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D1.3.2—Base substitution consequences

Base substitutions can be silent, missense or nonsense because genetic-code degeneracy changes how codon changes affect proteins, traits and disease phenotypes.

Syllabus
First assessment 2025
Objective
D1.3.2
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–4

Common command terms

  • Describe
  • Explain
  • Outline

Scoring notes

Common mistake
Stopping at the DNA substitution without tracing the codon and amino acid consequence.

Recent exam appearances

May 2025Paper2 ["HL"] · TZ19(a)[ 4 ]D1.3.2—Base substitution consequences
May 2023Paper1 ["HL"] · TZ211[ 1 ]D1.3.2—Base substitution consequences
November 2020Paper1 ["HL"] · TZ06[ 1 ]D1.3.2—Base substitution consequences
May 2014Paper2 ["HL"] · TZ26(b)[ 5 ]D1.3.2—Base substitution consequences
November 2013Paper1 ["HL"] · TZ011[ 1 ]D1.3.2—Base substitution consequences
Practice this objective

Coverage 2013–2025 · Updated 16 Jul 2026

Concept essentials

  • A substitution changes one base in a DNA sequence.
  • Degeneracy can make some substitutions silent.
  • A missense substitution changes one amino acid in a polypeptide.
  • A nonsense substitution can create a premature stop codon.
  • Sickle-cell anemia links a base substitution to altered haemoglobin structure.
ConceptIB Biology HL