AQA A-Level Biology: Gene Mutations
A clear revision guide to gene mutations for AQA A-Level Biology: substitution and deletion mutations, frameshifts, how they change proteins, and mutagenic agents.
A gene mutation is a change in the base sequence of DNA. Most arise spontaneously when DNA is copied, and while many have no effect at all, some change a protein enough to stop it working. This guide covers the two types you need, how they alter a protein, and what speeds them up.
What a gene mutation is
A gene mutation is a change in the sequence of bases in the DNA of a chromosome. Mutations happen spontaneously, most often as errors during DNA replication in interphase. The two types to know are substitution and deletion.
Their rate can be increased by a mutagenic agent (a mutagen), such as ultraviolet light or ionising radiation like alpha particles.
How a mutation can change a protein
The reason a change in DNA can matter is a chain of consequences. A change in the base triplets of a gene changes the codons on the mRNA, which changes the sequence of amino acids in the polypeptide (its primary structure). Because the amino acid sequence determines where the hydrogen, ionic and disulfide bonds form, the folding changes, and so the tertiary structure, the three-dimensional shape of the protein, changes too.
For an enzyme this is critical. If the shape of the active site changes, the substrate may no longer fit, no enzyme-substrate complex can form, and the enzyme cannot catalyse its reaction. The protein has become non-functional.
Substitution mutations
In a substitution, one base is swapped for a different one. Because the reading frame is not disturbed, only the single triplet containing that base is affected, so at most one codon, and therefore one amino acid, changes.
A substitution often has a small effect, or none at all, for three reasons:
- The code is degenerate, so the new triplet may still code for the same amino acid, leaving the protein unchanged.
- If one amino acid does change, it may sit somewhere that does not affect the folding, so the tertiary structure and function are unharmed.
- If the substitution falls within an intron, it is removed during splicing and never reaches the protein.
Deletion mutations
A deletion removes a base entirely, and this is usually far more damaging. Losing a base shifts every triplet from that point onwards along by one, a frameshift. Because all the codons after the mutation are now read differently, the amino acid sequence changes from that point on, not just at one position.
A comparison of a substitution mutation, which changes a single codon, and a deletion mutation, which causes a frameshift that changes every codon after the point of the mutation
This usually changes the primary structure drastically, which changes the positions of the bonds that hold the protein together, changing its tertiary structure and almost always destroying its function.
Mutations are not only a source of disease, though. By creating new alleles, they are the ultimate origin of all genetic variation, the raw material on which natural selection acts. That story continues in the meiosis and natural selection guides.