About Photograph 51
Photo 51 is an image of the more hydrated ‘B’ form of DNA. Franklin and Gosling had been experimenting with whether the humidity at which they kept the samples would affect the images. They had taken a series of images — photo 51 was taken at the highest humidity, around 92 per cent.
The darker patches indicate where the film has been repeatedly bombarded by diffracted x-rays from regular, repeating features within the molecule. The dark patches at the top and bottom of the picture, for example, represent DNA’s ‘bases’, the four parts of DNA which make up the genetic code — the patches are dark because there are so many bases all arranged in a regular fashion. You can work out the distance between bases in the structure by measuring the distance between the dark patches on the film and making a calculation based on how far the DNA sample was from the x-ray film and how it was orientated in the x-ray beam. In this case it’s 3.4 Ångstroms, a unit of measurement equivalent to 0.1 nanometre.
What about the cross shape of spots?
For people like Watson and Crick, who were already building models, this cross really spells out helix. Maurice Wilkins, who had worked on DNA separately from Franklin, showed this photo to Jim Watson when he came to visit and it really excited him — he raced back to Cambridge to the Cavendish Laboratory to tell Francis Crick about it. A lot has been said and written about that moment and some people think that Wilkins shouldn’t have shared the photo, but he had it legitimately as part of Rosalind’s papers (she was soon to leave for Birkbeck College) and he was keen that research on the structure progressed, particularly because he wanted the UK to beat Linus Pauling in the US to discovering the structure.
The reason that the cross indicates a helix is that the arms of the cross represent the planes of symmetry in a helix viewed from the side: the ‘zig’ and the ‘zag’, so to speak, of the turns of the helix. It’s difficult to see clearly, but there are ten blobs on each arm of the cross before you reach the large black patch at the top, and this tells you that there are ten bases stacked one on top of the other in each turn of the helix. In fact, one of the blobs is missing, the fourth if you count out from the centre of the pattern, and this indicates that one strand of DNA is slightly offset against the other.
If Franklin had all this information, why didn’t she suggest the structure?
Well, it’s difficult to say but one reason is probably that Rosalind had chosen to focus her attention on her x-ray photos of a less hydrated ‘A’ form of DNA, which appeared to show much more information and from which she hoped to calculate the structure directly, rather than build models. In fact, these photos of the ‘A’ form had revealed a key piece of information, namely that the two strands of DNA ran in opposite directions, although neither Rosalind nor the others had appreciated this, until Francis Crick realised its significance just before building the final model.
She didn’t turn her attention to photo 51 until early in 1953. You can see from her notebooks that once she did concentrate on it, she gleaned all the key information about the structure from it — I fully believe that given more time she would have cracked the structure. She was so close. Watson was surprised that she accepted the correctness of their model immediately upon seeing it — it must have been because she could see that it fitted so well with all of her evidence.
Maurice Wilkins and the x-ray diffraction camera he designed and built. Ray Gosling who was Maurice’s doctoral student before being reassigned to Rosalind Franklin by Randall (King’s Laboratory Director) while Wilkins was away on a vacation. Wilkins returned from vacation to learn that his camera, DNA and Gosling had been given over to Franklin. Gosling used the camera to take photograph 51 amongst of course, many other x-ray diffraction photographs of DNA. During this work Gosling continued to rely on Wilkins for technical guidance to improve the performance of the camera upto and subsequent to the taking of photograph 51.