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In a PCR type protocol, the high temperature stage of the cycle will cause dsDNA to denature, as well as any annealed primers. At the lower temperature the denatured dsDNA will remain denatured. As far as I understand, though, the primers are re-annealed.

My question is this: are there any enzymes that catalyses this re-annealing, or does this happen by chance?

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No enzyme is involved in base-pairing (annealing).

This does happens by chance, but the chances are governed by stoichiometric considerations.

The main such consideration is that the (low molecular weight) primers are added at a high concentration (greater than that of the DNA) and therefore have a greater chance of encountering the appropriate region of the DNA than its complementary strand does. (Reannealing of the denatured DNA strands can occur, but not over the time period of PCR.)

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