What is the polymerase chain reaction? Briefly state how it works.

The polymerase chain reaction (PCR) is a technique used by researchers to quickly amplify a specific sequence of DNA. The DNA to be copied (the target DNA) is placed in a tube along with short DNA sequences called primers, which are complementary to the ends of the target DNA. A supply of A,G,C and T nucleotides is also added to the mixture, as is a DNA polymerase enzyme that is capable of withstanding high temperatures (Taq polymerase). The mixture is then placed in a thermocycler-a device that can be programmed to alternate among various temperatures.
In PCR there are three temperature stages. The first stage heats the DNA up to the point where it separates into single strands. In the second stage, the temperature is lowered, allowing the primers to attach to the ends of the target DNA. In the third stage, the Taq polymerase copies each of the single strands into double stranded DNA, doubling the amount of the original target DNA. The process is then repeated, usually around 30 times, doubling the amount of target DNA each time. The final result can be billions of copies of the original target DNA.