PCR stands for polymerase Chain Reaction. The basic requirements of a PCR reaction are the following:

(i) DNA template: Any source that contains one or more target DNA molecules to be amplified can be taken as template.

(ii) Two nucleotide primers which are oligonucleotides, that hybridizes to the target DNA region.

(iii) Enzyme DNA polymerase which is thermo stable like tag polymerase.

(iv) Four types of dioxyribonucleotides called dNTPs.

The three essential steps for PCR technique are:

(i) Denaturation: The target DNA is heated to a high temperature (usually 94o to 96o C),resulting in the separation of the two strands each of which then acts as a template for DNA synthesis.

(ii) Annealing: In this step, the two oligo-nucleotide primers anneal to each of the single – stranded template DNA. This step is carried out a lower temperature.

(iii) Extension (polymerisation): The final step is extension, where in taq DNA polymerase synthesizes the DNA region between the primers, using dNTPs and Mg²⁺. By this the primers are extended towards each other so that the DNA segment lying between the two primers is copied. The optimum temperature for this step is 72o C. These three steps constitute one cycle of the reaction. The process is carried out for about 28 – 30 cycles beyond which its reliability decreases.