PCR - Polymerase Chain Reaction. DNA primers are designed that will base pair with sites a and b on opposite strands of the region of DNA that is to be amplified. Primers are mixed with duplexed DNA, and the mixture heated to 94°C to denature duplexes. The mixture is then cooled to 42°C, allowing duplexes to reform. Since primers are in vast molar excess to the target DNA, most of the time, a primer will base pair with a DNA strand, rather than target duplexing with target. Taq DNA polymerase and nucleotides are also included in the mixture. Since the optimal temperature for Taq polymerase is 72°C, the temperature is increased to °C, and the primers are extended, resulting in two complete duplex DNA molecules, one produced from each of the original strands. After allowing several minutes for polymerization to run to completion, the cycle is repeated: denaturation, annealing, and polymerization. In the 2nd cycle, each of the four daughter strands can now serve as a template for DNA synthesis, so four duplexed DNA molecules will be generated. Thus, for each cycle of PCR, the population of DNA molecules doubles. Typically, at least 25 cycles of PCR are done, which would give a theoretical amplification of 225= 3.4 million.

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