cell cycle

• G1 phase

  • protein synthesis
  • organelle replication
  • cell grow larger
• s phase

  • synthesis phase
  • dna is replicated in the nucleus
• G2 phase

  • cell continues to grow
  • 2nd growth phase
  • energy stores increased
• interphase

  • preparation for mitosis
  • consists of 3 stages: G1, G2 and S
• G0 phase

  • when the cells leave the cycle temporarily or permanently
• reasons for entering G0

  • a specialised cell can no longer divide
  • dna of the cell is damaged and permanently enters resting phase
  • cell becomes senescent
• where are the checkpoints

  • G1
  • G2
  • metaphase
• G1 checkpoint

  • end of G1
  • checks for cell size, nutrients, growth factors, DNA damage
  • if satisfied the cell will enter S phase
  • if unsatisfied it enters G0
• G2 checkpoint

  • end of G2
  • checks for DNA damage, DNA replication completeness
  • if satisfied, mitosis starts
  • if damaged, the cell cycle is halted and attempts to repair but if repairing it is unsuccessful it undergoes apoptosis (programmed cell death)
• spindle assembly checkpoint

  • at metaphase
  • checks that all chromosomes are attached to spindles and have aligned
  • mitosis only continues once it has passed this checkpoint
• chromosome are converted into 2 identical DNA molecules (chromatids) which are joined by centromeres
• • before replication - 1 chromatid per chromosome
  • after replication - 2 chromatids per chromosome
  • after mitotic division - 1 chromatid per chromosome
• order of phases in mitosis

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
• prophase

  • chromosomes condense and become visible
  • nuclear envelope breaks down
  • nucleolus disappears
  • centrioles move to opposite poles of the cell
  • microtubules assemble around centrioles, forming the spindle
• metaphase

  • spindle fibres attach to the centromeres of the chromosomes
  • chromosomes line up at the equator
  • each sister chromatid is attached to a spindle fibre from opposite poles
• anaphase

  • sister chromatids are separated as the centromeres divide
  • sister chromatids are pulled apart by the spindle fibres to opposite poles
  • spindle fibres shorten
• telophase

  • the chromatids reach opposite poles of the cell and uncoil/decondense, becoming chromosomes
  • nuclear envelope forms around each set of chromosomes
  • cell forms a cleavage furrow (point where the cell will divide)
• cytokinesis: the cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells.
• mitosis forms 2 diploid daughter cells
• meiosis forms 4 haploid gametes from diploid parent cells
• prophase 1

  • Dna condenses and chromosomes become visible
  • chromosomes are arranged in homologous pairs forming bivalents
  • crossing over of sister chromatids may occur at the chiasmata
  • centrioles migrate to opposite poles and form spindle
  • nuclear envelope breaks down, nucleolus disentegrates
• metaphase 1

  • the bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
  • independent assortment - the maternal and paternal chromosomes in each pair position themselves independently of the others
  • this means that the proportion of paternal and maternal chromosomes that end up on each side of the equator is due to chance
• homologous

  • each chromosome in a homologous pair has the same genes at the same loci
  • a pair of homologous pairs is called a bivalent
• anaphase 1

  • the homologous pairs of chromosomes are separated as microtubules pull chromosomes to opposite ends of the spindle
  • centromeres don’t divide
• telophase 1

  • chromosomes arrive at opposite poles
  • spindle fibres start to break down
  • nuclear envelopes form around the 2 groups of chromosomes and nucleoli reform
  • some plant cells skip telophase 1 and gp straight to meiosis 11
• cytokineses in meiosis 1, 2 haploid cells form
• prophase 11

  • the nuclear envelope breaks down
  • chromosomes condense
  • spindle forms at right angle to old one
• metaphase 11

  • chroosomes line up along the equator the spindle
• anaphase 11

  • centromeres divide and individual chromatids are pulled to opposite poles
  • there are now 4 groups of chromosomes that have 23 chromosomes
• telophase 11

  • nuclear membranes form around each group of chromosomes