#exonuclease activity

Repair Pathway

Proofreading during DNA replication involves which activity of DNA polymerase?

Replicative DNA polymerases such as Pol delta and Pol epsilon possess intrinsic 3' to 5' exonuclease domain distinct from polymerase active site. When incorrect nucleotide incorporated, duplex fraying moves primer terminus from polymerase to exonuclease site, hydrolyzing mispaired nucleotide before next incorporation. This proofreading activity is intrinsic exonuclease not 5' to 3' activity associated with removal of primers like Pol I. Endonuclease cleavage at internal sites and ligase activity sealing nicks are different repair steps unrelated to immediate correction during chain elongation,

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Proofreading 3' to 5' Exonuclease Activity

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Replication in Prokaryotes

Klenow fragment lacks which activity?

Limited proteolysis of Pol I by subtilisin yields large C-terminal Klenow fragment comprising polymerase and 3' to 5' exonuclease domains, removing N-terminal 5' to 3' exonuclease domain. Klenow retains templated synthesis and proofreading, widely used historically for DNA labeling, second strand cDNA synthesis, and Sanger sequencing before recombinant enzymes available. Loss of 5' to 3' activity prevents unwanted degradation of template during synthesis. Modern Klenow exo-minus version also lacks proofreading for higher efficiency incorporation of modified nucleotides and dideoxy terminators.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Tool Box: Klenow fragment preparation and activities, proteolysis of Pol I

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Replication in Prokaryotes

5′→3′ exonuclease activity is present in

DNA polymerase I is unique among E. coli polymerases for possessing two distinct exonuclease activities within one polypeptide. Its 3' to 5' proofreading domain removes misincorporated nucleotides from growing strand, while N-terminal domain carries 5' to 3' exonuclease that cleaves RNA primers and downstream DNA during nick translation. Pol II possesses only 3' to 5' activity, Pol III epsilon provides proofreading but no 5' to 3' function. Dual nuclease capability qualifies Pol I for Okazaki fragment maturation, gap filling, and base excision repair patch removal.

Ref: NCBI Bookshelf, Molecular Biology of Escherichia coli, DNA Pol I domain structure and exonuclease activities

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Replication Basics

DNA polymerase proofreading activity is

DNA polymerases achieve high fidelity through kinetic discrimination at active site and separate proofreading exonuclease function. When misincorporated nucleotide distorts primer-template duplex, synthesis stalls, transferring strand to exonuclease site located 30 Å distant, catalyzing hydrolysis in 3' to 5' direction removing unpaired base. This 3'->5' exonuclease activity is intrinsic to replicative polymerases Pol III ε subunit, Pol δ and Pol ε and Klenow fragment, distinct from 5'->3' exonuclease of Pol I involved in primer removal. Proofreading increases accuracy ~100-fold, complementing

Ref: Kunkel TA, Annu Rev Biochem 1988: DNA Polymerase Proofreading Activity is 3' to 5' Exonuclease

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PCR

Which DNA Pol has proofreading activity?

DNA polymerases vary in fidelity due to presence of 3' to 5' exonuclease domain. Tli polymerase, also called Vent polymerase from Thermococcus litoralis, possesses intrinsic proofreading activity, excising mispaired nucleotides and increasing accuracy approximately 5-10 fold over Taq. Taq from Thermus aquaticus and Tth from Thermus thermophilus lack proofreading, exhibiting higher error rates. For high-fidelity PCR, cloning, mutagenesis, and sequencing applications requiring accurate replication, proofreading enzymes like Tli, Pfu, and Q5 are preferred despite slower extension rates and higher

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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