination
  • Meiosis I begins after replication of parental chromosomes
  • 4 haploid cells are formed at the end of meiosis II
• Meiosis I


  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I
• Prophase I


  • Chromatin fibres become long slender chromosomes
  • Nucleus enlarges
  • Similar chromosomes start pairing together (synapsis)
  • Bivalent chromosomes split into similar chromatids
  • Recombination occurs between non-sister chromatids
• Meiosis II


  1. Prophase II

  2. Metaphase II

  3. Anaphase II

  4. Telophase II
• Meiosis conserves the chromosome number of each species
• Meiosis causes genetic variation in the population of organisms, which is important for evolution