Cell Division

• Cell Cycle


  The regulated sequence of events that occurs between one cell division and the next
• Cell Cycle


  1. Interphase

  2. Nuclear division (mitosis)

  3. Cell division (cytokinesis)

  
• Interphase


  1. G1 phase

  2. S phase

  3. G2 phase
• During Interphase, the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)
• G1 phase


  The gap between the previous cell division and the S phase
• S phase


  The phase of the interphase stage of the cell cycle where the DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)
• G2 phase


  The gap between the S phase and next cell division event
• During the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired
• Other preparations for cell division are made during the G2 phase (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
• Nuclear division (mitosis)


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
• Prophase


  • Chromosomes condense and are now visible when stained
  • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
  • Spindle fibres begin to emerge from the centrosomes (which consist of two centrioles in animal cells)
  • The nuclear envelope breaks down into small vesicles

  
• Metaphase


  • Centrosomes reach opposite poles
  • Spindle fibres (protein microtubules) continue to extend from centrosomes
  • Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
  • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
  • Each sister chromatid is attached to a spindle fibre originating from opposite poles
• Anaphase


  • The sister chromatids separate at the centromere (the centromere divides in two)
  • Spindle fibres (protein microtubules) begin to shorten
  • The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)
• Telophase


  • Chromosomes arrive at opposite poles and begin to decondense
  • Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
  • The spindle fibres break down
• Significance of mitosis


  • Growth of multicellular organisms
  • Replacement of cells & repair of tissues
  • Asexual reproduction
• Asexual reproduction


  Production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent
• Identifying mitosis in plant cells


  1. Growth in plants occurs in specific regions called meristems

  2. Root tip meristem can be used to study mitosis

  3. Root tip meristem contains cells undergoing mitosis

  4. Pre-prepared slides or temporary slides can be prepared using the squash technique
• Mitotic index

  The proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis
• Calculating mitotic index


  mitotic index = number of cells with visible chromosomes ÷ total number of cells