The double helix structure unzips which is catalysed by the DNA Helicase enzyme. DNA Helicase overcomes hydrogen bonding between nitrogenous bases. Two strands are produced, each one acts a template. Free DNA nucleotides align with exposed bases following complementary base pairing rules. DNA Polymerase catalyses the joining of adjacent nucleotides to from the sugar-phosphate backbone of the new DNA strand. Hydrogen bonds then form between the bases on each strand and the strands twist to form a double helix. 3 hydrogen bonds form between guanine (G) and cytosine (C), and 2 hydrogen bonds form between adenine (A) and thymine (T). Two identical DNA molecules are produced. Each of the new molecules contains a single strand from the original DNA molecule and a single new strand. Would amount to 6 marks. Each sentence in the first paragraph is a mark.