BIOLOGY · INTERACTIVE TOOL
Open a living replication fork — drag it, zoom in, and click helicase, primase, polymerase and ligase to see exactly what each one does. Then play the whole process step by step.
FREE · NO SIGN-UP · RUNS IN BROWSERA full, touchable schematic of DNA replication. Pick a story stage to build the fork piece by piece, switch on animation to watch it move, or tap any part to inspect it.
DRAG TO PAN · SCROLL OR PINCH TO ZOOM · TAP ANY PART TO INSPECT IT
The diagram above frozen in time — here is the same process told from start to finish, the way it unfolds inside a cell.
Initiator proteins bind an origin sequence and open a bubble; helicase is loaded onto the strands, creating two forks that race away from each other.
Helicase breaks the base-pair hydrogen bonds and advances; topoisomerase relieves the supercoiling ahead while SSB coats the exposed strands.
Primase lays a short RNA primer — the free 3'-OH handle that every DNA polymerase needs before it can start building.
Pol III extends one primer continuously toward the fork, reading its template 3'→5' and writing the new strand 5'→3' without stopping.
On the other template, Pol III works away from the fork in short Okazaki fragments, each started by a fresh RNA primer.
Pol I replaces primers with DNA, ligase seals the nicks, and proofreading plus mismatch repair leave roughly one error per billion bases.
Seven molecular machines, one job each. This is the cheat-sheet for every part you can click in the diagram.
The ring-shaped motor that pries the double helix open and drives the fork forward, one ATP per step.
Relieves the over-winding and supercoils that build up ahead of the fork so the DNA doesn't snap.
Hold the freshly separated strands open, straight and protected until polymerase arrives.
Writes the short RNA starting block that gives polymerase something to hold and extend.
Pol III extends both strands; Pol I later chews out RNA primers and fills the gaps with DNA.
Spends ATP to seal the backbone nicks between Okazaki fragments into one unbroken strand.
Everything people usually ask about this replication fork — what's real, what's simplified, and why the lagging strand works the way it does.
Pick a story stage — Origin, Unwinding, Leading, Lagging or Finishing — to watch the fork assemble piece by piece, or dive straight into the full diagram. Tap or click any enzyme or strand to see what it does plus a quick fact, drag to look around, and scroll, pinch, or use the zoom buttons to get closer. The Labels and Animate pills and the guided tour sit in the top-left of the canvas.
It's a carefully-styled schematic, not a microscope image. The enzymes, directions and order of events follow the standard textbook model of a bacterial replication fork, but real enzymes are nanoscale protein blobs that jostle and overlap — here they're drawn larger, spaced out and labelled so every part is easy to find, click and tell apart.
Both are built 5'→3', but the leading strand's template faces the fork the right way, so it's copied in one continuous stretch toward the fork. The lagging template faces the other way, so it's copied in short Okazaki fragments pointing away from the fork — each with its own primer — which are stitched together afterwards.
DNA polymerase can only add nucleotides to a free 3' end — it can't start fresh and can't work backwards. On the lagging strand, the fork keeps exposing new template "behind" the only direction polymerase can travel, so the cell re-primes and builds short chunks away from the fork, then joins them up.
The reaction that joins nucleotides needs a free 3'-OH on the last base of the growing strand — the incoming nucleotide attaches there. A DNA chain has only one 3' end, so growth can only run one way: toward the 3' end.
It plays gentle looping motion — the helicase motor spinning, nucleotide beads flowing down the growing strands, enzymes drifting — so the fork reads as a living machine rather than a static picture. Everything stays clickable while it runs.
No. The diagram runs entirely in your browser. Nothing you click, zoom or explore is ever sent to a server, logged, or stored — and it's completely free with no sign-up.
Yes. Tapping selects a part, dragging pans the view, and pinching zooms in or out, so the whole tool works with touch exactly as it does with a mouse.