Figure 3. A helix viewed from the side has a “zig-zag” appearance; the repeating zigs and zags give rise to the characteristic cross pattern of diffraction spots.
Such a pattern of spots is highly suggestive of a helical structure, and so when James Watson saw Photograph 51 in January 1953, it spurred both him and Francis Crick to attempt to build a model. The photograph was shown to him by Wilkins, who had a copy because he was soon to take over the work; Franklin was shortly to leave King’s for Birkbeck College. However, Franklin was unaware that the photograph had been shown to Watson, and Wilkins had assumed that Watson had seen earlier “helix cross” diffraction patterns taken by Franklin.
But Photograph 51 was so much clearer than any of the earlier images. Moreover, it contained further information: the vertical separation between the spots of the helix cross is one tenth of the distance from the centre of the pattern to the large, diffuse diffraction “spots” at the top and bottom of Photograph 51, which arise from the regular stacking of the bases in the middle of the double helix. From this one can conclude that there are ten stacked bases per turn of helix.
Their model building led Watson and Crick to the concept of base pairing, which immediately suggested how DNA could be copied and hence a molecular mechanism for heredity. Photograph 51, and other data from Rosalind’s work during her time at King’s, were key to building the structure of DNA but never properly acknowledged until much later, after her death.
Franklin had always resisted model building, believing that it should be possible to calculate the structure from the diffraction patterns, particularly using her A-form photographs. But this approach proved unfruitful. Unaware that Watson and Crick were building their model, Franklin returned to Photograph 51 in early 1953 and in March drafted a manuscript proposing that a “helical structure [was] highly probable”, most likely a double helix with ten bases per turn with the bases on the inside and phosphate groups on the outside.
She even deduced from the absence of the fourth spot in each arm of the helix cross (counting outwards from the centre of the pattern, Fig.1), that the two chains would be separated by 3/8 of the pitch of the double helix, as indeed they are (Fig.2). She was so close to the answer, but only days later, Watson and Crick announced their model of the double helix.
And thus, seventy years ago, when Franklin and Gosling’s paper appeared alongside that by Watson and Crick in Nature, her work, and in particular Photograph 51, appeared merely to confirm their model, whereas in fact it had played a crucial role in its construction.