Which step in PCR involves cooling the mixture to allow primers to attach?

In the process of Polymerase Chain Reaction (PCR), the correct step where the mixture is cooled to allow primers to attach is called annealing. During annealing, the temperature is lowered to enable the primers to bind or "anneal" to their complementary sequences on the single-stranded DNA templates. This binding is crucial because the primers provide a starting point for the DNA polymerase to synthesize new DNA strands.

Heating the reaction mixture to high temperatures in the denaturation step separates the double-stranded DNA into single strands. After that, cooling is necessary for the annealing step to occur, allowing the primers to specifically attach to the target sequences. Following this, the elongation step occurs at a higher temperature where DNA polymerase synthesizes new DNA strands by extending from the primers. Hence, understanding the process emphasizes that annealing is the step responsible for primer attachment due to the temperature condition that promotes this interaction.