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Polymerase Chain Reaction or PCR is a molecular technique used to generate a very number of copies of a piece of DNA. PCR has found numerous applications in areas such as, forensic science, microbiology, biotechnology, pharmaceutical industry, etc. The reaction consists of 3 steps: Denaturation, Annealing and Elongation. Each one of these cycles represents certain operation at a certain temperature and these cycles are repeated 20-40 times. In the denaturation step, the DNA piece is heated (at around 95 degrees Celsius) to break open the double strand (by breaking the hydrogen bond holding the two strands together) and obtain single strands. In the annealing step, lower temperature (of about 50-65 degrees Celsius) causes the primers to bind to the single strands. The primers are oligonucleotide molecules that are complementary to 3' ends of the DNA fragment. In the elongation step, DNA polymerase enzymes extend the DNA fragment by adding nucleotides in a particular direction (5' to 3' direction).
Depending on the temperatures used and the number of cycles, we can end up with thousand to millions of copies of starting DNA fragment.
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