THE CELL: Unraveling the Double Helix and the Secret of Life

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A number of such repressers have since been found in bacteria. Scientists have also isolated enzymes that turn the genes back on. These inducers, as they are called, work by unlocking the repressers on the segment of DNA. But even in E. coli, such switching can become bafflingly difficult: the repressers and inducers, for example, require controlling enzymes of their own. These enzymes, in turn, apparently need the help of still other molecules, such as the recently discovered sigma, rho and psi factors, in recognizing the appropriate genes. In fact, it is because of the very complexity of these processes that leading molecular biologists like Crick find the questions arising from cell differentiation so fascinating. How in the human embryo, for instance, are certain genes switched on so that by the end of the first week after conception identical cells have begun to grow into cells with differing characteristics?

SO FAR THESE fundamental questions are largely unanswerable, although some clues have been uncovered. For one thing, it is thought that in higher, multicellular forms of life, repressers may be a special class of proteins called histones; these are not found in bacteria. When histones are removed, Rockefeller University's Vincent Allfrey has found, RNA production soars by 400%, evidence that formerly repressed segments of DNA have become active. In addition, it has been learned that the cell membrane itself appears to play a crucial part in switching genes on and off. When a membrane is merely brushed by certain hormones—a large class of molecules that serve as intercellular messengers—the membrane will respond as though jolted by an electric probe. It will instantly send off a signal to the nucleus, triggering RNA production by the genes. That finding could eventually have medical application for diseases—like diabetes—resulting from vital genes that are inexplicably turned off.

Many more puzzles remain unsolved. Why are there small bits of DNA located outside the nucleus in energy-producing cell centers called mitochondria? Does this mean that there are other, unknown repositories of hereditary information? In spite of such questions and complications, the basic structure of DNA postulated by Crick and Watson 18 years ago has withstood the test of time remarkably well. More important, it has given man a profound new understanding of basic life processes—and the means to control and alter them.