DNA replicates via semi-conservative replication. This means one strand acts as the template strand. The enzyme DNA Helicase breaks the hydrogen bonds between base pairs in DNA. This unwinds the DNA, exposing the template strand. DNA free nucleotides are able to bind to the exposed template strand via complimentary base pairing. This process allows hydrogen bonds between bases to form. The sugar-phosphate backbone is formed by the formation of phosphodiester bonds between the phosphate group of one nucleotide and the deoxyribose sugar of the next nucleotide. This is catalysed by the enzyme DNA Polymerase, in the 5' to 3' direction.
Extension: This occurs continuously on the leading strand, but forms fragments called Ozaki fragments on the lagging strand. The fragments are then joined by DNA ligase.