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Chapter 17 : The Polymerase Chain Reaction
The polymerase chain reaction (PCR) has become a key tool in molecular biology research and in biotechnology applications. In the student activity, students use paper models to simulate the steps of the PCR. The model exercise demonstrates how DNA polymerase can be used to make multiple copies of a specific DNA fragment and shows how the technique can be used to detect a specific DNA molecule (such as the chromosome of a disease-causing microorganism) in a sample. Automated DNA sequencing is also based on PCR technology. PCR is a simple technique that combines in vitro DNA synthesis by DNA polymerase and hybridization. To start the reaction, the double-stranded sample DNA in the mixture is denatured into single strands by heating it. The mixture is then cooled so that hybridization of complementary strands can occur. In the next step, the DNA polymerase enzyme synthesizes a second DNA strand on each of the two original strands, using the free nucleotides in the solution. The annealed primer serves as a starting point. New DNA is synthesized from the 3' end of each primer, extending in only one direction. In early PCR methods, new DNA polymerase had to be added after each denaturation step, because the high heat necessary for denaturation destroyed the enzyme. This paper model very accurately demonstrates the steps of PCR and shows how a specific DNA segment can be amplified from a single copy. The second part of the activity illustrates how PCR is used as a diagnostic tool.