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As his source of DNA, Berg turned to a well-known laboratory tool known as SV40, short for simian virus 40 (so called because it was orginally found in monkeys). SV40's genetic structure is relatively simple—it seems to have no more than seven genes (vs. around 5,000 in the cell of a fruit fly and the 100,000 in a human cell). Thus SV40's genes could easily be identified and distinguished from the other DNA of the host cell.
To insert the genetic material into the bacterium, he used as his "vector," or carrier, another variety of virus called a lambda phage, which preys on bacteria. But first he had to cut open SV40's single circular DNA molecule. As his biochemical knife, he used certain enzymes, or helpers in chemical reactions, that cells normally use in such processes of their everyday life chemistry as digestion. Then he employed more enzymes to break into lambda's genes. Still other enzymes were painstakingly used to create the required mortise-like "sticky" ends to attach the two strips of DNA together.
By the time Berg and his team "glued" all this DNA back into a circle, they had achieved a scientific first: genetic material from two different organisms—in this case, two kinds of viruses—had been directly combined by human intervention. Recombinant DNA, or gene-splicing, was born. As its midwife, Berg shared the 1980 Nobel Prize in Chemistry.
The next phase Berg planned for his experiment brought on the hottest controversy that gene splicers have yet confronted. Berg wanted to insert the SV40 genes into the bacterium Escherichia coli, an inhabitant of the human intestine only about one ten-thousandth of an inch long. E. coli has been the regular guinea pig of the molecular biology lab for some 40 years. But a few scientists who learned of Berg's plans were shocked. SV40 seems harmless enough in monkeys. But it causes tumors in mice and hamsters and has turned test-tube cultures of human cells cancerous. What would happen if E. coli containing the monkey virus escaped from Berg's lab, established themselves in the human gut and went on multiplying? Would that plant a slowly ticking cancer time bomb?
Berg voluntarily dropped the planned experiment. Concerned about the possible escape of new and deadly pathogens, he helped persuade his colleagues to observe a self-imposed moratorium on such experiments. Even so, some university towns threatened to ban all recombinant DNA work. The voluble former mayor of Cambridge, Mass., Alfred Vellucci, spoke darkly, and inaccurately, of breeding "Frankensteins" in the labs at Harvard and M.I.T.
Under federal guidelines drawn up with the help of scientists led by Berg and adopted by the National Institutes of Health in 1976, gene splicing in university labs was strictly controlled. The new rules established levels of biological containment deemed appropriate to possible hazards. If a proposed experiment was low on the risk scale, it could be done on an open bench or perhaps on a special counter protected by a curtain of air. More dangerous experiments required sealed isolation chambers like those used in germ warfare research; only by reaching through a gloved compartment did the