Polymerase chain reaction (PCR) refers to an in vitro enzymatic method used to amplify a targeted segment of DNA. In a controlled series of temperature changes, a thermostable DNA polymerase synthesizes new strands from specific primers, doubling the amount of the selected region with each cycle.
Explanation
PCR was introduced in 1983 as a fast and selective way to multiply DNA fragments without cloning them into cells. The method relies on the ability of primers—short synthetic oligonucleotides—to bind to sequences flanking the target. A mixture containing the template DNA, primers, free nucleotides, buffer and a thermostable polymerase is placed in a thermal cycler. The temperature is raised to separate the DNA strands, lowered to permit primer binding and then set to an optimal level for the enzyme to extend the primers. This cycle is repeated 20–40 times, resulting in exponential amplification; after thirty cycles, a single molecule can yield more than a billion copies. Taq polymerase from the bacterium Thermus aquaticus was the first enzyme widely used because it withstands high temperatures; more recently, high-fidelity enzymes have improved accuracy. Variants of the technique include reverse transcription PCR, which converts RNA to DNA before amplification, and quantitative PCR, which measures fluorescence in real time to quantify the amount of product. PCR has become fundamental in molecular diagnostics, genome sequencing, cloning and genetic engineering.
Key Applications and Facts
PCR underpins many routine tests. Clinicians use it to detect genetic mutations responsible for inherited disorders and to identify infectious agents by amplifying pathogen DNA or RNA directly from clinical samples. In forensic investigations, PCR amplifies small amounts of DNA from blood, saliva or hair to generate a profile for identity matching. Researchers use PCR to insert or modify genes, sequence genomes and analyze gene expression. It also enables rapid detection of genetically modified organisms in food and helps monitor biodiversity by amplifying environmental DNA from soil or water samples. The technique’s sensitivity means contamination can lead to false positives, so careful laboratory practice and controls are essential.
PCR (Polymerase Chain Reaction) has transformed biology by making it possible to create large quantities of a precise DNA fragment quickly. Its adaptability has led to numerous variants tailored for specific purposes, and it remains a core method in research, medicine and forensic science.
Related Terms: DNA replication, Molecular diagnostics, Kary Mullis, qPCR, Thermal cycling