PCR

Polymerase chain reaction (PCR) is a molecular biology technique used to amplify a specific DNA segment. By starting from a small amount of DNA, PCR can generate millions to billions of copies, enabling analysis that would be impractical with the original material. The method relies on a DNA polymerase, two primers, nucleotides, and a suitable buffer.

During PCR, a DNA template is repeatedly cycled through three stages: denaturation, primer annealing, and strand extension. Each cycle doubles the amount of target DNA, leading to exponential amplification. The reaction typically uses a thermostable DNA polymerase that remains active across repeated heating and cooling.

Variants include reverse transcription PCR (RT-PCR), which starts from RNA and converts it to DNA; quantitative PCR (qPCR), which monitors amplification in real time with fluorescent signals; and digital PCR, which partitions the reaction to count target molecules. Multiplex PCR uses multiple primer sets to amplify several targets simultaneously.

PCR was developed in 1983 by Kary Mullis, and its development transformed research, medicine, forensics, and clinical diagnostics. It has become a routine tool in laboratories worldwide. The technique requires careful primer design and experimental controls to avoid nonspecific amplification and contamination.

Limitations include sensitivity to contamination, inhibitors present in samples, and potential for false positives or negatives if controls are not properly used. Despite these caveats, PCR remains a foundational method in molecular biology and has many specialized variants and applications.