A mutation is a sudden, permanent change in the nucleotide sequence of DNA. Mutations may alter the structure and function of the proteins encoded by the affected gene, with consequences ranging from no effect to serious disease.
| Type | Location | Heritable? | Consequence |
|---|---|---|---|
| Germ-line mutation | Gametes (egg/sperm) | Yes | Passed to all cells of offspring |
| Somatic mutation | Body (non-reproductive) cells | No | Affects only the individual; can cause cancer |
A point mutation is a change in a single nucleotide pair. There are two categories:
Substitutions can produce three outcomes:
| Outcome | Description | Effect on Protein |
|---|---|---|
| Silent mutation | Codon changes but encodes the same amino acid (due to degeneracy of the genetic code) | No change |
| Missense mutation | Codon changes to encode a different amino acid | Altered protein (may be functional or non-functional) |
| Nonsense mutation | Codon changes to a stop codon | Premature termination → truncated, non-functional protein |
A frameshift mutation occurs when nucleotides are inserted or deleted in a number that is not a multiple of three. This shifts the reading frame of all subsequent codons, usually producing a completely non-functional protein.
Example: Inserting one base near the start of a gene alters every codon downstream — the resulting protein bears no resemblance to the original.
If nucleotides are inserted or deleted in multiples of three, the reading frame is preserved and only one or a few amino acids are added or removed.
Sickle cell anaemia is a classic example of the significance of a single point mutation:
This example illustrates B-11-C-14: the sequence of amino acids is critical to protein function.
Mutagens are physical or chemical agents that increase the rate of mutation above the spontaneous background level.
| Type | Examples | Mechanism |
|---|---|---|
| Physical mutagens | UV radiation, X-rays, gamma rays | UV → thymine dimers; ionising radiation → strand breaks |
| Chemical mutagens | Ethidium bromide, nitrous acid, alkylating agents | Alter base structure or intercalate into DNA, causing mispairing |
| Mutation Type | Cause | Effect |
|---|---|---|
| Transition | Purine ↔ Purine or Pyrimidine ↔ Pyrimidine | May be silent, missense, or nonsense |
| Transversion | Purine ↔ Pyrimidine | May be silent, missense, or nonsense |
| Frameshift | Insertion/deletion (not multiple of 3) | Alters all downstream codons |
| Silent | Base substitution → synonymous codon | No change in protein |
| Missense | Base substitution → different amino acid | Altered protein (e.g., sickle cell) |
| Nonsense | Base substitution → stop codon | Truncated protein |