In this diagram, the diffraction of the X-ray beam causes an image with a helical arrangement to form as all the DNA molecules in a fibre are aligned in the same direction.
X-ray diffraction pattern obtained by M H F Wilkins and R Gosling in late 1950 showing a clear crystalline arrangement.
Before the new camera was set up, it was decided that Rosalind Franklin, who was joining the laboratory from Paris, would replace Wilkins in producing the x-ray diffraction images with the continued assistance of Raymond Gosling. Both Stokes and Wilkins continued working on the problem with the latter embarking on some rough tests with the old X-ray diffraction camera on various DNA specimens that produced an observed “X” crossed pattern. The X pattern of diffraction was created by the x-ray radiation scattering at right angles off the "zigzag" structure of the DNA chain. This interpretation was further supported when Franklin and Gosling produced the first “B” structure X-ray patterns in the late summer of 1951. This was a crucial development as it showed two observed states of DNA: crystalline “A” and semi-crystalline “B” (the best B structure diffraction photograph became known as “Photo 51”). The photos also supported the predicted observed readings of a helix that Alec Stokes had developed using the mathematical technique known as Bessel functions.
Plot of Bessel Functions for a smooth helix, named "Waves at Bessel-on-sea" by Alec Stokes who completed the calculations for the diagram over a single train journey.
It was now Maurice Wilkins and Rosalind Franklin disagreed over the direction of the research in finding the overall structure. Wilkins was keen on hypothetical model building while Franklin favoured a more systematic study of the structure. This parting of ways can be partially explained as stemming from the limitations of the x-ray diffraction process itself. For example, the evidence from the photos clearly pointed to a helical structure but this begged question: what type of helix? Helices in nature could occur in single, double and even triple strands and there was no clear indication, which was the right number. This is why the King’s College London attempt at model building proved to be a failure when the model made by Bruce Fraser showed a triple helix based on the chemical readings but was unable to fit with the rest of the x-ray data. A crucial piece of the puzzle was missing and related closely to DNA’s function of providing the genetic material for life: it was only when Jim Watson and Francis Crick came up with the base pair hypothesis that the double helix seemed the ideal form.
X-ray diffraction studies undertaken at King's College London provided part of the experimental structural data needed to solve the general structure of the DNA double helix. Yet, as important as these observations were other methods and disciplines were of equal importance in unravelling the overall structure, in particular the biochemical work of Erwin Chargaff and the biological insight of Jim Watson. X-ray diffraction work on DNA at King’s did not finish with the unveiling of the structure in March 1953 but continued for another decade as Wilkins and his team worked to test to the correctness of the "Watson-Crick" model.