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Unraveling the Nature of the Gene |
A major milestone along the trail to gene testing was the discovery of the structure of deoxyribonucleic acid (DNA), the molecule that contains genes. It had been known since the middle of the 19th century, when the monk Gregor Mendel conducted his famous pea-breeding experiments, that physical traits such as height and color were passed from one generation to the next via units of inheritance that later came to be called genes. But the physical character of the gene had eluded scientists until 1944, when the studies of Oswald Avery, Colin MacLeod, and Maclyn McCarty of New York's Rockefeller Institute provided the first experimental evidence that DNA transmits genetic information. They showed that all that was needed to transform harmless bacteria into a type that can cause pneumonia was their uptake of DNA from a pneumonia-causing strain of bacteria. That experiment suggested that genes were made of DNA, and it launched many researchers on a quest to determine the exact structure of DNA as a means of unraveling how genes exert their influence on all living things.
Two of these researchers, Rosalind Franklin and Maurice Wilkins, of King's College in London, studied the pattern generated when x-rays were scattered from DNA fibers. The photographic image immediately revealed that the DNA structure was regular and helical. With that information and knowledge of the chemistry of the DNA components, James Watson and Francis Crick, then at the Medical Research Council laboratories in Cambridge, England, began building molecular models that might account for the details in the photograph. The model that they ultimately proposed in 1953 contains two helically twisted strands connected to each other by a series of molecular "rungs." They suggested that each rung was composed of one of two chemical "base pairs" called adenine (A)-thymine (T) or guanine (G)-cytosine (C). These young scientists correctly surmised that it was the order of those A, T, G, and C bases on the DNA strand that spelled out the genetic endowment of every living organism. They also recognized that the two strands could be separated for copying--a simple mechanism for passing on genetic information from one generation to the next.
 A few years after Watson and Crick clarified the structure of DNA, several other researchers, notably Marshall Nirenberg, at the National Institutes of Health, and Har Gobind Khorana, at the University of British Columbia, deciphered the genetic code that all living cells use to translate the series of bases in their DNA into instructions for the production of the thousands of proteins that determine the cell's structure and carry out all its functions, including determining such genetic traits as eye color and susceptibility to cancer. The researchers discovered that each triplet of bases (CTG, for example) codes for one amino acid (in this case leucine) or for a signal to start or stop building the long chain of amino acids that creates a protein. |
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