the replisome
/ REP-li-sohm /
Copying DNA is not done by one lonely enzyme; it is a team effort. All the proteins needed to copy DNA at a replication fork gather into one large, coordinated assembly that travels along the DNA together. That whole molecular machine is called the replisome — think of it as the mobile factory parked at each fork.
The crew includes a helicase that unwinds the double helix, single-strand binding proteins that coat the opened strands, a topoisomerase relieving the twisting strain ahead, a primase laying down short RNA primers, and two copies of the DNA polymerase — one for the leading strand and one for the lagging strand — each clamped onto the DNA by a sliding clamp loaded by a clamp loader. Strikingly, the two polymerases are held together in one complex even though they copy strands running in opposite directions; the lagging-strand template is thought to loop out (the 'trombone' model) so that both new strands can be made in the same direction of fork movement by a single coordinated machine.
The replisome matters because it explains how copying is so fast and so coordinated: rather than separate enzymes finding the DNA one at a time, a single integrated machine unwinds, primes, copies, and proofreads in a tightly choreographed way, moving along at hundreds to a thousand or more bases per second in bacteria. It is one of the cell's most impressive molecular machines, and its parts are conserved in broad outline from bacteria to humans even though the specific proteins differ.
In E. coli the replisome is built around DNA polymerase III holoenzyme: a helicase (DnaB) at the front, primase (DnaG) hopping in to make primers, two or three polymerase cores tethered by beta sliding clamps, and SSB coating the single strands — all moving as one unit.
Many proteins, one coordinated machine at each fork.
The replisome is not a permanent organelle — it assembles at origins, runs along the DNA, and disassembles when the fork is done; the same logic applies in cells far more complex than E. coli.