What is a mutation?
A mutation is a change in the DNA that stays. DNA is the instruction book of the cell. It is written with four letters called bases: A, T, G and C.
The cell reads a gene three bases at a time. Each group of three is a codon. Each codon tells the cell which amino acid to add to a protein. If the letters change, the protein may change.
Mutations in body cells affect only that person. Mutations in sex cells (sperm or egg) can be passed to children.
Gene mutations (point mutations)
Substitution
One base is swapped for another. There are three possible results:
- Silent: the new codon means the same amino acid, so the protein does not change. This works because the genetic code has more than one codon for most amino acids (it is "degenerate").
- Missense: one amino acid changes. Example: sickle-cell anaemia (GAG → GTG, glutamic acid → valine in haemoglobin).
- Nonsense: the codon becomes a STOP codon, so the protein is cut short.
Insertion and deletion
A base is added (insertion) or removed (deletion). Because the cell reads in threes, every codon after the change is read wrongly. This is a frameshift. It usually makes a useless protein. If exactly three bases are added or lost, the frame is kept and only one amino acid is added or lost.
Chromosome mutations
These change large pieces of a chromosome or the number of chromosomes.
- Deletion: a piece is lost.
- Duplication: a piece is copied twice.
- Inversion: a piece breaks off and joins back upside down.
- Translocation: a piece moves to a different chromosome.
Changes in number: one extra or one missing chromosome (aneuploidy), for example Down syndrome (three copies of chromosome 21) or Turner syndrome (only one X). A whole extra set of chromosomes is polyploidy; it is common in crop plants such as wheat and bananas.
Causes and effects of mutations
Causes
Spontaneous mutations happen by chance when DNA is copied before cell division. Mutagens raise the rate: UV light, X-rays and gamma rays, and chemicals such as those in tobacco smoke. Some viruses can also change DNA.
Effects
- Harmful: genetic diseases (sickle-cell anaemia, cystic fibrosis, haemophilia) and cancer, when genes that control cell division are damaged.
- Neutral: most mutations have no visible effect.
- Useful: a few help survival, for example antibiotic resistance in bacteria, or lactose tolerance in adult humans.
Mutations are the only source of truly new alleles. Meiosis and fertilisation then mix them. Natural selection acts on this variation, so mutations are the raw material of evolution.
Try it
Write the sentence THE CAT ATE THE RAT. Change one letter (substitution), then remove one letter and re-split into threes (deletion). Which one ruins the whole sentence?
Key formulas and definitions
- Mutation – a lasting change in the base sequence of DNA
- Codon – 3 bases = 1 amino acid
- Substitution – one base swapped (silent, missense or nonsense)
- Insertion / deletion – base added / lost → frameshift
- Chromosome mutation – deletion, duplication, inversion, translocation
- Aneuploidy – one chromosome extra or missing (e.g. trisomy 21)
- Polyploidy – whole extra sets of chromosomes
- Mutagen – agent that raises mutation rate (UV, X-rays, chemicals)
Worked examples
1. The DNA codon GAG codes for glutamic acid. It mutates to GTG (valine). Name the type of mutation and its effect.
One base is swapped, so it is a substitution. One amino acid changes, so it is a missense mutation. In haemoglobin this causes sickle-cell anaemia.
2. A gene reads ATG-GAG-GTG-CAC. The 5th base (A) is deleted. Write the new codons.
New sequence ATGGGGTGCAC → ATG-GGG-TGC-AC… Every codon after the deletion changes: this is a frameshift.
3. A substitution changes CTT to CTC. Both code for leucine. What kind of mutation is it?
A silent mutation: the base changed but the amino acid did not, because the code has several codons for leucine.
4. Why does deleting three bases usually do less harm than deleting one?
Three bases = one codon. The reading frame stays the same, so only one amino acid is lost; the rest of the protein is normal.
Common mistakes
- Thinking every mutation is harmful. Most are neutral and a few are useful.
- Mixing up substitution and frameshift. Only insertion and deletion (not in multiples of 3) shift the frame.
- Thinking mutations in skin cells are passed to children. Only mutations in sex cells are inherited.
- Thinking a mutation happens because an organism "needs" it. Mutations are random; selection comes after.