Describe the sequence of events that occur during DNA replication.

DNA replication begins with the unzipping of the double stranded DNA by the enzyme DNA helicase, which breaks hydrogen bonds as it separates the strands. Another enzyme known as topoisomerase holds the two DNA strands apart whilst helicase works. Once the two strands are separate, DNA polymerase begins attaching free DNA nucleotides from the 5' end to the 3' end (replication ALWAYS occurs in the 5' to 3' direction). The nucleotides are added by complementary base pairing (A only binds to T and C only binds to G). One strand is known as the leading strand, as addition of nucleotides is continuous, whilst the other is known as the lagging strand because addition of nucleotides is fragmented and done in short sections called Okazaki fragments. Once DNA polymerase has added the fragments on the lagging strand, DNA ligase joins the fragments together to create a full strand. The process of DNA replication is semi-conservative because each new strand joins with one template strand to create a DNA helix that is half-old and half-new.