Two sets of primers and the enzyme DNA polymerase are required in each cycle of polymerase chain reaction. Role of primers and DNA polymerase : Primers are necessary to start the functioning of DNA polymerase. DNA polymerase extends the primers using the nucleotides provided in the reaction and the genomic DNA as the template. The segment of DNA can be amplified to approximately billion times. Such repeated amplification is achieved by the use of a thermostable is achieved by the use of a thermostable DNA polymerase, which remains active during the high-temperature induced denaturation of double stranded DNA. DNA polymerase used in PCR reaction is known as Taq polymerase, which is obtained from a bacterium Thermus aquaticus. It remains active during the high-temperature induced denaturation of double-stranded DNA.