Why is the DNA polymerase used in PCR described as heat-stable?

• A. It binds primers more quickly.
• B. It denatures after cycling.
• C. It increases the mutation rate.
• D. It is not denatured at 95°C.

Answer: (D)

The key idea is that PCR requires the enzyme to stay active after exposure to very high temperatures. During PCR, the DNA is heated to about 95°C to separate the strands, then cooled to allow primers to bind, and finally extended at around 72°C. Most enzymes would lose their shape and activity at such high heat, so a heat-stable (thermostable) polymerase is used. This kind of enzyme remains folded and active even after the high-temperature denaturation step, allowing many cycles of amplification without being replaced. That’s why the description fits: the polymerase is not denatured at 95°C.

The other statements don’t fit because they describe traits not related to surviving high heat. A polymerase that binds primers more quickly isn’t about heat stability, and a polymerase that denatures after cycling would fail to function in subsequent cycles. Increasing mutation rate isn’t a desired feature of PCR, and primer binding speed isn’t what makes the enzyme useful in PCR.