DNA

Perhaps the greatest medical breakthrough of the 20th century is the discovery of the structure of deoxyribonucleic acid (DNA—the molecular basis of heredity). Knowledge of DNA's chemical structure allowed scientists to understand for the first time how DNA replicates itself and passes information from one generation to the next. This monumental discovery has already revolutionized many aspects of medicine, permitting the development of a vast range of genetically engineered drugs, hormones, and other useful substances. Even more radical changes are afoot. In the new millennium, scientists are expected to have access to a complete map of the human genetic code, which should help them trace the genetic causes of all inherited diseases and search out possible cures.
The Swiss physician Friedrich Miescher isolated DNA for the first time in 1869, but the function of the chemical, which is found only in the nucleus of cells, was unknown. As the years passed, scientists learned that DNA contained phosphate, a sugar called deoxyribose, and four different compounds called nucleotide bases.
In 1944 the Canadian-born American physician and bacteriologist Oswald T. Avery and his colleagues showed, in a series of experiments on bacteria, that DNA transmitted genetic information. Prior to Avery's groundbreaking work, many biochemists believed that proteins were the source of genetic information.
By 1950 two groups of scientists were in hot pursuit of the structure of DNA. One of the groups was at Cavendish Laboratory in Cambridge, England. The other group, at King's College, London, consisted of Maurice Wilkins, a physicist, and Raymond Gosling, a graduate student. They were joined in 1951 by Rosalind Franklin, an expert in X-ray crystallography (a technique that uses a tiny beam of X rays to create images of the structural relationships between atoms and molecules of chemical substances). Photographs of images produced by X-ray crystallography are called X-ray diffraction photographs.
In 1950 Wilkins received a uniquely pure sample of DNA from a Swiss physicist. From this sample, he was able to pick out single DNA fibers with a glass rod. Wilkins and Gosling X-rayed these fibers in 1950, as did Franklin when she joined the laboratory in 1951.
However, a misunderstanding caused Wilkins and Franklin each to think they were in charge of X-ray crystallography, and they did not cooperate. When Franklin left the team in 1952, she was ordered to submit all of the X-ray diffraction photographs to Wilkins. One of these photographs showed that the DNA molecule had the shape of a double helix, a structure resembling a twisted ladder.
In the meantime the American biologist James Watson attended a meeting in Naples, Italy, in 1950, in which he saw one of Wilkins's X-ray diffraction photographs. Watson immediately thought the molecule might be a double helix. In the fall of 1951 he joined the team of scientists at Cavendish Laboratory, where he convinced a British biophysicist, Francis Crick, that a combination of model building—using plastic balls, wires and steel plates—and X-ray crystallography could lead them to the structure of DNA.
The double helix by itself, however, was not the only secret to the DNA molecule. Its entire chemistry needed to be explained. Watson, unbeknownst to Wilkins, was now convinced that DNA had a helical structure and was working feverishly with Crick on their increasingly complex model of the molecule, which they finished during the second week of 1953. This model incorporated all the known chemical components of DNA and closely matched the diffraction pattern observed in Wilkin's photograph. Watson and Crick accurately deduced that the two strands of the double helix separated before cellular division, providing templates, or patterns, for the creation of two new DNA molecules identical to the original.
Watson and Crick sent Wilkins a copy of their manuscript, which took advantage, of course, of what Wilkins and Franklin had done, and which Wilkins thought was his own. After reading their manuscript, Wilkins sent them a letter that began, “I think you are a couple of old rogues.…”
On April 25, 1953, the journal Nature published one article from the Cambridge laboratory and two from King's College in London on the molecular structure of DNA. Many felt that the key to life itself had been revealed. Wilkins, Watson, and Crick shared the 1962 Nobel Prize in physiology or medicine.
source: encarta encyclopedia