replication, complementary base pairing

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DNA-semi-conservative replication

  • 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.

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cell cycle

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

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