DNA Molecules Have Twisted Ladder Shapes

Earlier work had shown that DNA is composed of building blocks called nucleotides consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogen bases-adenine (A), thymine (T), guanine(G), and cytosine(C). Phosphates and sugars of adjacent nucleotides link to form a long polymer.

Other key experiments showed that the ratios of A-to-T and G-to-C are constant in all living things. X-ray crystallography provided the final clue that the DNA molecule is a double helix, shaped like a twisted ladder.

In 1953, the race to determine how these pieces fit together in a 3D structure was won by James and Francis at the Cavendish Lab in England. They showed that alternating deoxyribose and phosphate molecules form the twisted uprights of the DNA ladder.

The rungs of the ladder are formed by complementary bases-A always paired with T and G always paired with C.