By Rachel K. Sobel

If the discovery of DNA conjures up any names, it's those of James Watson and Francis Crick, the scientists who in 1953 first described the ladderlike double helix. But they're not the only innovators deserving credit for the molecular revolution now at hand. The largely unsung heroine in this story of discovery is British physical chemist Rosalind Franklin. For it was Franklin who, at King's College London, during the summer of 1952, produced an image of the inner anatomy of DNA.

Franklin used X-ray diffraction–a technique that involves shooting X-ray beams through a crystallized substance to produce a pattern of its atoms on film–to make the picture. Across town, meanwhile, Watson and Crick were attacking the problem by building molecular models. But when Watson got a sneak peek at Franklin's photo in early 1953 (thanks to a treacherous former colleague), he had a Eureka moment. "The instant I saw the picture my mouth fell open and my pulse began to race," wrote Watson in The Double Helix, his 1968 personal account of the discovery. He went back to Cambridge and told Crick that they had the answer: DNA was a double helix supported on the outside by a sugar-phosphate backbone.

It's clear that Franklin's deft skill with X-ray diffraction made a profound impact. "They absolutely had to have the information from Franklin's photograph," says Horace Freeland Judson, director of the Center for History of Recent Science at George Washington University. "It wasn't the only thing, but it was crucial." Yet it's still unclear if Franklin herself ever knew how much her picture played a role in the discovery. The famed paper gave only scant credit to her work, and Watson's account of the photo wasn't published until 1968, a decade after Franklin died from ovarian cancer. (She also missed being considered for the Nobel Prize in 1962 alongside Watson and Crick, since the rules prohibited such a posthumous award.)

"It's sad in hindsight," says Aaron Klug, once a coworker of Franklin's and now a senior scientist at the Medical Research Council in Cambridge, England. This kind of "thievery," he notes, is not that unusual in science. Even more heartbreaking: Franklin's lab notebooks show that she was merely a step and a half away from figuring out the structure on her own.