DNA helicase
/ HEE-li-case /
Before you can copy two pages stuck back-to-back, you have to peel them apart. DNA's two strands are wound around each other and held together by base pairs, so the very first job at a replication fork is to unzip them. The enzyme that does this is DNA helicase — the cell's molecular zipper-opener.
Helicase is typically a ring-shaped protein, often built from six subunits, that encircles one of the two DNA strands. Powered by chemical energy from ATP, it pulls itself along that strand and, like a wedge, forces the two strands apart as it goes, breaking the hydrogen bonds between the base pairs. It sits right at the tip of the fork and leads the way, opening the helix so the polymerases behind it always have single-stranded template to read. In bacteria the main replicative helicase is called DnaB.
Helicase matters because unwinding is the rate-limiting gateway to copying: nothing downstream can happen until the strands are separated. But unwinding has a cost — twisting the helix open ahead of the fork builds up torsional strain (overwinding), which is why topoisomerases must work just ahead to relieve it. Helicases are not unique to replication; related enzymes unwind nucleic acids in transcription, repair, and RNA processing, but the replicative helicase is the one that drives the fork forward.
Watch the fork tip: the helicase ring slides along one strand 5'-to-3', breaking A-T and G-C hydrogen bonds and splitting the helix into two single strands. Behind it, single-strand binding proteins immediately grab the bare strands so they cannot re-pair.
An ATP-driven ring unzips the helix at the fork tip.
Helicase breaks the hydrogen bonds between paired bases; it does not cut the sugar-phosphate backbone — that distinction separates it from nucleases and topoisomerases.