At what temperature does the extension phase of PCR typically occur?

The extension phase of PCR typically occurs at a temperature range of 68°C to 72°C. This phase is crucial as it is when the DNA polymerase synthesizes new strands of DNA by adding nucleotides to the primers that have bound to the target DNA template.

At this temperature, the DNA polymerase is optimally active, allowing for efficient and accurate extension of the DNA strands. Different DNA polymerases have specific optimal temperatures, but the range of 68°C to 72°C is widely used because it is suitable for many commonly used DNA polymerases such as Taq polymerase.

In contrast, the lower ranges of temperature (50°C to 55°C and 60°C to 65°C) are more typical for the annealing phase, where primers bind to the template DNA, while higher temperatures (75°C to 80°C) may exceed the optimal limits for most DNA polymerases, potentially leading to decreased extension efficiency or enzyme denaturation. Thus, 68°C to 72°C is the standard temperature range for the extension phase in PCR protocols.