In the autumn of 1951, James Watson (left) and Francis Crick (right) started work on unravelling the structure of DNA. It was known at the time that DNA was present in the nucleus of every living cell, and that it had something to do with heridity, but without a knowledge of its structure little more could be understood about how it actually worked. They approached the problem with the same methodology that had been pioneered by Linus Pauling, who after years of exhaustive study had earlier discovered that many proteins exhibited a helical structure. Their task was to devise a structure which would account for all the chemical and X-ray evidence, and at the same time be consistent with all the structural features of the units involved - such as the size and shape, bond angles and lengths, configurations and conformations. X-ray diffraction photographs of DNA fibres taken by Rosalind Franklin and Maurice Wilkins showed a distinctive X-shape, which was characteristic of a helix structure, but strong arcs on the meridian indicated a repeating structure 3.4 apart. And from the chemical evidence, it was known that part of the structure was comprised of 4 heterocyclic bases, adenine (A), guanine (G), cytosine (C) and thymine (T), somehow linked together with sugar units and phosphates. One of the biggest puzzles was that although the proportion of these bases varied from one DNA to another, it was always found that the number of A = T, and G = C. Adenine Guanine Cytosine Thymine The 4 bases which make up DNA(Click on each image to get its 3D molfile). Using molecular models, Watson and Crick devised a structure in which all of the building blocks fitted together without crowding or overlapping, and which permitted a great deal of stabilisation by Hydrogen bonds. Moreover, these Hydrogen bonds were of the kind that Pauling had shown to be the strongest and therefore the most important for determining structure in proteins, namely N...
