On the last day of February 1953, according to the story James Watson later told, Francis Crick walked into the Eagle pub in Cambridge and announced that he and Watson had "found the secret of life." It was characteristic bravado, but for once it was close to true. That morning the two men had settled on a structure for deoxyribonucleic acid — DNA — that immediately explained how living things copy their genetic instructions. The molecule was a double helix: two strands winding round each other like a spiral staircase, held together by paired chemical rungs.
The elegance of the answer lay in the pairing. The rungs were made of four bases — adenine, thymine, guanine and cytosine — and Watson and Crick realised that adenine always pairs with thymine, and guanine with cytosine. This meant each strand was a template for the other. Pull the two strands apart, and each half carries all the information needed to rebuild its partner. As their famously brief paper in the journal Nature put it, with studied understatement: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."
Photo 51
Watson and Crick were theorists who built physical models out of metal plates and rods; they produced little experimental data of their own. The crucial evidence came from King's College London, where Rosalind Franklin, an expert in X-ray crystallography, was producing extraordinarily sharp diffraction images of DNA fibres. One of them, known as Photograph 51, showed a distinctive X-shaped pattern that is the unmistakable signature of a helix. Franklin's meticulous measurements also pointed to the phosphate backbone being on the outside of the molecule.
In early 1953 Maurice Wilkins, Franklin's colleague at King's, showed Photograph 51 to Watson — without Franklin's knowledge. Watson later wrote that the moment he saw it, "my mouth fell open and my pulse began to race." A report containing Franklin's unpublished data also reached the Cambridge pair. How much this material shaped their final model, and how properly it was credited, has been argued over ever since. Franklin was not told that her results were being used, and the pairing insight and model-building were genuinely Watson and Crick's — but the structure could not have been built without her images.
There was a rival too. The great American chemist Linus Pauling, who had recently solved the structure of protein helices, published his own proposal for DNA in early 1953 — a triple helix with the bases on the outside. It was wrong, and the error galvanised the Cambridge team into finishing before Pauling could correct himself. A further clue came from the biochemist Erwin Chargaff, who had found that in any sample of DNA the amount of adenine roughly equals the amount of thymine, and guanine equals cytosine — "Chargaff's rules," which the base-pairing scheme neatly explained.
The credit that never settles
Three back-to-back papers appeared in Nature on 25 April 1953: Watson and Crick's model, and two experimental papers from the King's College groups, including one by Franklin and her student Raymond Gosling. To a casual reader the Cambridge model looked like pure theory confirmed by London's data, but the ordering on the page suggested the King's work merely supported a conclusion already reached — a framing that has coloured the credit debate for seventy years.
The full significance of the model took a few years to be proved. In 1958 two American scientists, Matthew Meselson and Franklin Stahl, ran an experiment often called "the most beautiful in biology," confirming that DNA really does copy itself by splitting into two strands that each serve as a template — exactly the mechanism Watson and Crick's understatement had hinted at. The double helix was not just elegant; it was correct.
Rosalind Franklin left King's, did notable work on the structures of viruses (having earlier conducted pioneering research on coal), and died of ovarian cancer in 1958, aged only thirty-seven. Four years later, in 1962, Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine. The Nobel Prize is not awarded posthumously, so Franklin could not have been included even had the committee wished to; whether she would have been recognised had she lived is one of science's great unanswerable questions. Her reputation has since been thoroughly rehabilitated, and Photograph 51 is now among the most famous images in the history of biology — a picture that unlocked the molecule of heredity, taken by a woman who was largely written out of the triumph.
Quiz nuggets
- James Watson and Francis Crick proposed the double-helix structure of DNA in 1953 at the Cavendish Laboratory in Cambridge.
- The base-pairing rule they identified is that adenine pairs with thymine and guanine pairs with cytosine.
- Rosalind Franklin's X-ray image "Photograph 51," taken at King's College London, provided crucial evidence of the helical structure.
- Linus Pauling published a rival, and incorrect, triple-helix model of DNA in 1953.
- Watson, Crick and Maurice Wilkins shared the 1962 Nobel Prize; Franklin had died in 1958 and the prize is not awarded posthumously.