PCR enzyme choice — why thermophilic DNA polymerases? What is the principal advantage of using DNA polymerases from thermophilic organisms (e.g., Taq, Pfu) in the polymerase chain reaction (PCR)?

Introduction / Context:
PCR repeatedly cycles DNA through denaturation, annealing, and extension. High temperatures (typically 90–98 °C) are required to denature double-stranded DNA. Enzymes from thermophiles remain active after such heat pulses, enabling continuous cycling without replenishing polymerase after each denaturation step, which was necessary in early PCR experiments with mesophilic enzymes.

Given Data / Assumptions:

• Denaturation requires very high temperatures.
• DNA polymerase must remain active across many cycles.
• Fidelity varies by enzyme family (A- vs B-family polymerases).

Concept / Approach:

Thermostable polymerases such as Taq (A-family) and Pfu (B-family) retain catalytic activity after repeated exposure to high temperatures, enabling automation in thermal cyclers. Speed and fidelity depend on the enzyme: Pfu has higher proofreading fidelity; Taq is faster but lacks 3'→5' exonuclease proofreading. Neither claim absolute zero error rates, and primers are still required to define target boundaries.

Step-by-Step Solution:

Verification / Alternative check:

Enzyme activity assays after repeated thermal cycling show sustained activity of thermophilic polymerases, whereas mesophilic enzymes are rapidly inactivated.

Why Other Options Are Wrong:

Speed (A) is context-dependent and not universally higher. Error-free replication (C) is unrealistic. Primers are essential in PCR (E).

Common Pitfalls:

Assuming thermostability equals highest fidelity; enzyme choice balances fidelity, processivity, and speed.

Final Answer:

They can withstand the high denaturation temperatures needed to separate DNA strands during PCR cycles