Polymerase chain reaction (PCR) is a way to make many copies of a sequence of DNA. This is done in a lab, using an enzyme called DNA polymerase. It is called chain reaction because the result of one cycle is used immediately for the next cycle. This allows exponential growth to happen.
PCR has many uses in a biological or biochemical setting. Because DNA is unique for every living thing, experimenters can often extract only small amounts of the DNA they are interested in from a specimen. These amounts are usually too little to be useful, and so a scientist will use PCR to make enough copies to start experimenting with. For this reason, it is one of the most common techniques used in genetics labs around the world, making it useful in experiments on many things, including gene therapy, infectious diseases, and even forensics.
This process was developed in 1983 by Kary Mullis. He was not the first person to develop the PCR process, but he was the first to make the process usable. He was awarded the Nobel Prize in Chemistry along with Michael Smith for their work in the PCR process.
The method consists of repeated heating and cooling, causing "melting" (separation of the two strands) and replication of the original DNA, also called a template. Short DNA fragments consisting of DNA sequences complementary to the ends of the template, called primers, and a DNA polymerase are key materials for selective and repetitive steps.
The technique proceeds in three steps:
- denaturation — The template strands that are bound together cannot be replicated, so the first step of PCR is to separate them by heating up the sample, breaking the hydrogen bonds between them.
- annealing — The sample is cooled just enough to allow the primers to bind to the ends of each of the two template strands
- extension — DNA polymerase attaches to the primers and makes a copy of each template strand.
After the first cycle, there are 4 DNA strands. The process repeats with the 4 DNA strands, which will go on to make 8 strands, then repeat itself again to make 16 strands. In this way, PCR doubles the amount of DNA in a sample after each cycle, making it possible to obtain millions of copies of a DNA strand overnight.
Images for kids
Ethidium bromide-stained PCR products after gel electrophoresis. Two sets of primers were used to amplify a target sequence from three different tissue samples. No amplification is present in sample #1; DNA bands in sample #2 and #3 indicate successful amplification of the target sequence. The gel also shows a positive control, and a DNA ladder containing DNA fragments of defined length for sizing the bands in the experimental PCRs.
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