3-2.1 Introduction:

Polymerase chain reaction (PCR) is a widely employed technique in molecular biology to amplify single or a few copies of DNA, generating millions of copies of a particular DNA sequence. The polymerase chain reaction results in the selective amplification of a target region of a DNA or RNA molecule. PCR has been extensively exploited in cloning, target detection, sequencing etc. The method consists of thermal cycles of repeated heating followed by cooling of the reaction mixture to achieve melting and primer hybridization to enable enzymatic replication of the DNA.

3-2.2 History:

By 1971, a “repair synthesis" process was reported which was an artificial system containing primers and templates that can allow DNA polymerase to copy target gene. The DNA polymerases initially employed for in vitro experiments were unable to withstand these high temperatures. In 1976, Chien et al discovered a novel DNA polymerase from the extreme thermophile Thermus aquaticus which naturally dwell in hot water spring (122 to 176^°F). The enzyme was named as Taq DNA polymerase which is stable upto 95^°C. In 1985, Kary Mullis invented a process Polymerase Chain Reaction (PCR) using the thermo-stable Taq polymerase for which he was awarded Nobel Prize in 1993.

Fig. 3-2.2: PCR Thermo cycler (Adapted from http://products.invitrogen.com/ivgn/product/4452300 )