Explain $PCR$ in short.

(N/A) $PCR$ stands for Polymerase Chain Reaction. In this reaction,multiple copies of the gene (or $DNA$) of interest are synthesized in vitro using two sets of primers (small chemically synthesized oligonucleotides that are complementary to the regions of $DNA$) and the enzyme $DNA$ polymerase.
The enzyme extends the primers using the nucleotides provided in the reaction and the genomic $DNA$ as a template. If the process of replication of $DNA$ is repeated many times,the segment of $DNA$ can be amplified to approximately $1$ billion times. Such repeated amplification is achieved by the use of a thermostable $DNA$ polymerase (isolated from a bacterium,Thermus aquaticus),which remains active during the high-temperature-induced denaturation of double-stranded $DNA$. The amplified fragment,if desired,can now be used to ligate with a vector for further cloning.