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Resonance

March 2004
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The April 25 1953 issue of the journal Nature carried three classic papers collectively entitled ‘Molecular Structure of Nucleic Acids’ published back to back. The paper by Rosalind Franklin and R G Gosling was preceded by the paper of Watson and Crick that announced their famous model of DNA and one from the Wilkins group. For most biologists, the paper by Watson and Crick is very lucid, as it is not constrained by hard data and interpretations, whereas the one by Franklin and Gosling is elegant but technical. All the same, among the three, only Franklin’s had real data relevant to the model, her beautiful X-ray photograph of B-DNA and the parameters of the double helix calculated from it. Yet, it is ironic that the paper gives the impression of being just an afterthought, with the statement “Thus, our general ideas are not inconsistent with the model proposed by Watson and Crick in the preceding communication”. With her insight and original data, Rosalind Franklin would have been the most likely person who would have solved the structure of DNA if Watson and Crick had not seen her data and come up with the model first. But her approach would have been more formal, deducing it from first principles using Fourier transforms, as she was not in favour of short-cut methods such as model building. The full text of her classic paper is reproduced below.

S Mahadevan

Sodium thymonucleate fibres give two distinct types of X-ray diagram. The first corresponds to a crystalline form, structure A, obtained at about 75 per cent relative humidity; a study of this is described in detail elsewhere [1]. At higher humidities a different structure, structure B, showing a lower degree of order , appears and persists over a wide range of ambient humidity. The change from A to B is reversible. The water content of structure B fibres which undergo this reversible change may vary from 40-50 per cent to several hundred per cent of the dry weight. Moreover, some fibres never show structure A, and in thew structure B can be obtained with an even lower water content.

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