DNA Replication
A semi-conservative copying machine
Summary
Before a cell divides, the double helix is opened up by helicase; each strand is read by DNA polymerase, which builds a new strand with complementary bases. The result: 2 identical double helices, each containing 1 old + 1 new strand.
In depth
DNA replication is a key step in cell division. The semi-conservative copying of DNA, the mechanism Watson and Crick hinted at in 1953, was proved by the Meselson-Stahl experiment in 1958. When the double helix opens, each strand serves as a template; DNA polymerase reads each template in the 3'→5' direction and builds its partner in the 5'→3' direction. The bases are added according to Watson-Crick pairing: A-T (2 hydrogen bonds), G-C (3 hydrogen bonds). Because the polymerase works in only one direction, the two strands are handled differently: the leading strand is made continuously, while the lagging strand is made in short Okazaki fragments that are later joined by DNA ligase. In a human cell all 3 billion base pairs are copied in about 8 hours, roughly 100,000 bases per second across the genome. The error rate is ~1 in 10⁹, incredibly low thanks to proofreading and repair mechanisms.
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