DNA Polymerases and Replication Fidelity

The enzymes that build new DNA, and the layered mechanisms — base selection, proofreading, and mismatch correction — that keep copying errors extraordinarily rare.

Definition

DNA polymerases are the enzymes that catalyse templated addition of deoxyribonucleotides to a growing strand; replication fidelity refers to the combined accuracy of nucleotide selection, proofreading, and mismatch repair that determines how rarely the wrong base is permanently incorporated.

Scope

This topic covers the catalytic properties of DNA polymerases and the multiple safeguards that produce high replication accuracy. It addresses nucleotide selection geometry, the 3'→5' proofreading exonuclease, the contribution of post-replicative mismatch repair to overall fidelity, and the diversity of polymerase families (replicative, repair, and translesion). It does not cover the structural detail of individual lesions, which belongs with repair pathways.

Core questions

• How does a polymerase choose the correct nucleotide to add at each position?
• What is proofreading and how much does it improve accuracy?
• How does mismatch repair add a further layer of fidelity after synthesis?
• Why do cells have several different polymerases with different roles?

Key theories

Layered fidelity model

Overall replication accuracy is the product of three steps — geometric selection of the correct base, 3'→5' exonucleolytic proofreading of misincorporated bases, and post-replicative mismatch repair — each multiplying the accuracy of the previous one.

Polymerase division of labour

Distinct polymerase families perform distinct jobs — high-fidelity replicative synthesis, gap filling during repair, and lower-fidelity translesion synthesis past damage — so that one enzyme need not optimise for conflicting demands.

Mechanisms

A replicative polymerase positions an incoming nucleotide in its active site, where correct Watson–Crick geometry is required for efficient catalysis, giving an initial selectivity. When an incorrect nucleotide is occasionally added, the distorted primer terminus is shuttled to a separate 3'→5' exonuclease site that removes it before synthesis resumes. Errors that escape proofreading leave a transient mismatch that the mismatch repair system detects, excises from the newly made strand, and resynthesises, multiplying the net accuracy.

Clinical relevance

Inherited defects in proofreading or mismatch repair raise mutation rates and predispose to certain cancers, and engineered high-fidelity polymerases are core reagents in DNA sequencing and amplification. Presented as significance, not as clinical or diagnostic guidance.

History

Arthur Kornberg's isolation of DNA polymerase I in the 1950s opened the enzymology of replication; subsequent work distinguished replicative from repair and translesion polymerases and quantified the contributions of proofreading and mismatch repair to the very low error rate described in modern texts.

Key figures

• Arthur Kornberg
• Thomas Kunkel

Seminal works

• watson2013
• alberts2014

Frequently asked questions

What is polymerase proofreading?

A built-in 3'→5' exonuclease activity that removes a wrongly added nucleotide immediately after it is incorporated, before synthesis continues.

Are all DNA polymerases equally accurate?

No. Replicative polymerases are highly accurate, while specialised translesion polymerases trade accuracy for the ability to copy past damaged bases.