DNA Structure Discovered
In 1953, James Watson and Francis Crick, working at the Cavendish Laboratory at the University of Cambridge, proposed that DNA is built as a double helix: two long strands wound around each other, held together by paired chemical bases. Their model explained how genetic information could be stored and how it could be copied, and it became the foundation of modern molecular biology, genetics and medicine.
By the early twentieth century, scientists knew that chromosomes carried heredity, but it was still widely assumed that proteins, with their greater chemical variety, were the likely carriers of genetic information. That view began to shift after Oswald Avery, Colin MacLeod and Maclyn McCarty reported in 1944 that DNA could transmit inherited traits in bacteria, and experiments by Alfred Hershey and Martha Chase in 1952 strengthened the case. Meanwhile, Erwin Chargaff had found that in DNA the amount of adenine roughly equals thymine, and guanine roughly equals cytosine, a pattern that would prove crucial.
The structural puzzle was being tackled in several places. At King's College London, Maurice Wilkins and Rosalind Franklin used X-ray diffraction to study DNA fibers. Franklin, with her student Raymond Gosling, produced exceptionally clear images, including the one known as Photograph 51. In California, Linus Pauling was also pursuing a model, and Watson and Crick feared being beaten by him.
Watson and Crick relied largely on building physical models rather than doing their own experiments. Drawing on Chargaff's ratios and on X-ray data, some of which reached them through Wilkins and through a Medical Research Council report that Max Perutz had received, they assembled a structure in which adenine pairs with thymine and guanine with cytosine. Their short paper appeared in the journal Nature in April 1953, alongside separate papers from Wilkins's group and from Franklin and Gosling presenting supporting experimental evidence. The paper closed with a famously understated remark that the pairing they proposed suggested a possible copying mechanism for genetic material.
The consequences were enormous. The model pointed toward how DNA replicates and how sequences of bases might encode instructions, leading in the following decades to the cracking of the genetic code, recombinant DNA techniques, DNA sequencing and eventually the Human Genome Project. Applications now range from medical diagnosis to forensic science and the study of evolution.
The episode also remains a subject of debate over credit. Watson, Crick and Wilkins received the 1962 Nobel Prize in Physiology or Medicine. Franklin had died in 1958, and Nobel Prizes are not awarded posthumously. Historians still discuss how much her data contributed and whether she was treated fairly, though most agree her work was essential to the discovery.