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Schematic showing the steps of the Polymerase Chain Reaction. Denaturation: DNA strand are separated at high temperature denaturation. Annealing or hybridization: Temperature is decreased and complementary primer sequences that flank the target sequences anneal to the target. Extension: Complementary nucleotides are added into the growing strand by the DNA polymerase enzyme. Extension usually continues until the template sequence has been completely copied. This process is repeated between 25-40 times, to generate a large number of copies from the original DNA template.