Mitosis

• cell cycle

  • the events that take place as one parent cell divides to produce two daughter cells
• cell cycle is the regulated sequence of events that occurs between one cell and the next

  • the cell cycle has three phases

  Interphase

  Nuclear Division (mitosis)

  Cell division/ Cytoplasmic Division (cytokenisis)

  the length of the cell cycle is variable depending on environmental conditions, the cell type and organism

  e.g onion root divide once every 20 hours(roughly) but human intestine epithelial cells divide once evens 10 hours (roughly)
• the movement from one phase to another is triggered by chemical signals called cyclins
• G1 (gap 1) phase-also called the growth phase - a Gl checkpoint control mechanism ensures that the cell is ready to enter the S phase and begin DNA synthesis

  • cell grow and increase in size
  • transcription of genes to make RNA occurs
  • organelles duplicate
  • biosynthesis e.g protein synthesis, including making enzymes needed for DNA replication in the S phase
  • the p53 (tumour suppressor) gene helps control this phase
• S (synthesis) phase of interphase - because chromosomes are unwound and the DNA is diffused, every molecule of DNA is replicated. There is a specific sequence to the replication of genes : housekeeping genes- those which are active in all types of cells, are duplicated first. Genes that are normally inactive in specific types of cells are replicated last
• S phase #2

  • Once the cell has entred this phase, It is committed to completing cell cycle
  • DNA replicates
  • when all chromosomes have been duplicated each one consists of a pair of identical sister chromatids
  • this phase is rapid, and because the exposed DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening
• G2 (gap 2) phase of interphase - special chemical ensure that the cell is ready for mitos is by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle

  • cells grow
• M Phase - a check point chemical triggers condensation of chromatin - halfway through cycle, the metaphase checkpoint ensures cell is ready for complete mitosis

  • cell growth stops
  • nuclear division consisting of stages : PMAT
  • cytokenesis
• interphase = G1 + S + G2
• the G1/S checkpoint also called the restriction point and the G2/M checkpoint
• Purpose of checkpoints

  • to prevent uncontrolled division that would lead to tumours (cancers)
  • to detect and repair damage to DNA (e.g damage caused by UV light

  Because the molecular events that control the cell cycle happen in a specific sequence, they also ensure that :

  • cycle cannot be reversed
  • DNA is only duplicated once during each cycle
• Significance of mitosis in life cycle : asexual reproduction, growth, tissue repair,
• Acetic Orcein - stains chromosomes
• Prophase

  • the chromosomes that have replicated during S phase of interphase + consists of two sister chromatids that thicken + shorten as DNA supercoils
  • nuclear envelope breaks down
  • centriole in animal cell divides into two new daughter ceAtrioles and go opposite directions to opposite poles
  • cytoskeleton protein (tubulin) threads form a spindle between these centrioles. The spindle has a 3D structure and is rather like lines of longitudinal lines of a virtual globe

  →In plant cells, tubulin threads are formed from cytoplasm
• Metaphase

  • the pairs of chromatids attach to the spindle threads at the equator region
  • they attach by their centromeres
• Anaphase

  • the centromeres of each pair splits
  • motor proteins, walking along the tubulin threads, pull each sister chromatid to opposite poles
  • because their centrosome goes first, the chromatids, now called chromosomes, assume a V shape
• Telophase

  • the seperated chromosomes reach the poles
  • new nuclear envelope froms around each set of chromosomes
  • the cell now contains two nuclei each genetically identical to each other + parent cell
• Cytokinesis

  • animal cell, plasma membrane folds inwards and 'nips in’ the cytoplasm
  • plant cell, an end plate form where the equator of spindle was, and new plasma membrane + cellulose cell wall material are laid down on either side along this end plate

  Two new daughter cells are now formed. They are genetically identical to each other + parent cell
• centriole - bundles of microtubules that form the spindle fibres during nuclear division
• centrosome - area of cell cytoplasm where centrioles are
• chromosome - DNA molecule that is unwound (without histone proteins)
• centromere - region of a pair of sister chromatids that attaches to spindle fibres