Before replication DNA has two complementary strands with each base being bonded to a specific partner: adenine to thymine and guanine to cytosine. Enzymes in the nucleus first separate the two helices, and each of the strands then acts
as a template onto which join complementary nucleotides from solution. The nucleotides are then bonded to form the new sugar - phosphate backbones, and two new DNA molecules result, each identical to the original template molecule.
