Biochemistry · Topic
Types of mutations !
A mutation is a change in the DNA sequence; depending on where and how it happens it may do nothing, change one amino acid or ruin the whole protein.
Updated 2026-11-01
Key facts
- Silent: changes the codon but not the amino acid (code degeneracy).
- Missense: changes one amino acid. E.g. HbS, Glu6Val in β-globin.
- Nonsense: creates a premature stop codon → truncated protein or degraded mRNA.
- Frameshift: insertion or deletion not a multiple of 3 → everything after it changes. E.g. Duchenne.
- Others: splice-site (β-thalassemia), trinucleotide repeat expansion (Huntington, fragile X), large deletions and rearrangements.
In detail
- Transition: purine for purine or pyrimidine for pyrimidine (A↔G, C↔T). Transversion: purine for pyrimidine.
- In-frame deletions (multiple of 3) remove amino acids without shifting the frame: they often give partly working proteins (Becker dystrophy, cystic fibrosis ΔF508).
- Loss of function: usually recessive (one healthy copy is enough). Gain of function or dominant-negative effect: dominant.
- Germline (heritable, in every cell) versus somatic (in one tissue only; the basis of most cancers). Mosaicism: a mix of cell lines.
- Mutagens: UV light (pyrimidine dimers), ionising radiation (breaks), alkylating agents, base analogues, intercalators (frameshifts).
- Polymorphism: a common variant (> 1% of the population), usually without disease effect.
- Also called
- Genetic variants
Clinical notes
- Sickle cell disease is the classic missense example; β⁰-thalassemia comes from nonsense or splice mutations.
- Whether a deletion keeps the reading frame largely explains why Becker dystrophy is milder than Duchenne.
- Repeat-expansion diseases show anticipation: the disease starts earlier and is more severe in each generation.
Related conditions
- Sickle cell disease
- Thalassemias (α and β)
- Duchenne muscular dystrophy
- Huntington disease
- Fragile X syndrome
Related
- DNA repair
- Translation and the genetic code
- RNA processing (cap, poly-A tail and splicing)
- Trinucleotide repeat expansion disorders
- Hemoglobin variants (HbA, HbA2, HbF, HbS, HbC)
Test yourself
Think of the answer, then tap the question to see it.
Why is a 3-nucleotide deletion usually milder than a 2-nucleotide one?
Deleting 3 removes one amino acid and keeps the reading frame; deleting 2 shifts the frame and changes the rest of the protein.
What kind of mutation produces haemoglobin S?
A missense mutation: codon 6 of β-globin changes from glutamic acid to valine.
What is genetic anticipation?
Earlier onset or greater severity in successive generations, typical of trinucleotide repeat expansions.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: What is a gene variant and how do variants occur?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); What kinds of gene variants are possible?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Frameshift Mutation (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.