THE COMPLEMENTARY STRUCTURE OF DNA

This paper reviews very briefly the recent work on the “structure” of the sodium salt of desoxyribonucleic acid (DNA). By structure is meant not the chemical formula but the spatial arrangement of the atoms. The structure revealed by X-rays is the general structure-that is, the featmes common to DNA from different sources, since the X-rays only respond clearly to the repeating part of the structure and effectively say nothing about the exact sequence of the bases, which probably does not repeat. The general chemical formula has been described in these PROCEEDINGS by Professor A. R. Todd’ and will be taken as known. The IL-ray work is carried out on fibers drawn from extracted DNA from a variety of sources. The early studies were by A&bury and Bell.2 Almost all the recent work has been done at King’s College, London, by Drs. Wilkins and Franklin and their colleagues. Preliminary studies are reported in Wilkins, Gosling, and Seeds3 More detailed reports on the X-ray data have been given in two papers by Franklin and Gosliag.4 Evidence for the helical nature of the structure and for some of its other features has been presented in a preliminary way by Wilkins, Stokes, and Wilson6 and by Franklin and Gosling.6 More detailed interpretations of the crystalline form have been given by Franklin and Gosling’ and by Wilkins, Seeds, Stokes, and Wilson.8 In addition, Wilkins and Randall9 have shown that the oriented sperm heads give a very similar X-ray pattern to that from extracted DNA.. The interpretation of their results offered by the experimentalists is in broad agreement with the type of structure proposed by Watson and Crick,l” though the precise dimensions of this model may be incorrect. A more detailed description of the proposed structure and an account of the methods used in arriving at it have been given by Crick and Watson.” As is well known, this structure consists of two helical phosphate-sugar chains, winding round the same axis, the chains running in opposite directions. The chains are linked together by hydrogen bonds between their bases, a base from one chain being paired off with the opposite base on the other chain. It is postulated that specific pairing occurs and that the only pairs that will normally fit into the structure are adenine with thymine, guanine with cytosine (or its derivatives). The model places no restriction on the sequence of bases along a single chain. If the sequence of the bases on one chain were known, the sequence on the other chain could be written down because of the specified pairing. Thus each chain, with its bases, can be regarded as the “complement” of the other. The salient features of this structure are as follows: a) There are two chains in the structural unit.12 This is strongly suggested by the observed density (which rules out three chains). Model-building shows that a single-chain structure is unlikely. The X-ray data also show “two-ishness.”