The replication fork in 3D

Drag to orbit, scroll to zoom. Watch the fork open and one new strand gets built backwards — in pieces. Click any part for a note.

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Strands — click to isolate

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Happening now

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RNA primers laid0 Okazaki fragments0 nicks sealed by ligase0 leading strand built0 lagging strand built0

What you are looking at

The replication fork — one parent duplex becoming two

The whole puzzle in one sentence: DNA polymerase can only ever add a nucleotide to a 3′ end, so it always builds 5′→3′ — but the two parent strands run in opposite directions, so that one rule sends one new strand chasing the fork and the other running away from it.

The strand that chases the fork is the leading strand: one polymerase clamps on and never lets go. The strand that runs away is the lagging strand: it has to wait for the fork to expose fresh template, start over with a new primer, and fill backwards — again and again. Those repeated pieces are Okazaki fragments.

The cast, in the order they act

Helicase Unzips the double helix, breaking the base pairs open. SSB proteins Coat the bare single strands so they can't snap back together. Primase Lays a short RNA primer — polymerase cannot start from nothing. Polymerase III Adds nucleotides to the 3′ end. Continuous on the leading strand, stop-start on the lagging one. Ligase Seals the nick left between fragments once the primers are replaced.

The result is two duplexes that are indistinguishable — no trace that one of them was assembled backwards, in pieces.

Check your understanding

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