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to work with Hermann Joseph Muller, who had been one of Thomas Hunt Morgan's associates in the famous "fly room" at Columbia University, and who had received a Nobel Prize for his discoveries in genetics. Watson's thesis adviser and principal mentor was Salvador Luria, who, along with Max Delbrück, had established bacterial genetics as the experimental system in which most of the major discoveries in molecular biology were to be made. Watson's thesis was on the effect of X rays on the multiplication of a bacterial virus, called phage.

Watson continued to study phage as a postdoctoral student in Copenhagen, Denmark where he worked from 1950 to 1951. While there, he met Maurice Wilkins, and for the first time saw the X-ray diffraction images generated in Wilkins's lab by Rosalind Franklin. Watson quickly decided to turn his attention to discovering the structure of important biological molecules, including DNA and proteins. By that time, DNA had been shown to be the genetic molecule, and it was believed that it somehow carried the instructions for making proteins, which actually perform most of the work in a cell.

The Structure of DNA

Luria arranged for Watson to continue his work at the Cavendish Laboratory in Cambridge, England, which was a center for the study of biomole-

?cular structure, and Watson arrived there in late 1951. At the Cavendish, he met Francis Crick, who, after training in physics, had turned his attention to similar structural questions. The two hit it off, and began collaborating on the structure of DNA.

Watson and Crick approached the problem by building models of the four nucleotides known to make up DNA. Each was composed of a sugar called deoxyribose, a phosphate group, and one of four bases, called adenine, thymine, cytosine, and guanine. They knew the sugars and phosphates alternated to form a chain, with the bases projecting off to the side. The X-ray images they had seen suggested the structure was a helix, and offered more information about dimensions as well. They also knew that the biochemist Erwin Chargaff had discovered that the amounts of adenine and thymine in a cell's DNA were equal, as were the amounts of cyto-sine and guanine.

After several failed attempts, more analysis of the X-ray images, and a fortuitous conversation with a biochemist who corrected one of their hypothesized base structures, they developed the correct model. The helix is formed from two opposing strands of sugar phosphates, while the bases hydrogen bonding weak project into the center. Weak bonding (called hydrogen bonding) bonding between the H between bases holds them together. The key, as Watson and Crick dis-oroup^md aCrUjtrogen or covered, was that the hydrogen bonds work best when adenine pairs with oxygen of another thymine, and guanine with cytosine, thus explaining ChargafPs ratios. The structure immediately suggested a replication mechanism, in which each template a master side serves as the template for the formation of a new copy of the oppos ing side, and they speculated, correctly, that the sequence of the bases was a code for the sequence of amino acids in proteins. They published their results in 1953, and received the Nobel Prize for physiology and medicine for it 1962, along with Wilkins (Franklin by then had died, and was therefore ineligible for the prize).

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