What are the steps in semi-conservative DNA replication?
- Initially, the DNA is present in the form of a double helix, comprising 2 polynucleotide strands joined by hydrogen bonds between complementary bases. The enzyme helicase unwinds and separates the 2 strands of DNA in the double helix by breaking hydrogen bonds between the base pairs. This is important so that both strands may act as templates for DNA replication.
2) In the nucleus free DNA nucleotides are present which join to the complementary exposed bases on each polynucleotide strand by complementary base pairing. Adenine forms 2 hydrogen bonds with thymine, while cytosine forms 3 hydrogen bonds with cytosine. Each strand therefore acts as a template to build a new strand.
3) The enzyme DNA polymerase then joins the new nucleotides to each other by condensation reactions, forming strong covalent phosphodiester bonds to produce the sugar-phosphate backbone.
4) Another enzyme winds the new strands up to form double helices, producing 2 identical molecules of DNA which are exact copies of the original molecule used for replication. As each new molecule of DNA contains one 'new' strand and one 'old' strand of DNA, the method of replication is termed 'semi-conservative'.
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