During PCR, at what temperature does the annealing of primers occur?

The process of primer annealing during PCR takes place at around 55°C, which is considered optimal for enabling the primers to bind to their complementary sequences on the DNA template. This temperature is lower than the denaturation temperature (typically around 94°C), which is used to separate the double-stranded DNA.

During the annealing phase, the lowered temperature allows the DNA primers, which are short sequences of nucleotides, to find and bind to the complementary sections on the single strands of DNA. If the temperature is too high, the primers may not bind effectively, resulting in lower yields of the desired DNA product. Conversely, if the temperature is too low, non-specific binding may occur, which can lead to the amplification of undesired products.

The other temperatures listed do not correspond to the correct annealing conditions: 94°C is associated with the denaturation phase, 72°C is generally used for the elongation step where DNA polymerase synthesizes the new DNA strand, and 37°C is typically too low for effective annealing of primers in the context of standard PCR protocols.