Question

Which of the following functions are associated with the DNA polymerase I
(A) Proof Reading
(B) Removal of Primer
(C) Nick Translation
(D) DNA Ligation
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (C) and (D) only.
• C. (A), (B) and (C) only.
• D. (B), (C) and (D) only.

Hint:

DNA Polymerase I is the only E. coli polymerase with 5' to 3' exonuclease activity.
This unique activity allows it to perform both primer removal and nick translation.
The remaining nick is subsequently sealed by DNA Ligase.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
DNA Polymerase I (Pol I) is a multi-functional enzyme in E. coli that plays a critical role in DNA replication and repair.
It possesses distinct enzymatic activities contained within a single polypeptide chain.

Step 2: Detailed Explanation:
DNA Polymerase I possesses three major enzymatic activities:
1. 5' to 3' Polymerase activity: Synthesizes DNA by adding nucleotides to the 3' end of a primer.
2. 3' to 5' Exonuclease activity (A): Performs proofreading by recognizing and removing mismatched bases at the 3' end.
3. 5' to 3' Exonuclease activity (B): Responsible for the removal of RNA primers from Okazaki fragments during lagging strand replication, or damaged DNA during repair.
- Nick Translation (C) is a combined function of Pol I.
The 5' to 3' exonuclease activity degrades the RNA primer or damaged DNA ahead of the enzyme, while the 5' to 3' polymerase activity simultaneously synthesizes new DNA behind it, effectively "moving" the nick forward.
- DNA Ligation (D) is the sealing of the remaining single-stranded nick (joining the 3'-OH and 5'-phosphate).
This ligation is performed by a completely separate enzyme called DNA Ligase, not DNA Polymerase I.
Thus, (A), (B), and (C) are associated with DNA Polymerase I, while (D) is not.

Step 3: Final Answer:
The functions associated with DNA Polymerase I are (A), (B), and (C) only.