What is a gene mutation?
A gene mutation is a change in the base sequence of DNA in a chromosome. Gene mutations can arise spontaneously during DNA replication and occur at random.
The sequence of DNA bases determines the sequence of amino acids in a polypeptide. A mutation can therefore alter the polypeptide produced by a gene, although this does not happen in every case.
From DNA to protein
A change in the DNA base sequence may alter a DNA triplet and therefore the corresponding mRNA codon, which may then change the amino acid incorporated into the polypeptide.
The effect ultimately depends on whether the mutation changes the amino acid sequence, three-dimensional shape or function of the protein.
Why some mutations have little effect
The genetic code is degenerate, meaning that more than one codon can code for the same amino acid.
Some mutations therefore leave the amino acid sequence unchanged, while others alter it only slightly.
Two gene mutation types
| Mutation | Change to the DNA sequence | Main consequence |
|---|---|---|
| Base substitution | One base is replaced by another. | The reading frame is retained, although the affected DNA triplet and corresponding mRNA codon may change. |
| Base deletion | A nucleotide, and therefore its base, is removed. | Deletion of a single nucleotide shifts the reading frame beyond the mutation. |