Ultimate Gene Machine

Imagine an amplifier powerful enough to convert the inaudible whir of butterfly wings into a mighty roar. That's what a new tool called PCR routinely does to the most infinitesimal snippets of DNA, the molecule that carries the genetic blueprint for all living things. Within the space of a few hours, an unprepossessing aluminum box stuffed with test tubes can create a billion copies of what started out as a single strip of DNA. A dividing cancer cell would take at least a month to perform the same stupendous feat. "This technique," marvels Dr. Harley Rotbart, a microbiologist at the University of Colorado School of Medicine, "can reproduce genetic material even more efficiently than nature."

PCR stands for polymerase chain reaction, polymerase being the enzyme that triggers the replication of DNA inside dividing cells. All PCR does is reproduce, in a test tube, this basic biological process, turning it into a chain reaction that can be endlessly repeated by having a machine alternately raise and lower the temperature in the test tube. "The beauty of PCR is that it's technically so simple," observes cell biologist Peter Parham of Stanford University.

Since the first working machine was developed six years ago by a team of Cetus Corp. researchers, including biochemist Kary Mullis, PCR has enabled researchers to study even the faintest, most fragmentary traces of DNA found in specks of dried blood, strands of hair, chips of bone. In the journal Nature last week, for example, a team of British researchers recounted how they successfully identified a teenage murder victim from skeletal remains eight years old. First they extracted DNA from bone cells in the dead girl's femur. Then they obtained DNA from blood samples donated by the couple believed to be her parents. Using a PCR machine as their microscope, they went on to magnify and examine the unique genetic markers the dead girl shared with her parents. The evidence helped to convict two men of the crime earlier this year.

To date, PCR has been used to compare the DNA of extinct animals with their closest living relatives. It has assisted the U.S. military in identifying the remains of soldiers who died during Operation Desert Storm. It is beginning to help physicians detect small numbers of cancer cells circulating in the bloodstream and make prenatal diagnoses of genetic diseases such as sickle- cell anemia, as well as ensure better matches between organ donors and transplant recipients.

PCR may also soon aid scientists in solving a number of historical mysteries. Among them: whether the man who drowned in Argentina in 1979 really was Nazi war criminal Dr. Josef Mengele, and whether Abraham Lincoln suffered from Marfan's syndrome, an inherited disease characterized by gangly limbs, poor eyesight and a weak heart. "The applications of this technology are literally as wide as your imagination!" exclaims University of Virginia geneticist Dr. Thaddeus Kelly.

Among the areas where PCR is starting to make important inroads: