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Polymerase Chain Reaction
  1. Karen E. Morrison
  1. Department of Clinical Neurosciences, The Medical School, University of Birmingham, Edgbaston, Birmingham; E-mail: k.morrison{at}


The polymerase chain reaction (PCR) is the single most important tool in biological sciences that has been developed in the last 20 years, with numerous applications in research and diagnosis. It is a test-tube method to selectively copy specific target DNA sequences – within a given source of DNA – and was ‘invented’ in 1986 following the identification of a specific DNA polymerase* that can withstand repeated heating to high temperatures. It came into widespread laboratory practice when efficient thermal cycling machines became available.


The five components of the basic PCR are:

  1. A source of template DNA containing the target sequence to be amplified (Fig. 1). This template can be either total genomic DNA containing exons* and introns*, or complex cDNA* mixtures.

  2. Two short single-stranded sequences of DNA termed ‘primers’ or ‘amplimers’, designed on the basis of previous knowledge of the DNA target sequence. They flank

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