Question

Which one of the following statements about the DNA polymerase III of E. coli is NOT correct?

• A. It catalyzes nick translation.
• B. Its absence is lethal to \( E. coli \).
• C. It synthesizes a complementary DNA strand using a single-stranded template.
• D. It possesses 3’ → 5’ exonuclease activity.

Hint:

DNA polymerase III is essential for replication and has both polymerase activity and proofreading (exonuclease) activity, but it does not catalyze nick translation.

Answer 1

The Correct Option is A

Step 1: Understanding the Function of DNA Polymerase III.
DNA polymerase III is a key enzyme in the replication of DNA in \( E. coli \). It synthesizes the complementary DNA strand using a single-stranded template and possesses a 3’ → 5’ exonuclease activity that allows it to proofread the newly synthesized strand. Its absence is lethal to \( E. coli \) as it is essential for DNA replication.
Step 2: Analyzing the Options.
- (A) It catalyzes nick translation: This is incorrect. Nick translation is performed by DNA polymerase I, not DNA polymerase III. DNA polymerase I has both 5’ → 3’ polymerase activity and 3’ → 5’ exonuclease activity, which allows it to replace the RNA primer with DNA during replication.
- (B) Its absence is lethal to \( E. coli \): This is correct. DNA polymerase III is essential for replication in \( E. coli \), and its absence leads to cell death.
- (C) It synthesizes a complementary DNA strand using a single-stranded template: This is correct. DNA polymerase III uses a single-stranded template to synthesize a complementary DNA strand during replication.
- (D) It possesses 3’ → 5’ exonuclease activity: This is correct. DNA polymerase III possesses 3’ → 5’ exonuclease activity, which enables proofreading and correction of errors during replication.
Step 3: Conclusion.
The correct answer is (A) because DNA polymerase III does not catalyze nick translation; this function is performed by DNA polymerase I.