Mutations
Changes to the nucleotide sequence of DNA can range from silent to catastrophic, depending on type and location.
Point mutations
- Silent mutation: a base change results in a codon that still codes for the same amino acid, due to redundancy in the genetic code. Usually has no effect on the protein.
- Missense mutation: a base change results in a codon coding for a different amino acid. Effects range from negligible to severe, depending on the chemical similarity of the new amino acid and its location in the protein - sickle-cell anemia results from a single missense mutation in the beta-globin gene.
- Nonsense mutation: a base change converts a codon into a premature stop codon, truncating the protein. This is usually severe, especially if it occurs early in the gene.
Frameshift and chromosomal mutations
Insertions or deletions of a number of nucleotides that is not a multiple of three cause a frameshift mutation: every codon downstream of the mutation is read incorrectly, typically producing a completely different (and usually nonfunctional) amino acid sequence, often ending in a premature stop codon. Because of this cascading effect, frameshift mutations are almost always more severe than point mutations.
Larger-scale chromosomal mutations - deletions, duplications, inversions, and translocations of whole chromosome segments - often arise from errors during meiosis (e.g., unequal crossing over) and can affect many genes at once, sometimes with severe developmental consequences.
Sources of mutation
Mutations arise from several sources: spontaneous errors during DNA replication that escape proofreading and mismatch repair (rare, but not zero), spontaneous chemical changes to DNA (like depurination or deamination), exposure to radiation (UV light, X-rays), and chemical mutagens that react with or intercalate into DNA.
Germline vs. somatic mutations, and evolutionary significance
Mutations occurring in germ cells (sperm- or egg-producing cells) are heritable - they can be passed to offspring and become part of the gene pool. Mutations occurring in somatic (body) cells affect only that individual and its descendant cell lineage, not future generations; many cancers result from an accumulation of somatic mutations in genes controlling cell division.
Despite their potential for harm, mutations are also the ultimate source of all new genetic variation in a population. Without mutation, natural selection would have no new raw material to act upon, and populations could not adapt to changing environments over evolutionary time.
Key terms
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- Point mutation
- A change involving a single nucleotide base pair in DNA.
- Frameshift mutation
- Insertion or deletion of a number of bases not divisible by three, shifting the reading frame.
- Germline mutation
- Mutation in a gamete-producing cell; heritable and passed to offspring.
- Somatic mutation
- Mutation in a non-reproductive body cell; not passed to offspring, can cause cancer.
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