Nib asked
How does DNA replicate before cell division?
Each new DNA molecule keeps one old strand and gets one new one (semiconservative copying), built by polymerase on each unzipped template: smoothly on the leading strand, in Okazaki fragments on the lagging strand.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1three guesses
Before a cell divides it copies its DNA. Three ideas competed: keep the old pair whole, one old strand per copy, or old and new mixed.
2the experiment
In nineteen fifty-eight Meselson and Stahl grew bacteria in heavy nitrogen, then switched them to light. After one copy, DNA formed one band in the middle.
After one copy: a single band of intermediate density, which rules out the conservative model.
3the verdict
That rules out the first idea. Two copies gave a middle band and a light band, which only one idea explains: one old strand per copy.
4the fork
Helicase unzips the double helix at a fork, breaking the hydrogen bonds. Now each old strand is a template, a master copy for building a new partner.
5the leading strand
Polymerase adds matching letters, A with T, G with C, but only onto a three-prime end. Below, that follows the fork, so it builds smoothly.
DNA polymerase only extends a 3-prime end, so the strand whose template runs toward the fork is built continuously.
6the lagging strand
The top strand runs the wrong way, so it's built backward in short pieces, Okazaki fragments, each started by primase, then glued by ligase.
7the whole idea
So here's the whole copy: helicase unzips, polymerase builds a new partner on each old strand, and every new molecule is half old, half new.
Each new double helix is half old and half new, so a cell can hand down a true copy while keeping one original strand.