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