26. Among the following which iS termed as proof reading activity of DNA polymerase?

(1) 5’à3′ polymerase activity

(2) 3’à5’ polymerase activity

(3) 5’à3′ exonuclease activity

(4) 3’à5’ exonuclease activity

Introduction

DNA replication is a highly accurate process, but errors occasionally occur when incorrect nucleotides are incorporated into the growing DNA strand. To maintain genetic fidelity, DNA polymerases possess a proofreading mechanism that detects and corrects these errors. This proofreading activity is specifically mediated by the 3′→5′ exonuclease function of DNA polymerase, which excises mispaired nucleotides from the 3′ end of the newly synthesized DNA strand.

What Is Proofreading in DNA Polymerase?

Proofreading is an intrinsic error-correcting process during DNA synthesis. After adding a nucleotide, DNA polymerase checks whether the base is correctly paired with the template strand. If a mismatch is detected, the enzyme reverses direction by one nucleotide and removes the incorrect base via its 3′→5′ exonuclease activity. This allows the polymerase to replace the wrong nucleotide with the correct one before continuing DNA synthesis.

Why 3′→5′ Exonuclease Activity Is Called Proofreading

• Directionality: The exonuclease activity removes nucleotides from the 3′ end of the DNA strand, opposite to the direction of synthesis (which is 5′→3′).

• Error Correction: By excising mismatched nucleotides, the enzyme prevents mutations from becoming permanent.

• Fidelity Enhancement: This mechanism reduces the error rate of DNA replication by about 100- to 1000-fold, ensuring genomic stability.

Other DNA Polymerase Activities (Not Proofreading)

Summary: Which Activity Is Proofreading?

Among the options:

1. 5′→3′ polymerase activity: DNA synthesis activity, not proofreading.

2. 3′→5′ polymerase activity: This activity does not exist; polymerization proceeds only 5′→3′.

3. 5′→3′ exonuclease activity: Removes primers, not proofreading.

4. 3′→5′ exonuclease activity: Correct answer; responsible for proofreading.

Correct Answer

(4) 3′→5′ exonuclease activity

Conclusion

The proofreading activity of DNA polymerase, crucial for maintaining replication fidelity, is mediated by its 3′→5′ exonuclease function. This activity allows the enzyme to remove incorrectly incorporated nucleotides from the 3′ end of the DNA strand, ensuring accurate DNA replication and preventing mutations.