Which of the following is the primary function of Taq polymerase in PCR?

Explanation

## Taq Polymerase: Structure and Function **Key Point:** Taq polymerase (Thermus aquaticus DNA polymerase) is a thermostable enzyme that synthesizes DNA in the 5' to 3' direction and LACKS 3' to 5' exonuclease activity, making it ideal for PCR. ### Why Taq is Chosen for PCR | Feature | Taq Polymerase | Standard DNA Polymerase (Pol III) | | --- | --- | --- | | **Direction of synthesis** | 5' to 3' | 5' to 3' | | **3' to 5' exonuclease** | Absent | Present | | **Thermal stability** | Stable up to 95°C | Denatures at ~65°C | | **Processivity** | ~30–50 nucleotides | High (>1000 nt) | | **Error rate** | ~1 in 10,000 | ~1 in 10^7 | **High-Yield:** The absence of 3' to 5' exonuclease activity in Taq is actually an advantage in PCR because: 1. It survives repeated heating cycles (denaturation at 94–95°C). 2. It adds an extra adenine (A) to the 3' end of PCR products, enabling TA cloning. **Clinical Pearl:** Modern high-fidelity polymerases (e.g., Pfu, Phusion) DO have 3' to 5' exonuclease activity and are used when error-free amplification is critical (e.g., cloning, sequencing prep). ### PCR Cycle Overview ```mermaid flowchart TD A[Double-stranded DNA template]:::outcome --> B[Denaturation at 94-95°C]:::action B --> C[Separation into single strands]:::outcome C --> D[Annealing at 50-65°C]:::action D --> E[Primers bind to template]:::outcome E --> F[Extension at 72°C]:::action F --> G[Taq polymerase synthesizes 5' to 3']:::outcome G --> H[Repeat 25-35 cycles]:::action H --> I[Exponential amplification]:::outcome ``` **Mnemonic:** **TAQ = Thermostable, Amplification-friendly, Quick** (no proofreading).