In polymerase chain reaction, polymerisation or extension step is catalysed by Taq polymerase enzyme. PCR is carried out in the following three steps:

(i) Denaturation: The double-stranded DNA is denatured by applying high temperature of for 15 seconds, each separated single-stranded strand now acts as template for DNA synthesis.
(ii) Annealing: Two sets of primers are added which anneal to the 3' end of each separated strand. Primers act as initiators of replication.
(iii) Extension: DNA polymerase extends the primers by adding nucleotides complementary to the template provided in the reaction.

A thermostable DNA polymerase (Taq DNA polymerase) is used in the reaction which can tolerate the high temperature of the reaction.

All these steps are repeated many times to obtain several copies of desired DNA.