CELL DIVISION

• THE CELL CYCLE IN EUKARYOTES

  INTERPHASE

  1. gap phase 0
  2. Growth/gap phase 1 - G1
  3. synthesis - S
  4. growth/gap phase 2 - G2

  MITOSIS

  1. Prophase
  2. metaphase
  3. anaphase
  4. telophase

  CYTOKINESIS

  
• INTERPHASE

  1. GAP 0 - resting phase, for cells not dividing
  2. GAP/GROWTH 1 - cell grows, new organelles, proteins and mRNA made
  3. SYNTHESIS - DNA replication, 2N —> 4N for mitosis
  4. GAP/GROWTH 2 - cell grows, new organelles, proteins and mRNA made

  CHECKPOINTS

  • G1 - checks if cell is big enough and proteins are made, if fail goes into G0
  • S - checks if DNA has been replicated
  • G2 - checks if it’s big enough to enter mitosis and if replication was correct
• 1, PROPHASE (MITOSIS)


  • chromosomes condense, becoming short and fat
  • become visible under microscopes
  • x-shaped chromosomes, joined by centromere, sister chromatids are identical
  • centrioles (bundles of protein) move to poles of cell
  • network of spindle fibres form across the cell
  • PROMETAPHASE: nuclear envelope starts to break down to release chromosomes into cytoplasm

  
• 2. METAPHASE (MITOSIS)

  • chromosomes line up along the centre / equator of the cell
  • attached to the mitotic spindle by their centromeres
  • arranged sideways and neatly
  • CHECKPOINT: to make sure all chromosomes are on the metaphase plate which is important to ensure each daughter cell gets the same number of chromosomes

  
• 3. ANAPHASE (MITOSIS)


  • spindle fibres contract
  • pulling the sister chromatids away from each other, towards the poles
  • centromeres are pulled first causing them to look v-shaped
  • 2 separate groups of chromatids

  
• 4. TELOPHASE (MITOSIS)


  • chromatids each the poles
  • uncoil and become long and thin again
  • new nuclear envelope forms around the two new full sets of chromosomes
  • forms two new nuclei, daughter nuclei
  • genetically identical

  
• CYTOKINESIS

  • when the cell physically separates / divides and the cytoplasm splits
  • contractile ring forms at the centre
  • makes an indentation called a cleavage furrow
  • pinches in and the cell divides into 2 identical daughter cells
  • starts in telophase

  
• MITOSIS: controlled process of cell division
• CANCER: uncontrolled cell division

  • when there’s a mutation in a gene that controls cell division, cells grow out of control, forming a tumour.
• MUTATION: a change in the base sequence of DNA
• MALIGNANT TUMOUR: grows rapidly and can invade healthy neighbouring tissues.
• BENIGN TUMOUR: grows slowly and doesn’t spread
• CANCER TREATMENT

  • some treatments control the rate of cell division in tumour cells by disrupting the cell cycle and killing tumour cells.
  • however these treatments don’t distinguish between cancer cells and normal cells that are dividing.
  • but it is more likely to kill tumour cells as they divide more frequently.
• CANCER TREATMENT

  CHEMOTHERAPY: prevents synthesis of enzymes that are needed for DNA replication in gap phase 1, meaning the cell cannot enter the synthesis phase, disrupting the cell cycle and causing the cell to kill itself.
• CANCER TREATMENT

  RADIATION THERAPY: radiation damages DNA in the synthesis phase. This meant at the next checkpoint, if severe damage was detected, the cell kills itself, preventing further tumour growth.
• CELL DIVISION IN PROKARYOTES - BINARY FISSION

  1. circular DNA and plasmids replicate (once for circular, more for plasmids)
  2. cell gets bigger and DNA loops move to opposite poles of the cell
  3. cytoplasm begins to divide and new cell walls begin to form
  4. cytoplasm fully splits and 2 daughter cells are made, each with one identical loop of circular DNA and with variable plasmids.

  
• VIRAL REPLICATION

  1. attach to host cell using their attachment proteins which bind to complementary receptor proteins on the surface membrane of the host cell.
  2. genetic material from virus is released into host cell.
  3. use host cells ‘machinery’ like enzymes and ribosomes to replicated the genetic material and proteins.
  4. viral components assemble inside host cell. Replicated viruses released from the host cell.

  • different viruses have different attachment proteins so attach to different receptors so therefore can only infect one kind of cell.