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Ample samples of specific genes.

Ample samples of specific genes

A new process for copying any selected part of a human chromosome can make a million copies in about two hours, according to a group of California researchers. Unlike the techniques previously in use, the procedure does not insert the human DNA into a microorganism. Instead, it uses enzymes in the laboratory simply to copy and recopy a designated stretch of DNA. The procedure is expected to be useful wherever small amounts of DNA must be analyzed, for example in diagnostic medicine, forensic science and laboratory research. "If there is not enough DNA, you can just make more of it," says Henry Erlich of the Cetus Corp. of Emeryville, Calif.

Cetus scientists have used the method to take as little as 33 nanograms of human DNA and produce enough of the betaglobin gene to detect carriers of thalassemia (a form of anemia) and to do some prenatal diagnoses. They are also using the procedure to study the proteins responsible for transplant rejection.

The strategy takes advantage of the natural process by which DNA replicates. An enzyme called a polymerase travels along a single strand of DNA, adding the correct components to make a new double strand. The starting point for the polymerase activity is a short DNA segment, called a primer. The prime must be bound to the strand serving as the template.

In the new method, the scientists dissociate DNA into its single strands. Next they mark the gene to be copied by adding two short stretches of laboratory-synthesized, single-stranded DNA that bind, one on each side of the gene, to provide two primers for the polymerase. Then the polymerase is added and allowed to synthesize a new DNA segment that includes the gene of interest. The resultant double-stranded stretch of DNA is dissociated, completing a "cycle" of the procedure.

Because each of these single strands--both the original full-length strands and the new fragments -- can bind one of the two primers and serve as a template for further DNA synthesis, when the cycle is repeated the DNA pieces accumulate exponentially.
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Title Annotation:new process for copying any selected part of a human chromosome
Author:Miller, Julie Ann
Publication:Science News
Date:Apr 5, 1986
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