DNA replication
Two daughter helices, each half-old half-new.
Semi-conservative model. Each new DNA double helix contains one original (parental) strand and one newly synthesised strand. Confirmed by Meselson and Stahl (1958) using density-gradient centrifugation with ¹⁵N-labelled DNA.
Process at the replication fork:
- Helicase unwinds the double helix at an origin of replication; breaks H-bonds between bases.
- Single-strand binding proteins prevent re-annealing.
- RNA primase lays a short RNA primer.
- DNA polymerase III adds nucleotides 5'→3', using each parental strand as a template (complementary base pairing).
- On the leading strand synthesis is continuous (toward fork).
- On the lagging strand synthesis is discontinuous — short Okazaki fragments are made 5'→3' AWAY from the fork.
- DNA polymerase I replaces RNA primers with DNA.
- DNA ligase seals the gaps with phosphodiester bonds.
Result: two identical double helices, each semi-conservative.
Why semi-conservative matters. Each cycle of cell division produces faithful copies of the genome — heritability of traits across generations depends on this fidelity.
- Semi-conservative — one old + one new strand.
- Helicase opens; polymerase extends 5'→3'.
- Lagging strand: Okazaki fragments + ligase.