Why is thermostable DNA polymerase used in PCR?

• A. To elongate primers faster
• B. To prevent mutations
• C. To function without magnesium
• D. To withstand high temperatures during denaturation

Answer: (D)

PCR cycles include a high-temperature denaturation step that melts the DNA strands. The enzyme used to copy DNA must stay active through these repeated heatings, so amplification can occur in every cycle.

Thermostable DNA polymerases, like those from heat-loving bacteria, are stable and functional at the temperatures used to denature DNA. This means the same enzyme can catalyze DNA synthesis during each extension phase without being inactivated by the heat. If a non-thermostable polymerase were used, it would lose activity during denaturation and amplification wouldn’t proceed efficiently. Other options aren’t the main reason: elongating primers faster isn’t about surviving heat, preventing mutations isn’t guaranteed by thermostability (and many thermostable enzymes lack proofreading), and polymerases still require magnesium as a cofactor to function. The essential point is the enzyme’s ability to withstand the high temperatures of denaturation.