01Semiconservative copying
Each new double helix keeps one original strand and one new strand (shown by Meselson and Stahl).
Helicase unzips DNA; topoisomerase relieves twisting ahead of the fork; primase lays down an RNA primer; DNA polymerase III adds nucleotides; DNA polymerase I replaces primers with DNA; ligase seals gaps.
02Leading vs. lagging
DNA polymerase only adds to the 3′ end, building 5′→3′. The leading strand is made continuously toward the fork. The lagging strand is made in short Okazaki fragments away from the fork, later joined by ligase.
Polymerase proofreads, giving about 1 error per 10 billion bases.
03Notebook box
Enzyme order: helicase → primase → DNA pol III → DNA pol I → ligase.
Worked example: A ligase mutation leads to what? → Okazaki fragments aren't joined; the lagging strand stays fragmented.