The first X-ray diffraction pattern of DNA was obtained in 1938. William Astbury and Florence Bell used X-ray fibre diffraction to study DNA extracted from biological material, producing evidence that the molecule had a regular, repeating structure.
Their work measured a repeating distance of roughly 3.4 angstroms between stacked nucleotides. Astbury described the bases as resembling a “pile of pennies,” an image that captured the apparent regularity of the molecule even though the researchers did not yet understand its helical form correctly.
Some historical accounts mention 1937 because Astbury began working with DNA samples and obtained early patterns around that time. However, the landmark results were presented and published in 1938, which is the standard answer to this quiz question.
The famous Photo 51 came much later, in 1952. Rosalind Franklin and Raymond Gosling’s clearer image revealed the helical character of DNA and helped Watson and Crick build their 1953 double-helix model.