PCR fundamentals — The thermostable DNA polymerase originally popularized for routine PCR was obtained from which source enzyme?

Introduction / Context:
Polymerase chain reaction (PCR) relies on repeated high-temperature denaturation steps. Early PCR protocols required adding fresh polymerase after each cycle because mesophilic enzymes were heat-inactivated. The breakthrough came with thermostable polymerases that survive denaturation temperatures, enabling automated cycling and revolutionizing molecular biology.

Given Data / Assumptions:

• “Originally popularized” refers to the first widely used thermostable DNA polymerase in PCR workflow.
• Thermostability is required to withstand ~94–98°C during denaturation.
• We distinguish DNA polymerases from RNA polymerases and mesophilic sources.

Concept / Approach:
Taq polymerase, isolated from Thermus aquaticus, is thermostable and became the standard enzyme for PCR in the late 1980s and 1990s. While E. coli DNA polymerase I (Klenow fragment) was used in very early manual protocols, it lacked thermostability. E. coli RNA polymerase transcribes RNA and is not used for DNA-dependent DNA synthesis in PCR.

Step-by-Step Solution:

Verification / Alternative check:
Thermocyclers paired with Taq enabled exponential amplification without manual enzyme addition, demonstrating its practicality and popularity.

Why Other Options Are Wrong:

• E. coli DNA polymerase: heat-labile; impractical for automated cycling.
• E. coli RNA polymerase: not a DNA-dependent DNA polymerase.
• All of these: incorrect because only Taq satisfies the thermostable PCR requirement.

Common Pitfalls:
Confusing later high-fidelity polymerases (e.g., Pfu, Phusion) with the original widely used enzyme; the question asks about the original popularized choice.

Final Answer:
Taq polymerase