DNA is a double-stranded molecule that contains two complementary chains. Each chain is made up of nucleotides, which are made up of a sugar (deoxyribose), a phosphate group and one of four nitrogen-containing bases: adenine (A), guanine (G), cytosine (C) or thymine (T). The two strands are held together by hydrogen bonds between the bases: A bonds to T, and C bonds to G.

The structure of DNA was first proposed in 1953 by James Watson and Francis Crick, who were awarded the Nobel Prize in 1962 for their findings.

Chromosomes are composed of DNA molecules packaged into a tight double helix with proteins called histones that help keep the DNA stable. These packages can be thought of as tiny beads on a string: The histones form loops around the DNA, like beads on a bracelet, while the DNA forms a backbone in between each loop.

Last modified: October 9, 2022