Which of the following reaction is required for proofreading during DNA replication by DNA polymerase III?

Options

A. 5' to 3' exonuclease activity
B. 3' to 5' exonuclease activity
C. 3' to 5' endonuclease activity
D. 5' to 3' endonuclease activity

Answer

To determine which reaction is required for proofreading during DNA replication by DNA polymerase III, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding DNA Replication**: DNA replication involves the synthesis of a new DNA strand complementary to the template strand. DNA polymerases are the enzymes responsible for this process.

2. **Role of DNA Polymerase III**: DNA polymerase III is the primary enzyme involved in DNA replication in prokaryotes. It synthesizes new DNA strands by adding nucleotides in the 5' to 3' direction.

3. **Proofreading Mechanism**: During DNA replication, mistakes can occur where incorrect nucleotides are incorporated into the new strand. Proofreading is a mechanism that allows the enzyme to correct these mistakes.

4. **Direction of Proofreading**: The proofreading activity of DNA polymerase III occurs in the opposite direction of DNA synthesis. While DNA synthesis occurs from 5' to 3', proofreading occurs in the 3' to 5' direction.

5. **Exonuclease Activity**: The proofreading function is facilitated by the exonuclease activity of DNA polymerase III. This activity allows the enzyme to remove incorrectly paired nucleotides from the newly synthesized strand.

6. **Conclusion**: The specific reaction required for proofreading during DNA replication by DNA polymerase III is the **3' to 5' exonuclease activity**.

### Final Answer:
The reaction required for proofreading during DNA replication by DNA polymerase III is **3' to 5' exonuclease activity**.

---