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*

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