DNA replication

DNA replication copies the entire genome so that both cells formed by division inherit the same genetic information. It is semiconservative: the two strands of the double helix separate, each acts as a template for a new partner strand, and every finished molecule holds one old strand and one new one. A classic experiment by Matthew Meselson and Franklin Stahl, using bacteria grown in heavy and then light nitrogen, confirmed this.

Copying begins at origins of replication; a bacterial chromosome usually has one, while human chromosomes have many. Helicase unwinds the helix, single-strand binding proteins keep the strands apart, and topoisomerase eases the twisting ahead of the fork. Primase lays down short RNA primers, and DNA polymerase extends them, adding nucleotides only in the 5' to 3' direction. The leading strand is therefore made in one continuous piece, and the lagging strand in short Okazaki fragments that DNA ligase later seals together.

Proofreading by DNA polymerase and later repair systems keep mistakes rare. In eukaryotes, replication happens during the S phase of the cell cycle, and the human genome takes several hours to copy.