CBSE Class 12-science Answered

Polymerase Chain Reaction (PCR) is a technique used to synthesize multiple copies of the desired gene in vitro within a short span of time.

Amplification of a gene sample of interest is carried out using PCR in the following three steps:

(a) Denaturation: The double stranded DNA is denatured by applying high temperature of about 95C for 15 seconds. Each separated single stranded strand now acts as template for the synthesis of new DNA strand.

(b) Annealing: Two sets of primers are added which anneal at the 3’ end of each separated strand. They mark the beginning of replication.

(c) Extension: A thermostable DNA polymerase, Taq polymerase extends the primers by adding nucleotides complementary to the template provided in the reaction.

The cycle of denaturation, annealing and extension is repeated several times to obtain several copies of desired DNA.