In a typical PCR cycle, which description matches the annealing step?

• A. one to several mins at 50-65 C, primers anneal to their complementary sequences on either side of the target DNA sequence
• B. one to several mins at 94-96 C
• C. higher temp, tag polymerase binds and synthesizes complementary DNA strand to each primer
• D. no change in temperature and time is required

Answer: (B)

During PCR, the annealing step is when the primers find and attach to their complementary sequences on the single-stranded DNA template. This happens at a moderate temperature, typically about 50–65°C, for a short period so hydrogen bonds form between the primer and target without promoting nonspecific binding. This is different from denaturation, which uses a high temperature around 94–96°C to separate the DNA strands, and from extension, where a DNA polymerase extends from the annealed primers at roughly 72°C to build the new DNA strand. So the description that emphasizes primers binding to their targets at a mid-range temperature best represents the annealing step.