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D1.1.9 (HL)—DNA proofreading

DNA proofreading explains how DNA is copied, checked, amplified or analysed through specific strand relationships, enzyme functions and molecular techniques.

Syllabus
First assessment 2025
Objective
D1.1.9
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1–4

Common command terms

  • Identify
  • Explain

Scoring notes

Common mistake
Saying helicase forms new strands instead of unwinding DNA and breaking hydrogen bonds.

Recent exam appearances

November 2025Paper2 ["HL"] · TZ19(a)[ 4 ]D1.1.9 (HL)—DNA proofreading
May 2025Paper1A ["HL"] · TZ19[ 1 ]D1.1.9 (HL)—DNA proofreading
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Proofreading Removes Many Replication Errors

HL only

DNA polymerase III proofreads a newly added base at the growing strand's 3′ terminal and corrects a mismatch before replication continues.

A non-complementary base pair distorts the new DNA. Polymerase III removes the mismatched terminal nucleotide, exposes the 3′ end again and inserts a nucleotide complementary to the template.

Mismatch at 3′ terminal → polymerase III detects it → incorrect nucleotide removed → correct complementary nucleotide added → 5′→3′ extension resumes.

If an incorrect nucleotide is added opposite a template G, proofreading removes it from the 3′ end and replaces it with C before the strand is extended further.

Proofreading greatly improves accuracy but does not eliminate every mutation. This objective is specifically polymerase III correction of a mismatched 3′ terminal.

DNA proofreading

HL only

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, multiple choice, commonly using Identify / Explain.

Command terms

Identify / Explain

What earns marks

Build the answer around this relationship: DNA proofreading depends on matching each strand, enzyme or laboratory step to its exact function.

Watch for

Saying helicase forms new strands instead of unwinding DNA and breaking hydrogen bonds.

Representative question

Question 1

[Maximum number: 4]

Explain how mutation is avoided during DNA replication.

HL Replication Details

HL only

DNA strands have 5' and 3' ends; DNA polymerase adds nucleotides to the 3' end, so new DNA forms 5' to 3'. Leading strand synthesis is continuous; lagging strand synthesis is discontinuous as Okazaki fragments using repeated RNA primers. In the prokaryotic model, primase starts, DNA polymerase III extends, DNA polymerase I replaces primers, and ligase joins fragments. DNA polymerase III removes mismatched nucleotides from the 3' end; proofreading improves copying accuracy and reduces mutations.

Concept essentials

  • DNA proofreading depends on matching each strand, enzyme or laboratory step to its exact function.
  • Template strands and complementary base pairing give dna proofreading its copying accuracy.
  • DNA proofreading is clearer when amplification, separation and profiling are not confused.
  • Molecular evidence for dna proofreading must be interpreted from bands, isotopes, primers or enzyme roles.