Explain semi-conservative DNA replication, identifying the key enzymes involved and summarising their functions.

The DNA double helix structure, proposed by Watson and Crick in 1953, was the basis of the hypothesis that DNA replicated in a semi-conservative manner. In 1958 the Meselson-Stahl experiment supported this idea through the use of isotopic nitrogen. In DNA replication, the double helix unwinds, forming 2 template strands onto which free nucleotides can bind. As such the two daughter DNA molecules each contain 1 strand from the parent DNA molecule and 1 newly formed strand. The reason this works is due to complementary base pairing relationship DNA nucleotides have: A (adenine) binds with T (thymine) and C (cytosine) binds with G (guanine). The main components of DNA replication include: - The parental strand: The strand that unwinds, forming 2 template strands that then become half of the daughter strands. - DNA gyrase (a DNA): an enzyme that relieves the tension of the DNA unwinding. - DNA helicase: an enzyme that unwinds the parental DNA strand by cleaving the hydrogen bonds between the complementary nucleotides. - Primase: an enzyme that forms the starting point of DNA replication, by adding an RNA primer complementary to a short segment of the DNA bases on the template strand. - RNA primer: a short strand of RNA that allows the replication to start, as DNA polymerase III can only add nucleotides to already existing ones. - DNA polymerase III: this is the enzyme that carries out the DNA replication, adding free nucleotides complementary to the template strand and forming the daughter molecules. - DNA polymerase I: replaces the RNA primer with the equivalent DNA sequence. - DNA ligase: an enzyme that catalyses the bond formation between the segments of DNA on the newly formed strand. Note: replication occurs in the 5' to 3' direction, thus there will be a leading and lagging strand. The lagging strand will create Okazaki fragments, that will be joined by DNA ligase.