DNA Replication

Follow a replication fork and discover why copying two strands takes different routes.

Experimental human helicase cutaway with cyan and mint fork DNA, a resolved cyan template segment inside the protein channel, and detailed grey protein helices and sheets.

Experiment · A fork moving right

Two strands. One chemical direction.

Follow new DNA from its primer to its growing 3′ end. Then remove ligase and inspect what remains after the RNA primers have been replaced.

DNA ligase

Open the forkSchematic · not to scale

Parental DNANew DNARNA primerPol = DNA polymerase

New-strand synthesis

5′ → 3′

On both strands

Displayed lagging fragments

Made by separate priming events

Internal junctions

Not matured

Within the displayed window

Open the fork

An established fork moves to the right. The original strands are antiparallel; new DNA needs a base-paired primer end.

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This generic fork schematic separates synthesis, primer replacement and ligation to make them inspectable; they overlap in cells. Protein composition, rates and strand looping are omitted. Ligase seals a backbone bond; it does not replace RNA primers or fill a missing stretch of DNA.

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Quick reference

Two templates, opposite directions

The strands of a DNA duplex run antiparallel. Each parental strand can serve as a template for a complementary new strand, leaving each daughter duplex with one parental and one new strand.

One direction for synthesis

DNA polymerases extend a primer at its 3′ end, so new DNA grows 5′ to 3′. At a moving fork, one new strand can be extended continuously while the other is made as successive Okazaki fragments.

Finish the backbone

After primers are removed and replaced with DNA, ligase seals the remaining nicks in the sugar-phosphate backbone. Without that joining step, adjacent pieces can be correctly paired to the template but remain unsealed.

Check your understanding Optional

After primer removal and replacement, what is ligase needed for?

Changing synthesis to the 3′ to 5′ direction.Separating the two parental strands.Sealing nicks between adjacent DNA fragments.

At the bench

Practise reverse complements and strand direction when reading templates and primers.

Open Sequence

Sources and model notes

Structure-based views of resolved experimental coordinates. Cutaways, category colours, camera choices and guided highlights are authored for teaching. Coordinates remain fixed; highlights are not experimental trajectories or simulation outputs.

RCSB PDB · Human replisome 7PFO

Experimental structure of a core human replication assembly; the model diagram is schematic.

HHMI · DNA replication

Background on continuous and discontinuous copying.