cell cycle

• prophase 1

  • each chromsome is supercoiled and forms sister chromatids
  • nuclear envelope breaks down
  • centriole duplicated and moves to opposite ends of cell. begins to synthesise spindle fibre
  • homologous chromosomes come together in homologous pair
  • crossing over occurs non-sister chromatids swap sections of their DNA- results in shuffling of alleles.
• Metaphase 1

  • sister chromatids allign in equator of cell
  • spindle fibres attach to centromere
  • independent assortment takes place by which homologous chromosomes are arranged randomly
  • this means arrangement of chromosomes in each daughter cell will not be the same as the mothers cell
• independent assortment


  random arrangement of homologous chromosomes. results in gametes that have unique combinations of chromosomes
• anaphase 1

  • members of each homologous pair of chromosomes are pulled apart
  • sister chromatids are not pulled apart
  • crossed over areas separate creating chromosomes with different alleles from the mother cell
• telophase 1

  • nuclear envelope develops
  • each cell contains half the number of chromosomes as a normal cell does
• cell cycle stages

  • metaphase
  • G0/G1 phase -interphase
  • S phase- interphase
  • G2 phase- interphase
• m-phase (mitotic phase)

  • cell growth stops
  • nuclear division occurs (mitosis)
  • cytoplasmic division occurs (cytokenisis)
• G0 phase

  • resting phase triggered during early G1
  • some cells may remain in this stage for a long time
  • some cells may enter temporarily
  • cells may undergo:
  • adoptosis : programmed cell death
  • differentiation: e.g adult neurons
  • senescene: cells will no longer divide
• G1 phase

  • cells grow and increase in size
  • DNA transcription occurs
  • organelles duplicate
  • biosynthesis occurs
• interphase

  • cell growth
  • DNA replication
  • cells carry out their functions
  • preparing for the next m-phase
• S phase

  • DNA is unwound from histone proteins
  • DNA is replicated
  • sister chromatids formed
  • Rapid phase- reduces chances of errors
• G2 phase

  • cells grow
  • preparing for m-phase by stimulating proteins which will help chromatin condense and spindle to form
• metaphase checkpoint


  check for chromosome spindle attachment
• G1 checkpoint


  checks for nutrients, growth factors and dna damage
• S checkpoint


  checks for mutations - mistakes in dna
• G2 checkpoint


  checks for cell size and dna replication
• prophase

  • duplicated dna (sister chromatids) shortens and thicks
  • nuclear envelope breaks down
  • centriole divides and two daughter centrioles move to opposite ends of cell
  • cytoskeleton protein threads from a spindle between centrioles
• metaphase

  • sister chromatids attach to spindle fibres in the middle of the cell
  • the point of attachment is called the centromere
• anaphase

  • sister chromatids split apart
  • spindle fibres pull them apart and they are moved to each side of the cell
• telophase

  • separated chromatids reach sides of the cell
  • new nuclear envelope forms
  • spindle structures break down
• • centrioles synthesise the spindle
  • spindle made of tubulin protein