(4 of 10)
Science is not interested in pursuing such bizarre fantasies; the real advances are exciting enough. About five years ago, California scientists learned how to combine genes from different organisms, regardless of how low or high they are on the evolutionary scale. Though the researchers added only one or two new genes to a bacterium's collection of thousands of genes, the creation of such hybrid molecules was a stunning feat. The accomplishment seemed to breach one of nature's more inviolable barriers. Even primates as closely related as gorilla and man are so different genetically that they cannot produce offspring. Thus it was not size alone that made King Kong and his ladylove a mismatch. The real species barrier is in the genes.
Molecular biology's wizards have managed to cross that obstacle in their work with bacteria. Unlike higher organisms, bacteria are single-celled creatures that usually reproduce not by sexual mating but by simply dividing. Thus their ability to acquire new and possibly advantageous genes would seem to be highly limited. But the tiny creatures have devised a cunning alternative. Besides their single, large, ringed chromosome (which is the repository of most of their genes), they possess much smaller closed loops of DNA, called plasmids—which consist of only a few genes. This extra bit of DNA—genetic small change, as it has been dubbed—serves a highly useful purpose. When two bacteria brush against each other, they sometimes form a connecting bridge. During such a "conjugation," a plasmid from one bacterium may be passed into the other.
These natural transfers can be crucial to the survival of the bacterium. It is through new plasmids, for example, that bacteria like Staphylococcus aureus have become resistant to penicillin. The plasmid acquired by the staph bug contained a gene that directs the production of a penicillinase, an enzyme that cracks apart invading penicillin molecules, making them ineffective. Different plasmids, sometimes passed from one bacterium to another, can order up still another kind of chemical weapon, a so-called restriction enzyme, which can sever the DNA of an invading virus, say, at a predetermined point.
Observing these bacterial tricks, molecular biologists began isolating various restriction enzymes. They had already discovered another type of bacterial enzyme, called a ligase (from the Latin word meaning to bind), which acted as a form of genetic glue that could reattach severed snatches of DNA. Using their new biochemical tools, the scientists embarked upon some remarkable experiments. As usual, they turned to their favorite guinea pig, a lab strain of E. coli, and soon they had learned to insert with exquisite precision new genetic material from other, widely differing organisms into the bacteria (see diagram).
E. coli did not merely accept the hybrid plasmids. When the bacteria reproduced—by dividing and thus doubling—at a rate of about once every 30 minutes, they created carbon copies of themselves, new plasmids and all. In only a day, one bacterium could make billions of duplicates of a transplanted gene.