The source of the DNA Rosalind Franklin used in her X-ray diffraction experiments was calf thymus. The highly purified sample had been prepared by Swiss chemist Rudolf Signer and became known as Signer DNA.
Signer distributed the material at a Faraday Society meeting in London in 1950. Maurice Wilkins received some of it and, after Franklin began work at King’s College London, the sample was passed to her and Raymond Gosling. Its purity and long, well-ordered fibers made it especially useful for producing interpretable diffraction patterns.
Franklin’s experiments did not involve DNA isolated from sperm, despite the familiar association between sperm cells and genetic material. Nor was her famous Photo 51 made from a bacterial plasmid or human tissue. Using the calf-thymus DNA, Franklin and Gosling studied the A and B forms of DNA and gathered evidence about the molecule’s helical structure, hydration, and phosphate backbone.