3.4 DNA Replication

Class Presentation:

DNA Replication animations:

St. Olaf’s DNA Replication (very clear)

Another clear one from Wiley.

HHMI BioInteractive has a whole bunch of them.

Nicely illustrated one from Harvard.

Nobel.org’s DNA Replication Game

And another complementary base-pairing game from Pfizer


A bit more complex:

John Kyrk’s complicated molecular animation.

The Meselsohn Stahl experiment from Sumanas and from LearnersTVb (They discovered that DNA replication was semi-conservative).


Revision materials:

Click4Biology pages: Core & HL

Wikipedia page


  1. HI Steve,

    It appears as if the animations from St Olaf’s and Wiley are no longer working or have been moved. However, the Harvard one is still fine.

  2. Thanks for the heads-up, Liam. They’re all working now.

  3. Dear Stephen,
    Seems like the class presentations are not showing, is there a specific reason or could just be my browser? I tried to check if they were on the OCC but I didn’t find them either. Thanks for your prompt response. Blessings

    • I would refresh the page. If that doesn’t work, clear your ‘cache’ settings in whichever browser you’re using.

  4. Hi Stephen. I am having a problem with answering this question:
    “explain why dna must be replicated before mitosis”
    can you please help me?


    • Hi Maia,

      The two daughter cells need to be genetically identical and to achieve that, an exact copy of all the DNA needs to be made. This takes place in the synthesis stage of the cell cycle, then the nucleus divdes into two identical diploid nuclei through mitosis, before the cell itself finally divides in cytokinesis.

      I hope this helps.


  5. Afternoon Stephen,
    Why is the end product of DNA replication in a human somatic cell “23 pairs of sister chromatids” and not 46 pairs of sister chromatids?
    Thank you

    • Hi Aaron,

      On the Essential Biology sheet, 46 pairs of sister chromatids (or even 23 pairs of pairs of sister chromatids) would make more sense. Thanks for pointing it out.


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