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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
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–2

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

November 2024Paper2 ["SL"] · TZ26(a)[ 4 ]D1.3.2—Base substitution consequences
November 2022Paper2 ["SL"] · TZ04(d)[ 2 ]D1.3.2—Base substitution consequences
May 2013Paper1 ["SL"] · TZ213[ 1 ]D1.3.2—Base substitution consequences
November 2012Paper2 ["SL"] · TZ04(b)[ 2 ]D1.3.2—Base substitution consequences
May 2010Paper1 ["SL"] · TZ213[ 1 ]D1.3.2—Base substitution consequences
Practice this objective

Coverage 2010–2024 · 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 SL