While they were not the first to study DNA, their model was the first to accurately depict the structure of the molecule as a double helix. They used X-ray diffraction images taken by Rosalind Franklin and Maurice Wilkins, along with their own research and models, to piece together the correct structure.
While Rosalind Franklin's X-ray diffraction images were crucial to the discovery, she unfortunately did not share in the Nobel Prize awarded to Watson, Crick, and Wilkins in 1962.
