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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
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper2
Typical marks1–5

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 2023Paper1 ["SL"] · TZ214[ 1 ]D1.2.11—Mutations changing protein structure
November 2014Paper2 ["SL"] · TZ05(c)[ 8 ]D1.2.11—Mutations changing protein structure
May 2014Paper2 ["SL"] · TZ13(c)[ 3 ]D1.2.11—Mutations changing protein structure
May 2011Paper2 ["SL"] · TZ16(c)[ 5 ]D1.2.11—Mutations changing protein structure
May 2011Paper1 ["SL"] · TZ217[ 1 ]D1.2.11—Mutations changing protein structure
Practice this objective

Coverage 2011–2023 · Updated 16 Jul 2026

A Mutation Can Change Protein Structure

A point mutation can change one mRNA codon, replace one amino acid and alter how a polypeptide folds or functions.

The effect depends on code degeneracy, mutation position and the chemical properties of the original and replacement amino acids. Some substitutions are synonymous; others alter interactions stabilizing protein structure.

In sickle-cell disease, a point substitution changes a β-globin codon from GAG to GUG in mRNA, replacing glutamic acid with valine. The hydrophobic replacement promotes abnormal haemoglobin association and changes red-cell shape under low oxygen.

DNA base substitution → changed mRNA codon → possible amino-acid replacement → altered side-chain interactions → altered protein structure/function.

A mutation is not automatically harmful or structure-changing: degeneracy can make it synonymous, and some amino-acid replacements have little effect.

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.