How do primers anneal to ssDNA? 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?

**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.)