DNA Replication

  • Created by: Jacqui2
  • Created on: 23-05-17 17:07

DNA Replication

Replication of DNA happens via the following sequence - 

  • The double helix unwinds
  • The hydrogen bonds between the bases break (DNA helicase) producing two seperate chains
  • Free DNA nucleotides bind with complementary nucleotides on both of the seperrate chains (A&T) (C&G)
  • DNA polmerase joins together the bound nucleotides through condensation reactions and the formation of phospo
  • diester bonds to form a chain
  • The other strand is replicated in small sections. The polynucleotide pieces produced are then joined together by DNA ligase
  • Each new strand is an exact copy of the old one
  • Because one of the chains in each new molecule was present in the old molecule, this process is called semi-conservative replication*
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Evidence for DNA Replication

Know as Semi-Conservative because each new DNA double helix contains one complete polynucleotide strand.

Meselson and Stahl produced evidence to support this:

  • They grew E.coli made with the isotope 15N
  • The bacterium took up the isotope and so all its DNA contained 15N
  • They were then transferred to a growth medium containing a lighter isotope 14N, where they reproduced
  • Extracts of DNA from the first offspring shown a lower density, since half of the DNA was made form the original strand and the other half of the new strand
  • At succeeding generation times, the DNA extracts were found to have a lower proportion of 15N
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