Multiplying genes by leaps and bounds.

TISSUE FROM A WOOLLY MAMMOTH, w h o last walked the Siberian steppes some 40,000 years ago, would not seem to be a likely source of DNA for sequence analysis. After all, DNA breaks down very readily, and even though the mammoth had been frozen in the Siberian ice until it was chipped out about 10 years ago, its DNA had undergone extensive degradation. With the aid of a new gene amplification method, however, researchers were able to produce enough mitochondrial DNA to determine a nucleotide sequence. The results showed, not surprisingly, that the mitochondrial DNA of this extinct species is very similar to that of its close relative, the modern elephant. The ability to do the analysis at all nonetheless illustrates one of the great strengths of the gene amplification method, which goes by the name "polymerase chain reaction" (PCR). The technique works with broken down DNA, as well as with intact DNA. Pieces as small as 50 to 100 base pairs can be amplified. "The nice thing about the PCR is that it [the DNA] does not have to be in good shape," says Norman Arnheim of the University of Southern California in Los Angeles. This opens the way to a variety of applications in addition to analyzing ancient DNA samples. One that is advancing very rapidly is the forensic use of the technology to identify--or exonerate--suspects in rape and murder cases. The biological evidence found at the scene of violent crimes, including bloodstains, semen, and hair, is often of such poor quantity or quality that it can not be analyzed by other forensic techniques. But the DNA in the samples can be amplified and then compared with the suspect's DNA to obtain an identification. (The forensic applications of PCR-aided DNA analysis and other new DNA-typing technologies that are beginning to be used in court will be discussed more fully in a second article.) The gene amplification technique is also being applied in clinical medicine. It can be used, for example, to simplify and speed up the prenatal diagnosis of genetic diseases; to