Questions: DNA Replication Accuracy and Proofreading

Question 1

A mutation disables only the 3'→5' exonuclease proofreading activity of DNA polymerase, leaving nucleotide selectivity and mismatch repair fully intact. What would you predict about the organism's overall mutation rate?

• A. No change — the remaining two layers compensate entirely for the loss of proofreading
• B. A moderate increase of roughly 100-fold, since one of three multiplicative error-correction layers is lost
• C. An immediately lethal increase in mutations, since proofreading is the essential checkpoint
• D. A decrease in mutation rate, since removing exonuclease activity prevents deletion errors

Answer: (B)

The three layers — nucleotide selectivity, proofreading, and mismatch repair — each improve fidelity by roughly 100-fold and operate in series. Losing proofreading removes its ~100-fold contribution, raising the overall error rate from ~1 in 10⁹ to ~1 in 10⁷. The remaining two layers still function, so the increase is not catastrophic. This 'layered redundancy' design means each layer is necessary but not solely responsible for genomic stability.

Question 2

What is the primary physical basis for DNA polymerase's nucleotide selectivity — its ability to favor correctly matched bases over mismatches?

• A. Hydrogen bonding strength: correct base pairs form more hydrogen bonds than mismatches
• B. The precise geometric fit of a correct Watson-Crick base pair in the polymerase active site
• C. Electrostatic repulsion between mismatched bases and the template backbone
• D. Recognition of a specific chemical signature on the incoming nucleotide's sugar moiety

Answer: (B)

DNA polymerase uses geometric discrimination, not just hydrogen bonding strength. A correct Watson-Crick pair (A-T or G-C) has a precise shape that fits snugly into the active site, positioning the 3'-OH for efficient catalysis. A mismatch distorts this geometry, dramatically slowing the chemical reaction even if some hydrogen bonds still form. This is why G-T wobble pairs, which can form two hydrogen bonds, are still strongly discriminated against — the geometry is wrong even if the bonding is partial.

Question 3

The 3'→5' exonuclease proofreading activity that corrects mismatches during DNA replication is located within the same enzyme molecule as the polymerase active site.

• T. True
• F. False

Answer: True

In DNA polymerases I and III (and their eukaryotic counterparts), the 3'→5' exonuclease domain is a physically separate but integral part of the same polypeptide or holoenzyme complex. When a mismatch is detected — signaled by the distorted geometry at the 3' end of the growing strand — the mismatched end is shifted into the exonuclease domain, the error is excised, and synthesis resumes. This 'built-in backspace' function allows immediate correction without requiring a separate enzyme to find and fix the mistake.

Question 4

Mismatch repair in bacteria identifies the newly synthesized strand (rather than the template) for correction by detecting nicks and cuts in the new DNA.

• T. True
• F. False

Answer: False

In bacteria, the newly synthesized strand is identified by the absence of methylation. The template strand is methylated at GATC sequences by Dam methylase; the new strand is not yet methylated immediately after synthesis. MutH recognizes this hemimethylated state and cuts the unmethylated (new) strand, directing excision to the correct strand. Eukaryotes use strand discontinuities (nicks) rather than methylation for this purpose. Confusing the two organisms' mechanisms is a common error.

Question 5

Why are three successive error-correction layers necessary for DNA replication fidelity, rather than simply engineering a more accurate polymerase active site?

Model answer: Each successive layer catches errors that escaped the previous one, and the layers multiply their effects. Nucleotide selectivity alone achieves about 1 error per 10⁵ nucleotides — impressive but still far too high for a billion-base genome. Proofreading adds another ~100-fold improvement to reach ~1 in 10⁷. Mismatch repair adds a further 100–1000-fold to reach ~1 in 10⁹–10¹⁰. No single mechanism can achieve this level of accuracy on its own because each approach has fundamental physical and kinetic limits. The layered architecture exploits redundancy: rare errors slipping through each layer are caught by the next.

This principle of layered quality control appears throughout biology. Each mechanism operates via a different physical principle — geometric discrimination, exonuclease excision, and post-replication scanning — so their failure modes are largely independent. Losing any one layer still permits the others to function, while combining all three achieves an extraordinary overall fidelity that no single mechanism could approach.