Cell division

• Prophase:

  • chromosomes condense and coil
  • nuclear membrane breaks down and disappears
  • centrioles move to opposite poles
• Metaphase:

  • chromosomes move by spindle fibres to the centre (metaphase plate)
• Anaphase:

  • centromeres divide
  • chromatids are separated by the shortening spindle fibres
• Telophase:

  • two new sets of chromosomes assemble at each pole
  • nuclear envelope reforms around each set
  • chromosomes uncoil
• G1 checkpoint:

  • at end of G1 phase
  • checks cell size, DNA damage, nutrients and growth
  • cell either begins DNA replication or enters a resting state
• G2 checkpoint:

  • at end of G2 phase before mitotic phase
  • Checks cell size, DNA replication and DNA damage
  • cell begins mitosis if checkpoint is passed
• Spindle Assembly Checkpoint:

  • checks chromosomes are attached to spindles and are aligned
  • mitosis can then proceed
• G0:

  =when cell leaves the cycle

  Cell may leave for:

  • differentiation
  • damage to DNA
• Cytokinesis in plant cells:

  • vesicles from Golgi apparatus begin to assemble in the centre
  • vesicles fuse with each other and the cell-surface membrane
  • cell divides into two
  • new sections of cell wall form along new sections of membrane
• Cytokinesis in animal cells:

  • cleavage furrow forms around middle of cell
  • cell-surface membrane is pulled inwards by cytoskeleton
  • cell divides into two
• Stages of meiosis:

  • Meiosis I (reduction division)
  • Meiosis II
• Meiosis l:

  =pairs of homologous chromosomes are separated into two cells (each cell will now only contain one full set of genes so they are haploid)
• Meiosis ll:

  =pairs of chromatids present in each daughter cell are separated, forming two more cells (four haploid daughter cells are produced in total)
• Meiosis l:

  Prophase l: (same as prophase in mitosis)

  • chromosomes condense and coil
  • nuclear envelope and nucleolus disappear
  • spindle formation begins

  However, difference is the homologous chromosomes pair up and as the chromosomes are brought together the chromatids entangle (crossing over occurs)
• In meiosis l, during metaphase l independent assortment can occur and cause genetic variation.
• Meiosis l:

  Metaphase l:

  =same as in mitosis but it is homologous pairs of chromosomes assembling at the metaphase plate instead of individual chromosomes

  The orientation of each pair is random and independent of any other pair (independent assortment) and can lead to many different combinations of alleles.
• Meiosis l:

  Anaphase l:

  = unlike in mitosis, the homologous chromosomes are pulled to opposite poles and the chromatids stay joined to each other
• Meiosis l:

  Telophase l: (same as in mitosis)

  • chromosomes assemble at each pole
  • nuclear envelope reforms
  • chromosomes uncoil
  • cell undergoes cytokinesis
• Meiosis ll:

  Prophase ll:

  • chromosomes condense
  • nuclear envelope breaks down
  • spindle formation begins
• Metaphase 2:

  =same as mitosis, individual chromosomes assemble at metaphase plate

  (due to crossing over, chromatids are no longer identical so there is independent assortment and more genetic variation)
• Anaphase 2:

  =same as mitosis, chromatids of the individual chromosomes are pulled to opposite poles
• Telophase 2:

  • chromatids assemble at each pole
  • chromosomes uncoil + nuclear envelope reforms
  • cytokinesis results in four genetically different haploid cells