2.6

• Prophase

  • chromosomes condense and are visible when stained
  • chromosomes have 2 identical sister chromatids that are joined at centromere
  • centrosomes move to opposite sides of the cell
  • nuclear envelope breaks down
• Metaphase

  • centrosomes reach opposite sides
  • chromosomes line up at the equator of the spindle
  • spindle fibres reach chromosomes and attach to centrosomes
  • each sister chromatid is attached to spindle at opposite ends
• Anaphase

  • the sister chromatids seperate at the centromere
  • spindle fibres begin to shorten
  • the seperated sister chromatids are pulled to opposite poles by spindle fibres
• Telophase

  • chromosomes arrive at opposite poles and start to decondense
  • nuclear envelope begins to form around each set of chromsomes
  • the spindle fibres start to break down
  • new nucleoli forms within each nucleus
• Cytokenisis follows M phase, once nuclues has divided into 2 genetically identical nuclei. In animal cells, the cell surface membrane pinches inwards creating a clevage furrow in the middle of the cell ehich contracts, dividing the cytoplasm in 2. In plant cells, vesicles gather along equator of spindle. Vesicles merge with each other to form new cell surface membrane: layers of cellulose are laid to create the cell wall.
• Apoptosis is when a cell self destructs.
• G0 is when a cell leaves the cycle, either temporarily or permently.
• Some cells leave the cycle due to differentation, the cell becomes specialised and no longer needs to divide.
• The movement from one phase to another is triggered by chemical signals callled cyclins
• Checkpoints are used to prevent uncontrollable division: detect and repair any damages in the DNA
• During G2, tublin protein is produced to create microtubules for the spindle fibres. The mitochondria grows and any DNA replicated during phase S is checked for errors. During the whole of interphase the cell grows.
• P53 gene is known as the tumour suppressor gene.
• The products of meiosis are haploid cells
• Chromsomes are arranged side by side inoto homologous pairs called a bivalent . They can cross each other at an intersection point called the chiasma
• During Metaphase 1, maternal and paternal chromosomes in each pair move away from each other this is called an independent assortment
• Prophase 2

  • nuclear envelope breaks down and chromosomes condense
  • a spindle forms at right angle to the old one
• Metaphase 2

  • chromosomes line up in a single file alog the equator of the spindle
• Anaphase 2

  • centromeres divide and individual chromatids are pulled to opposite poles by spindle fibres
  • this creates 4 groups of chromosomes
• Telophase 2

  • nuclear envelope form around each group of chromosomes
• Cytokenisis

  • cytoplasm divides as new cell surface membrane are found creating 4 haploid cells
• During phase M the cell stops growing. During G0 many cells go through apoptosis (programmed cell death) differentiation . During G1 the cell grows and increases in size and the transcription of genes to make RNA occurs, organelles also duplicate. During S DNA is replicated. During G2 the cells grow once again and special chemicals ensure the cell is ready for mitosis by stimulating the protein that allows chromosomes to condense.