44. Among the following which DNA polymerase lacks proof reading activity?

(1) DNA polymerase α primase activity

(2) DNA polymerase Ꟙ

(3) DNA polymerase ϵ

(4) Both DNA polymerase α and ϵ

Introduction

DNA polymerases synthesize DNA strands during replication and repair. Many polymerases have an intrinsic 3′→5′ exonuclease activity that provides proofreading capability, removing incorrectly incorporated nucleotides to maintain replication fidelity. However, not all DNA polymerases have this proofreading function. Understanding which polymerases lack proofreading is crucial for insights into replication accuracy and mutation rates.

Key Points About Proofreading Activity

• 3′→5′ exonuclease activity allows DNA polymerases to excise mismatched nucleotides immediately after incorporation.

• Polymerases lacking this activity have higher error rates during DNA synthesis.

• In eukaryotes, DNA polymerase α initiates replication but lacks proofreading activity, whereas DNA polymerases δ and ε have proofreading functions.

• In bacteria, DNA polymerase I and III have proofreading activity; DNA polymerase II also has it, but polymerases IV and V do not.

DNA Polymerase α and Proofreading

• DNA polymerase α is responsible for synthesizing the RNA-DNA primer during replication initiation.

• It has limited processivity and lacks 3′→5′ exonuclease proofreading activity.

• This lack of proofreading results in a higher error rate during primer synthesis, but its role is primarily to initiate DNA synthesis rather than to perform bulk replication.

DNA Polymerase δ and ε and Proofreading

• DNA polymerase δ (lagging strand) and DNA polymerase ε (leading strand) possess 3′→5′ exonuclease activity, enabling high-fidelity DNA synthesis.

• These polymerases ensure replication accuracy by proofreading and correcting errors during elongation.

Evaluating the Options

Correct Answer Based on the Query

Among the options:

• DNA polymerase α lacks proofreading activity.

• DNA polymerase δ and ε have proofreading activity.

Therefore, the polymerase that lacks proofreading activity is:

(1) DNA polymerase α primase activity

Summary

• DNA polymerase α initiates DNA synthesis but does not proofread.

• DNA polymerase ε and δ have proofreading exonuclease activity.

• The absence of proofreading in DNA polymerase α explains its higher error rate during initial primer synthesis.