Cell division

• All organisms, even the largest, start their life from a single cell
• Cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide
• Cycles of growth and division allow a single cell to form a structure consisting of millions of cells
• Cell cycle

  The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells
• Phases of the cell cycle

  • Interphase
  • M Phase (Mitosis phase)
• Phases of interphase

  • G1 phase
  • S phase
  • G2 phase
• G1 phase

  Interval between mitosis and initiation of DNA replication
• S phase


  Period during which DNA synthesis or replication takes place
• G2 phase


  Proteins are synthesised in preparation for mitosis while cell growth continues
• Some cells in adult animals do not divide further and exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle
• In animals, mitotic cell division is only seen in the diploid somatic cells, with few exceptions like male honey bees
• Plants can show mitotic divisions in both haploid and diploid cells
• M Phase


  The phase when the actual cell division or mitosis occurs
• Stages of karyokinesis (nuclear division) in mitosis

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase

  • Chromosomal material condenses to form compact mitotic chromosomes
  • Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell
• Metaphase

  • Spindle fibres attach to kinetochores of chromosomes
  • Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles
• Anaphase


  • Centromeres split and chromatids separate
  • Chromatids move to opposite poles
• Telophase

  • Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements
  • Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei
  • Nucleolus, golgi complex and ER reform
• Cytokinesis

  1. In animal cells, a furrow appears in the plasma membrane that deepens and joins in the centre dividing the cell cytoplasm into two

  2. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls
• Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis.
• Mitosis is very essential to understand the significance of this division in the life of an organism.
• Mitosis usually results in the production of diploid daughter cells with identical genetic complement.
• The growth of multicellular organisms is due to mitosis.
• Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
• A very significant contribution of mitosis is cell repair.
• Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.
• Key features of meiosis


  • Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication
  • Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase
  • Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes
  • Four haploid cells are formed at the end of meiosis II
• Phases of Meiosis I

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I
• Prophase I

  1. Leptotene

  2. Zygotene

  3. Pachytene

  4. Diplotene

  5. Diakinesis
• During leptotene stage the chromosomes become gradually visible under the light microscope.
• During zygotene stage chromosomes start pairing together and this process of association is called synapsis.
• Electron micrographs of zygotene stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.
• The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
• During pachytene stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads.
• Crossing over is the exchange of genetic material between two homologous chromosomes.
• Crossing over is an enzyme-mediated process and the enzyme involved is called recombinase.
• Crossing over leads to recombination of genetic material on the two chromosomes.
• Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
• The X-shaped structures formed due to crossing over are called chiasmata.
• During diakinesis the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.