cell division

• the cell cycle can take a long time and the duration of the cell cycle depends on the cell

  • in some mammalian cells it can take up to 24 hours
  • in other cells it can take much longer, months
• some cells can exit the cell cycle either temporarily or permanently

  at certain key points the cell checks the cell cycle should continue (checkpoints). if a cell fails a checkpoint it enters G0 phase and leaves the cell cycle
• G1 phase - cell replicates its organelles which requires a lot of proteins, so a high level of transcription and translation take place. the size of the cell also increases so the 2 daughter cells are the correct size.
• S phase - all of the chromosomes are replicated so the cell needs to synthesise a great deal of DNA
• G2 phase - the cells replaces all the energy stores used during the S phase, the cell continues to carry out transcription and translation. if any chromosomes have been damaged they are repaired during G2. the cell also continues to grow
• M phase (mitotic phase) - the chromosomes are separated into 2 separate nuclei in cytokinesis and the cell divides in 2
• reasons the cell will enter G0

  • fully differentiated cells - cells that stay the same for the rest of their life (adult neurones)
  • DNA has become damaged - at a certain point the DNA can't divide anymore (senescent cells)
  • cells enter G0 temporarily - cells can be triggered to enter the cell cycle again (B memory cells during an infection)
• checkpoints in the cell cycle

  • G1 checkpoint - checks if the cell is the correct size and checks for DNA damage. if the checkpoint is passed the cell goes S phase and carries out DNA replication
  • G2 checkpoint - checks for DNA damage and if the cell is the correct size. if passed the cell moves into the mitotic phase
  • metaphase checkpoint - checks the chromosomes are assembled correctly on the mitotic spindle. if passed the cell completes mitosis and proceeds to cytokinesis
• mitosis starts with one diploid cell and ends up with 2 genetically identical diploid cells

  • mitosis takes place when we want identical copies of the cell, e.g. when the organism grows or repairs itself
  • used for a sexual reproduction
• mitosis is used to

  • replace cells
  • repair tissues
  • growth of the organism
  • a sexual reproduction
• mitosis is part of a larger process - the cell cycle

  interphase - cells replicate the DNA, grows in size, organelles are replicated and the energy store is increased (during interphase you cannot see the chromosomes in the nucleus as they are not visible as distinct structure. see a dark circle (chromatin))

  mitotic - nuclei divides into 2 daughter nuclei and then the cell divides into 2 daughter cells each with a nucleus
• mitosis - division of the nuclei

  cytokinesis - division of the cell
• chromatin - chromosomes have a loose open structure meaning the DNA is accessible for transcription and translation
• humans have 23 pairs of chromosomes
• 4 stages of mitosis

  prophase

  metaphase

  anaphase

  telophase
• prophase

  • chromosomes condense and become visible in the nucleus
  • each chromosomes consists of 2 sister chromatids joined at the centromere
  • nucleolus disappears and the nuclear membrane starts to break down
  • centrioles move to either side of the cell
  • proteins start to form spindle fibres which will attach to the centromere of each chromosome
• metaphase

  • spindle fibres have completely formed
  • chromosomes are lined up in the centre/equator of the cell
• anaphase

  • the centromeres divide in 2 and the spindle fibres begin to shorten. this pulls the sister chromatids to opposite ends of the cell
• telophase

  • chromatids have reached the poles of the cell - scientists refer to them as chromosomes again
  • spindle apparatus breaks down and the nuclear membrane begins to reform
  • chromosomes uncoil back to their chromatin state and the nucleolus reappears in each nucleus
• cytokinesis

  • central cell membrane is pulled inwards by the cytoskeleton, a groove which gradually deepens. the membrane then separates to form 2 separate cells. mitosis has finished
• interphase - the DNA replicates

  46 chromosomes goes to 92 chromosomes