Firstly, the double stranded helical structure of the DNA is unwound by an enzyme called DNA helicase. This process of separation requires the breaking of the hydrogen bonds which link the base pairs. After the two strands are separated, free floating nucleotides form complementary pairs (A with T and G with C – purine to pyrimidine.) to form new strands. The process of DNA replication is termed semi-conservative as the original strands are used as a template for the formation of the new ones. Finally, the enzyme DNA polymerase forms the phosphodiester bonds which link the sugar phosphate back bone of the newly replicated DNA.