Second step of PCR; involves binding of primers to denatured DNA strands at around 50°C. What is this step?

• A. Denaturation
• B. Annealing
• C. Extension
• D. Mammalian cells

Answer: (B)

This step is annealing. After the DNA strands are separated in the first step, the temperature is lowered to about 50°C to allow short primers to find and bind to their complementary sequences on the single-stranded DNA. Primers provide starting points for DNA synthesis, so binding at the right temperature is essential for specific attachment and efficient amplification. If the temperature is too high, primers won’t bind; if it’s too low, they may bind non-specifically. Once primers are bound, the next phase is extension, where DNA polymerase copies the target region from the primer ends. The option involving mammalian cells is not part of PCR.