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The polymerase chain reaction, PCR, is the amplification of a sequence of DNA using the cell's own system of DNA replication. Geneticists and medical researchers use PCR to determine if certain genes are present in a DNA sample and to create enough DNA to analyze the sequence.

PCR is setup like a cooking recipe. A portion of DNA is added to a mix of enzymes and primers, small pieces of DNA in the sequence of interest, and then cycled through temperatures that activate and deactivate the enzyme allowing the DNA pieces to separate, anneal, and elongate. The enzyme is Taq polymerase. Polymerase is the enzyme cells use to replicate, or copy, DNA strands. Taq polymerase is a polymerase found in hot springs and withstands the temperatures needed to separate the double stranded DNA strands. This freed up researchers from having to add more enzyme each cycle of PCR.

Machines now cycle small tubes of samples through the heat and cold allowing the DNA strands to build up. As the program progresses through each cycle the polymerase and primers are reused, making copies of the DNA strands starting at the sequence where the primers bind. To amplify large portions of DNA many primers are used. To amplify portions of a single gene as few as two are sometimes used. By the end of 30-40 cycles there should be enough DNA to run the PCR mixture on a gel and visualize the size of DNA amplified. Because the sequence of the primers was known the researcher can determine if the result is positive or negative. If the researcher is aiming to determine differences in the samples, the DNA can then be used in further testing and purification procedures.

This same process can be used with RNA, which aids in determining the activity of a gene as well as discerning the genomes of certain infectious agents such as HIV. An additional step is needed to convert RNA to DNA, but then continues in the same manner. Since RNA is converted directly to proteins the presence of RNA in a blood or tissue sample can determine if a gene is turned on or off. In this way researchers have been able to discover the environmental influences of abnormalities and disease. They have also been able to determine the genes involved in certain disorders such as hypertension, diabetes, and enzyme deficiencies.

Learn more about this author, Alicia M Prater PhD.
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