DNA Double Helix

In February 1953, working at the University of Cambridge, James Watson and Francis Crick built a physical model of DNA's molecular structure that solved a problem biology had been circling for years: how a molecule could carry hereditary information and copy itself accurately from one generation of cells to the next. Their model showed two long strands, each a chain of alternating sugar and phosphate groups, wound around each other in a double helix, with four kinds of nitrogen-containing bases, adenine, thymine, guanine, and cytosine, projecting inward from each strand like the rungs of a twisted ladder.

The structure's real insight was in how the bases pair up. Adenine on one strand always pairs with thymine on the other, and guanine always pairs with cytosine, a rule the biochemist Erwin Chargaff had already noticed in the relative quantities of the four bases without knowing why it held. Watson and Crick recognized that this fixed pairing meant each strand contained enough information to reconstruct its partner, so that if the two strands separated, each could serve as a template for building a new complementary strand, an elegant, self-evident mechanism for how a cell copies its genetic material every time it divides.

Watson and Crick published their model in a short paper in Nature in April 1953, closing with a now-famous line of understatement noting that the pairing they had found suggested a possible copying mechanism for genetic material. The semi-conservative nature of that copying, each new DNA molecule retaining one original strand and one newly built one, was confirmed experimentally five years later by Matthew Meselson and Franklin Stahl, in an experiment often described as one of the most beautiful in the history of biology.

The double helix model Watson and Crick built in 1953 depended on experimental data neither of them had collected. At King's College London, the chemist Rosalind Franklin, working with her graduate student Raymond Gosling, had produced extremely sharp X-ray diffraction images of DNA fibers, using a technique she had refined specifically to get better results than earlier attempts. One image in particular, known ever since simply as Photograph 51, taken in 1952, showed a distinctive X-shaped diffraction pattern that was strong, specific evidence for a helical structure and allowed key dimensions of that helix to be calculated directly from the image.

Franklin's King's College colleague Maurice Wilkins showed Photograph 51 to James Watson without Franklin's knowledge or permission in January 1953. Around the same time, Crick's colleague Max Perutz gave Crick access to an unpublished report Franklin had submitted to a Medical Research Council review committee, which contained her own precise unit-cell measurements for DNA. Watson later acknowledged in his memoir The Double Helix that seeing the photograph was decisive, writing that the moment he saw it his mouth fell open and his pulse began to race. Franklin herself was not consulted, credited in the acknowledgments only in general terms, and was not told how directly her unpublished data had shaped the Cambridge model.

In 1962, Watson, Crick, and Wilkins shared the Nobel Prize in Physiology or Medicine for the discovery. Franklin had died in 1958, at thirty-seven, of ovarian cancer, quite possibly related to her extensive occupational exposure to X-rays, and the Nobel Prize is never awarded posthumously, so the question of whether she would have shared it can never be formally tested. Her role has been substantially reassessed since, and she is now widely credited as an essential, and at the time uncredited, contributor to one of the twentieth century's central discoveries.