replication

Meaning: Replication is the copying of DNA before cell division. It is semiconservative: each new double helix keeps one parental strand and gains one new strand. Helicase unwinds the helix at an origin, forming replication forks, and topoisomerase relieves the supercoiling ahead of the fork. Primase lays down an RNA primer, and DNA polymerase extends it by adding nucleotides to the 3′ end, so new DNA is built 5′→3′. The leading strand is made continuously; the lagging strand is made as Okazaki fragments, which ligase joins after the primers are replaced. (In psychology, replication means repeating a study to see whether the result holds.)

On the MCAT: Passages describe a missing or inhibited enzyme and ask what fails: without ligase, lagging-strand fragments stay unjoined. Meselson–Stahl density labeling uses ¹⁵N and ¹⁴N: after one generation all DNA has intermediate density; after two, half is intermediate and half is light. Bacteria usually start from one origin on a circular chromosome, while eukaryotes use many origins.

Common confusion: Students think the lagging strand is built 3′→5′. Polymerase adds nucleotides in the 5′→3′ direction on both strands. The lagging strand is also built 5′→3′, but away from the fork, in pieces.