Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how does dna replicate before cell division
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.
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.
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.
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.
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.
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.
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.