DNA replicates by a semi-conservative method. When replication needs to occur, DNA helicase separates the two strands of DNA by breaking the hydrogen bonds between the strands. Each free strand now acts as a template. Free-floating DNA nucleotides are attracted to their complementary base pairs: A pairs with T and G pairs with C. DNA polymerase catalyses condensation reactions to occur in a 5 prime to three prime direction to form the sugar-phosphate backbone of the new complementary strand to the template. Hydrogen bonds then from between the paired strands and two new identical DNA molecules are formed. Replication is complete.