Polymerase chain reaction includes three steps - denaturation, annealing and elongation or extension. Denaturation - the process by which the DNA strands unwind and denature to form two separate strands. This step occurs at a very high temperature. Annealing - the process by which the DNA primer attaches to the DNA template strand by complementary base pairing. Elongation - it is the process of addition of nucleotides. A thermostable enzyme called Taq polymerase. This enzyme is extracted from the bacteria called Thermus aquaticus.