AI Tutor
Every single time your skin heals from a scratch, billions of new cells must be created — and each one needs an exact duplicate of your 3-billion-letter genetic instruction manual. How does a cell copy that much information in just a few hours without turning its DNA into an unreadable knot?
To solve that puzzle, scientists first had to figure out whether cells copy DNA by photocopying the entire original molecule or by splitting it in half.

The Semiconservative Rule

In 1958, biologists Matthew Meselson and Franklin Stahl grew bacteria in heavy nitrogen isotopes to trace how DNA splits, proving the semiconservative model of replication. In this process, the double helix unzips so each original strand acts as a template for a brand-new partner.
The word semiconservative means "half-conserved": each freshly minted double helix consists of one conserved parent strand and one newly built daughter strand.
In the broader cell cycle context, DNA replication occurs strictly during the Synthesis (SS) phase of eukaryotic interphase, prior to mitosis. This ensures that both resulting daughter cells inherit identical diploid genomes when the cell divides.
Knowing that the strands separate is one thing, but how does a cell physically wedge open two tightly coiled strands in the middle of a crowded nucleus?

Unzipping the Helix: DNA Helicase

DNA features an antiparallel architecture: the two parental strands run side-by-side in opposite directions, one 5′→3′5' \rightarrow 3' and the other 3′→5′3' \rightarrow 5'. This opposing orientation dictates asymmetric fork mechanics during replication, as enzymes can only process DNA in one direction.
An enzyme called DNA helicase acts like a molecular zipper pull, breaking the weak hydrogen bonds between base pairs (AA with TT, and CC with GG) to unwind the double helix. The active, Y-shaped opening where the strands split apart is called the replication fork.
Now that both single strands are exposed, how does the cell read the exposed letters and construct the matching partners?