Discovery of the Double Helix

The components of DNA were known by 1900—the sugar, phosphate, and bases—but the technology did not exist then to determine how they were put together. The credit for that discovery went mainly to

James Watson and Francis Crick in 1953 (fig. 4.4). The events surrounding their discovery of the double helix represent one of the most dramatic stories of modern science—the subject of many books and a movie. When Watson and Crick came to share a laboratory at Cambridge University in 1951, both had barely begun their careers. Watson, age 23, had just completed his Ph.D. in the United States, and Crick, 11 years older, was a doctoral candidate. Yet the two were about to become the most famous molecular biologists of the twentieth century, and the discovery that won them such acclaim came without a single laboratory experiment of their own.

Others were fervently at work on DNA, including Rosalind Franklin and Maurice Wilkins at King's College in London. Using a technique called X-ray diffraction, Franklin had determined that DNA had a repetitious helical structure with sugar and phosphate on the outside of the helix. Without her permission, Wilkins showed one of Franklin's best X-ray photographs to Watson. Watson said, "The instant I saw the picture my mouth fell open and my pulse began to race." It provided a flash of insight that allowed the Watson and Crick team to beat Franklin to the goal. They were quickly able to piece together a scale model from cardboard and sheet metal that fully accounted for the known geometry of DNA. They rushed a paper into print in 1953 describing the double helix, barely mentioning the importance of Franklin's two years of painstaking X-ray diffraction work in unlocking the mystery of life's most important molecule.

For this discovery, Watson, Crick, and Wilkins shared the Nobel Prize in 1962. Nobel Prizes are awarded only to the living, and in the final irony of her career, Rosalind Franklin had died in 1958, at the age of 37, of a cancer possibly induced by the X rays that were her window on DNA architecture.

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Photos Rosalind Franklin Dna Work

Figure 4.4 Discoverers of the Double Helix. (a) Rosalind Franklin (1920-58), whose painstaking X-ray diffraction photographs revealed important information about the basic geometry of DNA. (b) One of Franklin's X-ray photographs. (c) James Watson (1928-) (left) and Francis Crick (1916-) (right), with their model of the double helix.

Figure 4.4 Discoverers of the Double Helix. (a) Rosalind Franklin (1920-58), whose painstaking X-ray diffraction photographs revealed important information about the basic geometry of DNA. (b) One of Franklin's X-ray photographs. (c) James Watson (1928-) (left) and Francis Crick (1916-) (right), with their model of the double helix.

The essential function of DNA is to serve as a code for the structure of polypeptides synthesized by a cell. A gene is a DNA nucleotide sequence that codes for one polypeptide. The next section of this chapter explains in detail how the genes direct polypeptide synthesis. All the genes of one person are called the genome (JEE-nome); geneticists estimate that a human has about 35,000 genes. These account for only 3% of our DNA; the other 97% does not code for anything. Some of the noncoding DNA serves important organizing roles in the chromatin, and some of it is useless "junk DNA" that has accumulated over the course of human evolution. The latest triumph of molecular genetics is the human genome project, an enormous multinational effort that led to the mapping of the base sequence of the entire human genome. Its completion (in all but some fine details) in June 2000 was hailed as a scientific achievement comparable to putting the first man on the moon.

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