The Structure of DNA — The Watson–Crick Double Helix

Once the three landmark experiments of Griffith, Avery-MacLeod-McCarty, and Hershey-Chase had jointly established that DNA is the molecule of heredity, the next essential question was structural: what is the actual three-dimensional shape of the DNA molecule, and how does that shape allow it to store and faithfully pass on genetic information? This question was answered in 1953 by James Watson and Francis Crick, working at Cambridge, who proposed the now-famous double-helical model of DNA — built substantially on X-ray diffraction data of DNA fibres produced by Rosalind Franklin and Maurice Wilkins, together with an important quantitative chemical regularity discovered earlier by Erwin Chargaff.

A DNA molecule is a polymer built from repeating monomer units called nucleotides, and each nucleotide is itself made of three chemically linked parts: a nitrogenous base, a five-carbon (pentose) sugar called deoxyribose, and a phosphate group. DNA contains four different nitrogenous bases, divided into two structural classes: the purines, adenine (A) and guanine (G), which have a larger, two-ring chemical structure; and the pyrimidines, cytosine (C) and thymine (T), which have a smaller, single-ring structure. A nitrogenous base linked to the deoxyribose sugar alone (without a phosphate) is called a nucleoside; when a phosphate group is further linked to the sugar, the resulting unit is called a nucleotide. Successive nucleotides are joined together into a long chain, or polynucleotide strand, by phosphodiester bonds, which link the 3'-carbon of one sugar to the 5'-carbon of the next sugar's phosphate group — giving every polynucleotide strand a distinct chemical direction, or polarity, running from its 5' end to its 3' end.

Chargaff's rule, an empirical finding from earlier chemical analysis of DNA samples from many different organisms, states that in any double-stranded DNA molecule the amount of adenine is always equal to the amount of thymine (A = T), and the amount of guanine is always equal to the amount of cytosine (G = C) — a regularity that, at the time, hinted strongly at a specific, fixed pairing between these particular bases, though its structural explanation had to await Watson and Crick's model. …