Development of the polymerase chain reaction revolutionized research by making it possible to create millions of copies of a given region of DNA in only hours. The technique is used to amplify specific regions of DNA and detect specific sequences, applications that overlap somewhat with those of cloning and DNA hybridization. While PCR is much more rapid and sensitive than the other techniques, however, the key enzyme (Taq polymerase) and equipment used in the patented process are relatively expensive.
The critical aspect of PCR is the ability to amplify a specifically selected region of DNA, even in an impure sample such as soil or saliva (figure 9.11). After PCR amplification, DNA fragments in the specimen can be separated by gel electrophoresis and visualized by staining with a dye that fluoresces when bound to DNA. By amplifying a chosen sequence that is unique to a given organism, it is possible to detect the presence of that organism in a specimen. For example, if a sequence found only in Neisseria gonorrhoeae can be amplified from a vaginal secretion, then N. gonorrhoeae must be present in that specimen. Likewise PCR can be used to detect HIV in a sample of blood cells. The exquisite sensitivity of PCR, however, is also its greatest drawback. Great care must be taken not to contaminate a sample with an external source of the DNA sequence to be amplified. In the example of
DNA from Patient A
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