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What is new is that scientists are now able to manipulate directly the very substance that makes up genes: DNA, often called the master molecule of life. Coiled in the chromosomes of all living cells, DNA consists of only a handful of chemical building blocks—a sugar, a phosphate and four bases, adenine (A), thymine (T), guanine (G) and cytosine (C). But its simplicity is deceptive. In DNA's precise architecture—the famed double helix unraveled by Watson and Crick in 1953—lies the secret of how the molecule conveys the message of heredity from one generation to the next.
The twisted, double-stranded DNA, as frequently noted, resembles a spiral staircase, with each step formed by a pair of bases—A always binding with T, G always with C. In fact, it acts more like computer tape. Every three steps serve as a code word for one of the 20 amino acids found in all life on earth. Strings of code words, in turn, provide the sequence for linking these amino acids into proteins, the basic building blocks of living things. DNA thus carries the entire genetic blueprint for assembling any organism, from bacterium to man.
Though the double helix helped unlock many of the mysteries of DNA, even more are still unexplained. How do genes turn on and off—or, in the language of molecular biology, "express" themselves? What about cell differentiation? At a critical moment early in the life of an embryo, identical cells miraculously (no other word will do) begin to take on specialized roles—some forming tissue for the heart, for example, others that of the liver or skin. Each of these different cells still contains all the original instructions for producing the entire organism, but somehow unneeded genes are switched off. How does this differentiation come about? Do certain genes order up particular proteins that serve as "on" and "off" switches?
To answer such questions, scientists in labs around the country began looking for new ways to examine the genetic machine in action. One of them was Biochemist Paul Berg, 54, of Stanford University. Berg wanted to study genes of higher organisms. But their complement of genes tends to be dizzyingly complex, involving thousands of steps along strands of DNA. Instead he and his colleagues plotted an experiment involving viruses, which are nothing more than a short strip of nucleic acid, usually cloaked in a wrapper of protein. When they invade a living cell, viruses substitute their own genes for their victim's DNA and crank out duplicates of themselves. Berg's clever strategy was to exploit this mischief-making ability by using a virus to invade a bacterium. He hoped that there the new genes from the virus would begin producing proteins unlike any