 As PCR progresses, the DNA generated is itself used as a template for replication, setting in motion a chain reaction in which the original DNA template is exponentially amplified.Almost all PCR applications employ a heat-stable DNA polymerase, such as Taq polymerase, an enzyme originally isolated from the thermophilic bacterium "Thermus aquaticus".If the polymerase used was heat-susceptible, it would denature under the high temperatures of the denaturation step.