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four, eight, sixteen and so on so that after 20 cycles approximately 10^6
copies can be generated from the original.
Two key components essential to the successful application of PCR are
precision automated thermal cycling equipment and a heat stable DNA
polymerase which will survive the DNA denaturation step and catalyse
subsequent extension of the primers. The latter, known as Taq polymerase,
is obtained from the very thermophilic bacteriumThermus aquaticus.
There have been numerous embellishments of the basic PCR tech-
nique. These include:-

 Multiplex PCR which uses several primer pairs for the simultaneous

amplification of a number of targets.

 Nested PCR which improves the sensitivity of the reaction by first

amplifying a larger sequence using a pair of outer primers followed

Figure 10.6 The polymerase chain reaction

392 Methods for the Microbiological Examination of Foods