Taq polymerase
Summary
Taq polymerase is a heat-stable DNA polymerase from the hot-spring bacterium Thermus aquaticus. It survives the roughly 95 °C denaturation step of PCR, so it does not need to be re-added each cycle. The testable point is that PCR uses it because of its thermostability.
Detail
Meaning: Taq polymerase is a DNA polymerase from Thermus aquaticus, a hot-spring bacterium. Like other DNA polymerases, it uses a template and adds dNTPs to the 3′ end of a primer, building DNA 5′ → 3′. Its key feature is thermostability: it stays functional after repeated heating to about 95 °C. It works fastest near 72 °C, needs Mg²⁺, and lacks 3′ → 5′ proofreading, so it makes more errors than high-fidelity polymerases.
Equation: N = N₀ × 2ⁿ, where N is the number of double-stranded copies, N₀ is the starting number of template molecules, and n is the number of cycles (all unitless), assuming perfect doubling each cycle.
On the MCAT: A PCR passage lists denaturation near 95 °C, primer annealing near 50–65 °C, and extension near 72 °C. Expect questions on why Taq is chosen, or on what happens with a non-thermostable polymerase such as one from E. coli (it denatures at 95 °C, so amplification fails). Reagents supplied: template, two primers, dNTPs, Mg²⁺.
Common confusion: Heat, not Taq, separates the strands, and Taq does not make primers. Taq is useful because it survives denaturation, so it need not be added every cycle.
Sources
- Lehninger Principles of Biochemistry
- Molecular Biology of the Cell
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.