The polymerase chain reaction (PCR) is a widely used molecular biology technique used to amplify a specific segment of DNA (deoxyribonucleic acid) through a series of temperature-controlled cycles. PCR enables the exponential amplification of a targeted DNA sequence, making it possible to generate millions to billions of copies of the DNA segment of interest from a small starting amount.
The PCR process typically involves three main steps:
Denaturation: The double-stranded DNA template is heated to a high temperature (usually around 95°C), causing the two strands to separate and form single-stranded DNA molecules.
Annealing: The reaction mixture is cooled to a temperature typically between 50°C to 65°C, allowing short DNA sequences called primers to bind (anneal) to complementary sequences at each end of the target DNA segment.
Extension (elongation): The temperature is raised to around 72°C, the optimal temperature for the DNA polymerase enzyme to synthesize new DNA strands by extending from the primers along the template DNA, using nucleotides provided in the reaction mixture.
By repeating these cycles (usually 20-40 times), each cycle doubles the amount of DNA, resulting in an exponential increase in the number of copies of the target DNA sequence. PCR is employed in various applications, including molecular biology research, medical diagnostics, forensics, environmental testing, and genetic engineering.