Scientific_American_Moxon_and_Wills_1999

Scientific_American_ - AUTHOR TITLE SOURCE E Richard Moxon and Christopher Wills DNA Microsatellites Agents of Evolution Scientific American 280

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AUTHOR: E. Richard Moxon and Christopher Wills TITLE: DNA Microsatellites: Agents of Evolution? SOURCE: Scientific American 280 no1 94-9 Ja '99 (C) 1999 Scientific American, Inc. All rights reserved. For subscription information please contact 800-333-1199; web site: http://www.sciam.com. Further reproduction of the Works in violation of the copyright law and without the express permission of the publisher is prohibited. Repetitive DNA sequences play a surprising role in how bacteria--and perhaps higher organisms--adapt to their environments. On the downside, they have also been linked to human disease A human's genetic code consists of roughly three billion bases of DNA, the familiar "letters" of the DNA al- phabet. But a mere 10 to 15 percent of those bases make up genes, the blueprints cells use to build proteins. Some of the remaining base sequences in humans--and in many other organisms--perform crucial functions, such as helping to turn genes "on" and "off" and holding chromosomes together. Much of the DNA, however, seems to have no obvious purpose at all, leading some to refer to it as "junk." Part of this "junk DNA" includes strange regions known as DNA satellites. These are repetitive sequences made up of various combinations of the four DNA bases--adenine (A), cytosine (C), guanine (G) and thymine (T)--repeated over and over, like a genetic stutter. In the past several years, researchers have begun to find that so-called microsatellites, those containing the shortest repeat sequences, have a significance disproportionately great for their size and perform a variety of remarkable functions. Indeed, scientists are discovering that the repetitive nature of microsatellites makes them particularly prone to grow or shrink in length and that these changes can have both good and bad consequences for the organisms that possess them. In certain disease-causing bacteria, for example, the repeat sequences promote the emergence of new properties that can enable the microbes to survive potentially lethal changes in the environment. Some microsatellites are also likely to have substantial effects in humans, because at least 100,000 occur in the human genome, the complete complement of DNA in a human cell. Although the only function assigned so far to human microsatellites is negative--causing a variety of neurological diseases--microsatellites may be surviving relics of evolutionary processes that helped to shape modern humans. While some investigators search for the reasons humans carry so much repetitive DNA, many are now learning to exploit microsatellites to diagnose neurological conditions and to identify people at risk for those disorders. They are also finding that microsatellites change in length early in the development of some cancers, making them useful markers for early cancer detection {see box on page 98}. And because the lengths of microsatellites may vary from one person to the next, scientists have even begun to use
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Scientific_American_ - AUTHOR TITLE SOURCE E Richard Moxon and Christopher Wills DNA Microsatellites Agents of Evolution Scientific American 280

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