Which enzyme is used to synthesize new DNA strands during PCR, and why is its heat stability important?

• A. A DNA ligase; because ligation occurs at high temperatures.
• B. A thermostable DNA polymerase (e.g., Taq) is used; heat stability allows repeated denaturation and annealing cycles without enzyme inactivation.
• C. RNA polymerase; because it copies DNA at high temps.
• D. Restriction enzyme; because it cuts at high temps.

Answer: (B)

PCR relies on a DNA polymerase to synthesize new DNA strands, and the enzyme must survive repeated heating. A thermostable DNA polymerase, like Taq, is used because the process includes cycles that heat the sample to denature the DNA (around 95°C). After denaturation, the temperature is lowered for annealing and then raised again for extension, so the enzyme must remain active throughout these cycles. If the enzyme were not heat-stable, it would become inactivated during the high-temperature step and amplification would stop. The other enzymes listed don’t synthesize DNA during PCR: a ligase only seals nicks, RNA polymerase copies RNA, and restriction enzymes cut DNA.