Which of the following steps is/are catalysed by Taq polymerase in a PCR ?

Options

A. Denaturation of template DNA
B. Annealing of primers to template DNA
C. Extension of primer end on template DNA
D. All of these

Answer

To determine which steps in the Polymerase Chain Reaction (PCR) are catalyzed by Taq polymerase, we need to understand the three main steps involved in PCR: denaturation, annealing, and extension.

### Step-by-Step Solution:

1. **Understand the PCR Process**:
- PCR is a technique used to amplify DNA, allowing for the creation of millions of copies from a small initial sample. It consists of three main steps: denaturation, annealing, and extension.

2. **Denaturation**:
- In this step, the double-stranded DNA is heated to a high temperature (typically around 94-96°C). This heat causes the hydrogen bonds between the DNA strands to break, resulting in two single-stranded DNA molecules.
- **Role of Taq Polymerase**: Taq polymerase is not involved in the denaturation step. Its role is not to break the DNA strands but rather to synthesize new DNA strands.

3. **Annealing**:
- After denaturation, the temperature is lowered to allow primers to bind (anneal) to the single-stranded DNA templates. This step prepares the DNA for the next phase.
- **Role of Taq Polymerase**: Taq polymerase does not catalyze the annealing process. It is not involved in the binding of primers to the DNA strands.

4. **Extension**:
- In this final step, the temperature is raised again (to around 72°C) to allow Taq polymerase to synthesize new DNA strands by adding nucleotides to the primers. This is where the actual replication of the DNA occurs.
- **Role of Taq Polymerase**: Taq polymerase is actively involved in the extension step, as it catalyzes the addition of nucleotides to form new DNA strands.

5. **Conclusion**:
- Taq polymerase catalyzes only the extension step of the PCR process. It does not play a role in denaturation or annealing.

### Final Answer:
Taq polymerase catalyzes the **extension** step in PCR.

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