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PCR reaction is used to amplify DNA fragment. Each reaction requires a DNA template, buffer, dNTP mix and a unique pair of primers, one ''forward'' primer and one ''reverse'' primer. The DNA polymerase should be added last to the reaction, just before starting the PCR programme.

My question is, why is DNA polymerase added right before the PCR starts? Is it because it can begin to amplify DNA before adding the reaction to the thermocycler? How could it amplify the DNA when the DNA is not denaturated until added to the thermocycler?

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True, the template is double stranded, but the primers could begin amplifying themselves, and any other stray DNA. You don't want any nonspecific amplification in the early stages that could compete with the target fragment. Also, even though Taq polymerase is a very hardy enzyme, it is standard practice to treat enzymes with special care, on ice so they don't degrade (although I have forgotten them on the bench overnight, and they survived that).

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