How was the structure of DNA discovered?

The double helix structure of DNA was worked out in 1953 at Cambridge by James Watson and Francis Crick. They did not rely on a single experiment. They combined physical model building, chemical data on the ratios of DNA's bases, and X-ray diffraction images made at King's College London by Rosalind Franklin, Raymond Gosling and Maurice Wilkins. Their model and Franklin and Gosling's data were published side by side in Nature on 25 April 1953. [1] [2] [3]

In the early 1950s the Cambridge pair made physical models to narrow down the possibilities, an approach advanced by Linus Pauling. The King's College team, Wilkins and Franklin, took an experimental route based on X-ray diffraction images. Chargaff had earlier shown that DNA composition varies among species but that adenine roughly equals thymine and guanine roughly equals cytosine. He could not explain why. Crick learned of these findings in the summer of 1952 and added them to the model. [1] [3]

Franklin found that DNA exists in two forms depending on humidity, and this helped her deduce that the phosphate part of the molecule lies on the outside. Her wet, or B-form, images showed the characteristics of a helix. Photograph 51, taken with Gosling in May 1952 after a 62-hour exposure, was the best of these. Its cross-shaped pattern of spots is strongly suggestive of a helix, and its spacing indicates ten stacked bases per turn. [1] [2]

Watson saw the photograph in January 1953, when Wilkins showed him a copy. According to King's College London, Franklin was unaware of this, and the image spurred Watson and Crick to build their model. In March 1953 Franklin drafted a manuscript proposing that a helical structure was highly probable, probably a double helix with the bases inside and phosphates outside. [2]

Watson and Crick's first model, made after Watson attended a Franklin lecture in 1951, failed, and their unit head told them to stop DNA research. Their later model had two nucleotide chains, one running up and the other down. Matching base pairs interlocked in the middle, which kept the distance between the chains constant. They were briefly misled by an incorrect understanding of the structure of thymine and guanine. After American scientist Jerry Donohue made a suggestion, Watson cut new cardboard shapes, and A fitted with T and C with G, joined by hydrogen bonds. They were also concerned about being scooped by Pauling, whose proposed DNA structure turned out to be wrong. [1] [3]

The base-pairing idea immediately suggested how DNA could be copied, giving a molecular mechanism for heredity. King's College London notes that Franklin's data were key to the structure but were not properly acknowledged until after her death, and that her paper appeared merely to confirm the model when it had in fact helped build it. [2]