DNA-->Mitosis

• DNA molecule unwinds and unzips. The enzyme helicase causes the hydrogen bonds between the base pairs to break down.
• Free DNA nucleotides pair up with the exposed bases:
  Ademine and Thymine
  Cystine and Guanine 
  -Purines always pair with Pyrimidines 
• New hydrogen bonds form between the pairs
  Two between A and T, Three between C and G
• DNA polymerase forms new phosphodiester bonds between nucleotides, creating new polynucleotide chains
• Enzymes proof read the new DNA molecules to check them for mistakes and correct them. 

Phosphodiester bonds form between the deoxyribose of one molecule and the phophate of the next.

If DNA damaged by radiation,
P21 bins to CDK's during G1 instead of cyclins,
This stops the CDK'S from activating,
So the cell cycle stops.

A protein, P53, leads to the synthesis of P21 of which blocks cell division.
If the gene for P53 is defective, no P53 is produced. So, cell division continues.

G1: more cytoplasm
       protein synthesis
       build organelles
S: DNA replication
G2:growth and division of organelles
      building up of energy stores (ATP) 

 G1, G2, M = checkpoints