When a cell prepares to divide, the two strands of the double helix separate, serving as a template, for the creation of a new strand. The new strands are formed by adding nucleotides, resulting in the formation of two DNA molecules, both composed of an original strand and a newly synthesized strand. For this reason, DNA replication is referred to as asemi-conservative process. Before DNA replication can occur, the two strands of the molecule must separate, a process carried out by a group of enzymes called helicases. The helicase unwinds the double helix and separates the two strands by breaking the hydrogen bond. Once helicase has unwound the double helix, replication can begin. Another enzyme, DNA polymerase always moves along the template strand in the same direction, adding one nucleotide at a time. DNA polymerase brings nucleotides into the position where hydrogen bonds could form. Each time a nucleotide is added to the new strand, which Is complementary to the template strand. The rule that one base always pairs with another is called complementary base pairing and ensures that the two new DNA molecules are identical in their base sequences to the parent molecule that was replicated.
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