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D1.2.11—Mutations changing protein structure

Mutations can alter codons, change amino acid sequence, disrupt protein structure and produce disease phenotypes such as sickle-cell anemia in humans.

Syllabus
First assessment 2025
Objective
D1.2.11
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–6

Common command terms

  • Outline
  • Explain

Scoring notes

Common mistake
Stopping at the DNA mutation without tracing the change through mRNA codon and amino acid sequence.

Recent exam appearances

May 2025Paper1A ["HL"] · TZ111[ 1 ]D1.2.11—Mutations changing protein structure
May 2019Paper2 ["HL"] · TZ27(c)[ 8 ]D1.2.11—Mutations changing protein structure
May 2019Paper2 ["HL"] · TZ12(b)[ 2 ]D1.2.11—Mutations changing protein structure
May 2018Paper2 ["HL"] · TZ14(a)[ 2 ]D1.2.11—Mutations changing protein structure
November 2013Paper2 ["HL"] · TZ08(c)[ 6 ]D1.2.11—Mutations changing protein structure
Practice this objective

Coverage 2012–2025 · Updated 16 Jul 2026

A Mutation Can Change Protein Structure

A DNA mutation can alter an mRNA codon, amino-acid sequence and ultimately protein folding or function, although some changes have little effect.

Outcome depends on mutation type, code degeneracy, location and the chemical difference between residues. Folding translates sequence change into structure change.

Assess: DNA change; codon consequence; amino-acid change; structural or functional effect.

Replacing a charged residue in an enzyme's active site can alter binding and reduce catalytic activity.

A mutation is not automatically harmful; synonymous or distant changes may leave function largely unchanged.

Mutations changing protein structure

Assessment in practice

1–6 marks
How it is assessed

This objective is assessed through structured response, commonly using Outline / Explain.

Command terms

Outline / Explain

What earns marks

Build the answer around this relationship: A base substitution can change one mRNA codon.

Watch for

Stopping at the DNA mutation without tracing the change through mRNA codon and amino acid sequence.

Representative question

Question 1

[Maximum number: 8]

Explain the cause of sickle cell anemia and how this disease affects humans.

Core Protein Synthesis

  • Transcription: RNA polymerase builds complementary mRNA from the DNA template; A pairs with U and C with G.
  • Translation: ribosomes read mRNA codons 5′ to 3′ while tRNA anticodons deliver specific amino acids for peptide-bond formation.
  • Genetic code: codons are triplets; the code is degenerate and almost universal, with start and stop signals. Use mRNA—not DNA—when reading a code table.
  • Information flow: codon order determines amino-acid sequence, which determines protein folding and function.
  • Expression and variation: cells regulate which genes are transcribed. A mutation may change a codon, primary structure and phenotype, as in sickle-cell haemoglobin.

Concept essentials

  • A base substitution can change one mRNA codon.
  • A changed codon may replace one amino acid in a polypeptide.
  • Sickle-cell haemoglobin contains valine instead of glutamic acid at a key position.
  • Protein structure changes can alter cell shape and physiological function.
ConceptIB Biology HL