THE PROCESS OF REPLICATION
During DNA replication, the two strands of double helix unwind. Each strand serves as a template for the formation of a new, complementary strand.

DNA helicase enzymes "'walk" along the DNA molecule, opening the double helix as they move. Once the strands are separated, helix- destabilizing proteins bind to single DNA strands, preventing re-formation of the double helix until the strands are copied. Enzymes called topoisomerases produce breaks in the DNA molecules and then reconnect the strands, relieving strain and effectively preventing tangling and knotting during replication.

DNA polymerase adds new nucleotides to a growing strand of DNA. Because DNA polymerase must adhere to an existing template, an RNA primer—four or five nucleotides—is first created at the site of replication. The RNA primer is synthesized by primase, an enzyme that is able to start a new strand of RNA opposite a DNA strand. After a few nucleotides have been added, the primase is displaced by DNA polymerase, which can then add subunits to the 3' end of the short RNA primer.