How does the polymerase chain reaction (PCR) work?

PCR is a method to isolate and amplify a specific fragment of DNA that you would like to study. The reaction is set up with template DNA, primers, DNA polymerase enzyme and free nucleotides. Primers are short sections of DNA that are complimentary the beginning of the DNA fragment you are interested in - DNA polymerase binds these primers, and is able to extend the copy by adding free nucleotides that are complimentary to the template sequence. First, the mixture is heated to 95^oC to separate the 2 strands of the template DNA, by breaking the hydrogen bonds between them. The mixture is then cooled to about 60^oC to allow the primers to anneal (bind) to the now separated strands. Finally, the mixture is heated to 72^oC, as this is the temperature the DNA polymerase used works optimally at - it can then extend the copy strands by binding the sequence of bases that are complimentary to the template sequence. 2 complementary strands bound to the 2 template strands are formed. This process is then repeated, as the mixture is heated to 95^oC and all 4 strands are used as templates. This is repeated over several hour to form millions of copies of the desired DNA fragment.