Explain briefly PCR.

PCR stands for polymerase chain reaction. In this reaction, multiple copies of the gene of interest are synthesized in vitro under three steps.
(i) Denaturation : In this, double stranded DNA is converted to the single stranded often achieved by heating or alkaline conditions. This is called "melting" of DNA.
(ii) Annealing : The two sets of primers (small chemical synthesized oligonucleotides that are complementary to the regions of DNA) undergo biochemical process of annealing at an optimum temperature of $40-65^0$C.
Extension: The enzyme DNA polymerase extends the primers using the nucleotides provided in the reaction and the genomic DNA as template.
In the process of replication of DNA is repeated many times, the segment of DNA can be amplified to approximately billion times. Such repeated amplification is achieved by the use of a thermostable DNA polymerase and the amplified fragment if desired can be used to ligate with a vector for further cloning.