ABOUT SCIENCE

DISCOVERY OF THE DOUBLE HELIX - By the early 1950s, scientists knew that the genetic material of eukaryotic organisms was contained in their chromosomes. Scientists also knew that, of the two types of molecules found in chromosomes, proteins and DNA, the genetic material was DNA. The structure of DNA was still a mystery, however, and it represented the biggest unsolved problem in biology. In 1951, Francis Crick and James D. Watson began tackling the problem at Cambridge University in England. Their strategy was to build a model of DNA that would be consistent with available experimental evidence. Meanwhile, both Rosalind Franklin and Maurice Wilkins of King’s College in London were taking X-ray photos of DNA.

By early 1951, Franklin had gathered data on two forms of DNA. These differed in their water content there was a dry form, A, and a wet form, B. Franklin photographed the B form, and the photograph very clearly revealed a helix. However, she did not publish this finding and chose instead to concentrate on the A form. Later in 1951, Watson attended a seminar in which Franklin spoke about her findings. He and Crick used what they learned there to develop their first model of DNA, a triple helix that turned out to be incorrect. The real breakthrough, which came in January 1953, was sparked by two key events Wilkins shared Franklin’s photograph of the B form of DNA with Watson, and a copy of a report on Franklin’s experimental findings on DNA made its way to Watson and Crick. Watson and Crick began to build DNA models and to test them against Franklin’s photographs and data. Within a few weeks, they and thymine, and guanine and cytosine, paired up between the strands.

The Watson Crick model of DNA was published in April 1953, along with two papers offering supporting evidence, one by Wilkins and his collaborators and one by Franklin and her assistant. Acceptance of the Watson–Crick model was immediate and widespread. For their discovery of the double helix, Watson, Crick, and Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine. Franklin had died by then, of ovarian cancer that probably resulted from radiation exposure related to her work. She never knew how important her results had been in allowing Watson and Crick to develop their model. fact, Franklin’s importance to the discovery of DNA’s structure was unknown until Watson told the story in his memoir The Double Helix.