Which enzyme is used for DNA synthesis in PCR and remains active at high temperatures?

• A. Denaturation
• B. Extension
• C. Annealing
• D. Taq Polymerase

Answer: (D)

DNA synthesis during PCR is carried out by a DNA polymerase that can tolerate the high temperatures used in amplification. Ordinary enzymes would denature during the heat steps, but a thermostable polymerase from a heat-loving organism remains active after the temperature jumps. Taq polymerase, isolated from Thermus aquaticus, works during the extension step to synthesize new DNA by adding nucleotides to the growing strand in the 5' to 3' direction. This temperature resilience lets PCR cycle quickly through denaturation, annealing, and extension without needing fresh enzyme after each cycle. The other terms refer to stages—denaturation (separating strands), annealing (primers binding), extension (synthesis by the polymerase)—not enzymes themselves.