Word: plasmids
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...special trick to make the experiment work," Gilbert says. The insulin gene is inserted into a plasmid at a site before the end of another gene, which codes for a protein known as penicillinase. The insulin and penicillinase proteins are synthesized in a fused form when the genes are read together...
...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...
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...
After the plasmids are separated from the chromosomal DNA in a centrifuge, they are placed in a solution with a chemical catalyst called a restriction enzyme. This enzyme cuts through the plasmids' DNA strips at specific points. It leaves overlapping, mortise-type breaks with "sticky" ends. The opened plasmid loops are then mixed in a solution with genes-also removed by the use of restriction enzymes-from the DNA of a plant, animal, bacterium or virus. In the solution is another enzyme called a DNA ligase, which cements the foreign gene into place in the opening of the plasmids...
Finally, the chimeras are placed in a solution of cold calcium chloride containing normal E. coli bacteria. When the solution is suddenly heated, the membranes of the E. coli become permeable, allowing the plasmid chimeras to pass through and become part of the microbes' new genetic structure. When the E. coli reproduce, they create carbon copies of themselves, new plasmids -and DNA sequences-and all. Thus they become forms of life potentially different from what they had been before-imbued with characteristics dictated not only by their own E. coli genes but also by genes from an entirely different...