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 the near future.

Cells work quickly

Every day, your body makes billions of new cells. Each time a cell divides, it must first copy all its DNA—the genetic information contained within its nucleus. In a human cell, that’s 6.2 billion nucleotides.

The process cells use to copy DNA is called DNA replication. Using this activity, copying one human cell’s worth of DNA would take more than 95 years.* Yet the molecular machines in your cells accomplish this feat in 6 to 8 hours.

This speed comes from two factors. One, cellular machinery is fast. It adds about 50 nucleotides per second!** Two, DNA replication begins at multiple places along each chromosome. Molecular machines pull the two DNA strands apart and copy them in both directions at once.

Watch a real-time 3d animation of molecular machines copying DNA. DNA replication is a tightly coordinated process that relies on multiple proteins working together.

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

The two strands of a DNA molecule are held together through complementary base pairing: A pairs with T, and C pairs with G.

A DNA molecule consists of two strands. Each strand is made up of a string of tiny building blocks called nucleotides. There are four types of nucleotides in DNA, represented by the letters A, T, C, and G. The nucleotides in opposite strands are joined together through complementary base pairing: A pairs with T, and C pairs with G.

Because of complementary base pairing, each DNA strand contains within it the information needed for building its partner. During DNA replication, the two DNA strands are pulled apart, and each original serves as a template for building a new partner strand. This process ensures that DNA is copied accurately, with just a few errors (out of 6.2 billion letters!) for each round of replication.

Structure of a nucleotide

The models on this page are based on the molecular structure of real nucleotides. The grey and white circles on the models represent partial positive and partial negative charges that form hydrogen bonds between complementary bases. These bonds work kind of like tiny magnets to hold the two DNA strands together. 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.

APA format:

Genetic Science Learning Center. (2016, March 1) Build a DNA Molecule. Retrieved September 23, 2022, from

CSE format:

Build a DNA Molecule [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2016 [cited 2022 Sep 23] Available from

Chicago format:

Genetic Science Learning Center. "Build a DNA Molecule." Learn.Genetics. March 1, 2016. Accessed September 23, 2022.