Recombinant DNA and biotechnology have opened a new era of diagnostic testing and have made detecting many genetic diseases possible. The basic tool of DNA analyses is a fragment of DNA called the DNA probe. A DNA probe is a relatively small, single-stranded fragment of DNA that recognizes and binds to a complementary section of DNA on a larger DNA molecule. The probe mingles within the mixture of DNA molecules and unites with the target DNA much like a left hand unites with a right. After the probe unites with its target, it emits a signal like that from a radioactive isotope to indicate that a reaction has occurred.
To work effectively, a sufficiently large amount of target DNA must be available. To increase the amount of available DNA, a process called the polymerase chain reaction (PCR) is used. In a highly automated machine, the target DNA is combined with enzymes, nucleotides, and a primer DNA. In geometric fashion, the enzymes synthesize copies of the target DNA, so that in a few hours billions of molecules of DNA exist where only a few were before.
Using DNA probes and PCR, scientists are now able to detect the DNA associated with HIV (and AIDS). This has yielded a direct test for AIDS that is preferred over the AIDS antibody test. Lyme disease and genetic diseases such as cystic fibrosis, muscular dystrophy, Huntington's disease, and fragile X syndrome can be identified by DNA probes. (Cystic fibrosis is a respiratory disease in which mucus clogs the respiratory passageways and makes breathing difficult; muscular dystrophy is a disorder of the nervous system in which destruction of nerve fibers leads to erratic muscular activity; Huntington's disease is a disease of the nervous system accompanied by erratic movements and nervous degeneration; and fragile X syndrome is a disease of the X chromosome accompanied by a form of mental retardation.)
Segments of DNA called restriction fragment length polymorphisms (RFLPs) are the objectives of the tests with gene probes. RFLPs are apparently useless bits of DNA located near genes associated with the diseases. By locating RFLPs, the biotechnologist can locate the disease gene. DNA probes also detect microorganisms in the environment and identify viral and bacterial pathogens.
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