Build a DNA Molecule

Where is Build A DNA Molecule?
Our Build A DNA Molecule interactive was built using Adobe Flash which is no longer supported in modern web browsers. We are actively converting our old Flash pieces into new formats that will be supported long into the future. We hope to have Build A DNA Molecule back online in Spring 2021.

Molecular machines copy DNA

Your body produces billions of new cells every day. Each time one of your cells divides, it must first copy the genetic information contained within its nucleus. Copying the genetic information in one cell using this activity would take more than 95 years*, yet molecular machines in your cells accomplish this feat in about 6 to 8 hours.

In order to speed up the copying process, DNA replication begins at multiple locations along each chromosome. The two DNA strands are pulled apart and copied in both directions at the rate of about 50 nucleotides per second.**

It would take nearly 5,000 strands of DNA laid side by side to equal the width of a human hair. At the magnification shown here (about 7 million X), an average human chromosome would be about 621 kilometers (385 miles) long, or roughly the distance between San Francisco and Los Angeles, CA.

See the size of a nucleotide as compared to other objects

Complementary base pairing

These models are based on the molecular structure of real nucleotides. The grey and white circles on the models represent partial positive and negative charges that form hydrogen bonds between complementary bases. These bonds work kind of like tiny magnets to hold the two DNA strands together. Complementary base-pairing ensures that DNA strands are copied accurately, with just a few errors for each round of replication. Forces between neighboring nucleotides stack the bases on top of one another and twist the DNA strands into a double-helix.


*Assuming a rate of 2 base pairs per second x the 6 billion base pairs you inherit from your parents.

**DNA replication in one direction is straight-forward. But replication in the other direction happens a little differently. For an explanation, see the the animations below.

Animations of DNA replication on HHMI's Biointeractive: Basic detail and Advanced detail

APA format:

Genetic Science Learning Center. (2016, March 1) Build a DNA Molecule. Retrieved January 26, 2022, from

CSE format:

Build a DNA Molecule [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2016 [cited 2022 Jan 26] Available from

Chicago format:

Genetic Science Learning Center. "Build a DNA Molecule." Learn.Genetics. March 1, 2016. Accessed January 26, 2022.