Mitosis - The Eukaryotic Cell Cycle

• Mitosis- How eukaryotes divide

  Interphase (not part of mitosis, js part of cell cycle)

  • accounts for 90% of cell cycle
  • Has 3 phases
  • G1 (first gap phase)
  • G0 (ap zero phase)
  • S (synthesis phase)
  • G2 (gap 2 phase)

  Prophase

  Metaphase

  Anaphase

  Telophase

  Cytokinesis (not part of mitosis, js part of cell cycle)
• G1 phase

  • increasing the volume of its cytosol
  • synthesising proteins
  • replicating its organelles
• S phase


  during the S phase, the cell replicates its DNA

  • turning one chromosome into 2 genetically identical sister chromatids
  • held together by a centromere (so still 1 chromosome)
• G2 phase


  final stage of interphase where the cell continue to grow and prepare itself for mitosis.

  • increasing volume of the cytosol
  • synthesising proteins in preparation for mitosis
• G0 phase


  cells that are not required to replicate rest in the G0 phase. so after G1, cells that don't replicate go to G0

  • either quiescent (dormant and can re-enter the cell cycle)
  • terminally differentiated (remain in G0 indefinitely)
• Prophase

  • first phase of mitosis.
  • the chromatin condenses into distinct chromosomes, whilst the centrioles begin migrating to the poles of the cell.
  • spindle fibres begin to form
  • the nucleus breaks down and thus, the nucleolus disappears
• Metaphase

  • the spindle fibres are fully formed and attached to the centromeres of each chromosome.
  • this allows for the chromosomes to be guided to the equator and line up
• Anaphase

  • the spindle fibres contract and pull away, splitting the centromere and pulling the sister chromatids to the opposite poles of the cell.
• Telophase

  • the chromosomes densely pack together, at either side of the cell
  • new nuclear membranes form, producing 2 identical nuclei.
  • the spindle fibres disappear and the chromosomes decondense
• Cytokinesis

  • the cytoplasm divides and the organelles evenly distribute themselves before splitting into 2 identical daughter cells.
  • in animals, a cleavage burrow pinches the plasma membrane into 2 cells
  • in plants, a cell plate forms at the equator before separating
• REPLICATION OF OTHER ORGANELLES:

  • mitochondria and chloroplast divide by binary fission independently of mitosis.
  • other organelles (endoplasmic reticulum and ribosomes) are built within each cell during the G1 and G2 phases of interphase. the building is orchestrated by enzymes and other molecule machines in the cytoplasm under the direction of the nucleus.
• Regulating the Cell Cycle

  the cell cycle has 3 checkpoints where the cell inspects itself for errors before proceeding to the next stage

  • checkpoints at the end of the g1 and g2 phase, and during metaphase

  if any errors are detected, the cell can pause for repairs. if the damage is irreparable, then the cell undergoes programmed cell death
• The G1 Checkpoint

  • verifies that the cell as grown to the correct size, has synthesised enough protein for DNA replication
  • checks if the DNA had been damaged during mitosis cell growth, and checks if there are enough nutrients and oxygen (favourable conditions for mitosis)
• The G2 Checkpoint

  • ensures that DNA has replicated properly in the S phase, and that the cell has enough resources for mitosis
• The Metaphase Checkpoint

  • the cell checks the formation of the spindle fibres.
  • if the chromosomes are lined up in the correct location, the cell proceeds to anaphase