III.

Applications

Because only minute amounts of relatively crude DNA samples are required for PCR, it is a valuable tool for research in biology, clinical medicine, and forensic science (the application of scientific investigation to law).

In biological research, PCR has accelerated the study of gene function, gene mapping, and evolution. Gene-function research uses PCR to create copies of individual genes, the activities of which can then be studied and more precisely defined. Gene mapping (see Human Genome Project) relies on PCR to create many copies of specific regions of human DNA. These regions can then be examined to see if they are linked to genetic diseases, such as cystic fibrosis.

The study of evolution has also benefited from PCR. For example, scientists have used PCR to study DNA from insects trapped for thousands of years in amber. Even though much of the DNA has lost its structure, there is still enough intact DNA remaining to be multiplied by PCR for comparison with the DNA of present-day insects.

In medicine, PCR is particularly useful in prenatal testing for genetic diseases. DNA samples obtained from a fetus by amniocentesis, in which a small sample of fluid is drawn from the mother's uterus, can be tested by PCR in just a few hours. Previously, it was necessary to culture fetal cells, or grow them in a special nutrient medium, for several weeks before biochemical tests could be performed. PCR testing has also been used on cells taken from hours-old embryos fertilized in vitro (in a test tube) to identify an embryo that was free of disease. The embryo was then implanted in the mother's uterus, and a normal pregnancy resulted.

Other medical applications of PCR include identifying viruses, bacteria, and cancerous cells in human tissues. PCR can even be used within single cells, in a procedure called in situ (in-site) PCR, to identify specific cell types.

In forensic science, PCR has revolutionized the process of criminal identification. PCR-based DNA-typing tests can create detailed DNA “fingerprints” (see DNA Fingerprinting) that can definitively identify individuals. Such tests can also exclude or implicate suspects based on small amounts of blood, skin, hair, or semen left at a crime scene.

PCR has been used to trace industrial waste and other products. Small amounts of known DNA are inserted into batches of explosives, petroleum products, poisons, and other waste at their source to create a tag that will last indefinitely. Such DNA tags can be recovered from oil slicks or other pollutants found in public waterways (see Water Pollution), for example, and then multiplied by PCR and compared with a listing of manufacturers' DNA tags. A match can provide strong evidence implicating polluters.