How DNA's double helix structure was discovered

The structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands twisted around each other. James Watson and Francis Crick determined this structure in 1953, building upon almost a century's worth of research from the scientific community. Their work was influenced in particular by two developments: first, the conclusion in the 1920s that DNA was made up of four nitrogenous bases: adenine, guanine, cytosine, and thymine; second, the discovery in 1950 that pairs of these bases—which form the subunits, or nucleotides, of DNA strands—occur in equal amounts.

Each nucleotide consists of a deoxyribonucleic sugar molecule attached to a phosphate group and one of the four bases. Adenine always links to thymine by two hydrogen bonds, and guanine always joins to cytosine by three.

Watson and Crick built stick-and-ball models of DNA to test their theories about its composition. An X-ray photograph of DNA produced by Rosalind Franklin helped them to realize its double-helix structure. The two strands of DNA bond together by the base pairs and twist around each other like a spiraling ladder.