PCR (Polymerase Chain Reaction)

IT is a technique in which cycles of denaturation, annealing with primer, and extension with DNA polymerase, are used to amplify the number of copies of a target DNA sequence by more than 100 times in a few hours. American molecular biologist Kary Mullis developed the techniques of PCR in the 1970s. For his ingenious invention, he was awarded the 1993 Nobel Prize in physiology or medicine.


PCR amplification of DNA is like any DNA replication by DNA polymerase in vivo. (in lving cells) The difference is that PCR produces DNA in a test tube. For a PCR reaction to proceed, four components are necessary: template, primer, deoxyribonecleotides (adenine, thymine, cytosine, guanine) and DNA polymerase. In addition, part of the sequence of the targeted DNA has to be known in order to design the according primers. In the first step, the targeted double stranded DNA is heated to over 194°F (90°C) for denaturation. During this process, two strands of the targeted DNA are separated from each other. Each strand is capable of being a template. The second step is carried out around 122° (50°C). At this lowered temperature, the two primers anneal to their complementary sequence on each template. The DNA polymerase then extends the primer using the provided NUCLEOTIDES. As a result, at the end of each cycle, the numbers of DNA molecules double.

PCR was initially carried out manually in incubators of different temperatures for each step until the extraction of DNA polymerase from thermophilic bacteria. The bacterium Thermus aquaticus was found in Yellow Stone National Park. This bacterium lives in the hot springs at 203°F (95°C). The DNA polymerase from T. aquaticus keeps its activity at above 95°C for many hours. Several additional heat-resistant DNA polymerases have also now been identified.