Caveat Academics · Study Skills · Free tool

Watch DNA copy itself,
one enzyme at a time.

An animated replication fork you can step through: helicase, primers, the leading and lagging strands, Okazaki fragments and ligase, with a check at every stage and base-pairing practice.

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One replication fork, step by step

Move through the eight stages or play them in order. Each stage changes the model, explains what happened, and asks one question.

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Six enzymes, six jobs

Most test questions on replication come down to which enzyme does what, and in which direction.

UnzipsHelicase

Breaks the hydrogen bonds between base pairs and opens the fork.

StartsPrimase

Builds a short RNA primer so polymerase has a 3' end to add to.

BuildsDNA polymerase III

Adds nucleotides 5' to 3', matching each to the template, and proofreads.

SwapsDNA polymerase I

Removes RNA primers and fills the space with DNA.

SealsDNA ligase

Forms the backbone bond that joins Okazaki fragments.

RelievesTopoisomerase

Cuts and rejoins DNA ahead of the fork so it does not overwind.

What this resource is

Replication is easy to memorize as a list of enzymes and hard to picture. Watching the fork open, the leading strand run ahead and the lagging strand fill in backward makes the direction rule stick: new DNA always grows 5' to 3'.

Part of Academics · Study Skills in the Caveat Suite

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Outcome
An animated, steppable model of DNA replication at one fork, with an explanation and a check question for each of eight stages, and base-pairing practice for DNA strands and RNA primers.
Who it is for
High school biology students, including those preparing for AP Biology, and the teachers, tutors and families who review with them.
Format
A browser model with eight stages, eight checks and unlimited base-pairing practice. Print study notes with every stage and answer.
Access
Free. No account, no sign-up. Your progress stays in this browser.
Time
About 20 minutes for all eight stages and a few practice strands.
Space
Academics · Study Skills primary space
Support
Questions or a problem with this tool: team@caveatacademic.com

Sources

  • OpenStax, Biology 2e, section 14.4, "DNA Replication in Prokaryotes," and 14.5, "DNA Replication in Eukaryotes" (primer length, Okazaki fragment sizes, enzymes): openstax.org
  • Meselson, M. and Stahl, F. W. (1958), "The replication of DNA in Escherichia coli," Proceedings of the National Academy of Sciences 44(7): 671-682: pnas.org
  • National Human Genome Research Institute, Talking Glossary of Genomic and Genetic Terms (DNA replication, Okazaki fragment): genome.gov

The model is simplified for high school biology: it shows one side of one fork, short primers and fragments only a few bases long, and the bacterial enzyme names (DNA polymerase III and I). Real forks move in both directions from each origin, eukaryotes use different polymerases for the same jobs, and the lagging strand template loops so one polymerase complex can copy both strands.