DNA Replication and the Replication Fork

DNA has to be copied base by base, and that's only possible if the two strands are pulled apart so each base is exposed. The enzyme helicase does the prying, breaking the hydrogen bonds between paired bases and moving along the molecule like a zipper pull, opening up a Y-shaped region called the replication fork. Because the two strands are complementary, not identical, each one carries enough information on its own to specify a full new partner strand through base pairing: A with T, G with C.

Unwinding the helix doesn't come for free. As helicase unwinds DNA at the fork, the sections of DNA still wound up ahead of it get twisted tighter, like over-twisting a rope by spinning one end. This buildup of torsional strain, called supercoiling, would eventually stall the fork if left alone. Topoisomerase fixes this by cutting one or both strands ahead of the fork, letting the DNA swivel to release the twist, then resealing the cut. Helicase creates the problem; topoisomerase solves it, just ahead of where helicase is working.

Once the strands are separated, each old strand acts as a template, and new nucleotides pair up against it to build a brand new complementary strand. This is why replication is called semiconservative: every finished double helix has one original strand and one newly made strand, never two of a kind. This setup connects directly to the next piece of the machinery, DNA polymerase, which actually reads each template strand and adds the matching nucleotides, working alongside primase and ligase to finish the job.