Mutations are heritable changes in the nucleotide sequence of DNA that serve as the ultimate source of all genetic variation. While some mutations are neutral or beneficial and contribute to evolutionary adaptation, others disrupt gene function and cause genetic disorders or cancer.
Types of Point Mutations
Point mutations are changes in a single nucleotide. A transition substitutes a purine for a purine (A↔G) or a pyrimidine for a pyrimidine (C↔T), while a transversion substitutes a purine for a pyrimidine or vice versa. Transitions are more common than transversions due to the spontaneous deamination of 5-methylcytosine to thymine. Silent (synonymous) mutations change a codon to another that encodes the same amino acid, having no effect on the protein sequence. Missense (non-synonymous) mutations change a codon to encode a different amino acid, which may be conservative (similar chemical properties) or non-conservative (different properties). Nonsense mutations create a premature stop codon, leading to truncated proteins that are often non-functional and targeted for nonsense-mediated mRNA decay.
Insertions and Deletions
Insertions and deletions (indels) of one or more nucleotides can have varying effects depending on their size and location. Frameshift mutations occur when the indel length is not a multiple of three, shifting the reading frame and altering all downstream codons, typically producing a completely non-functional protein. In-frame indels, where the number of nucleotides inserted or deleted is a multiple of three, add or remove whole codons without altering the reading frame. Trinucleotide repeat expansions, such as CAG repeats in Huntington disease and CTG repeats in myotonic dystrophy, involve the unstable expansion of repetitive sequences beyond a pathogenic threshold, with longer repeats causing earlier onset and more severe disease (anticipation).
Causes of Mutation
Spontaneous mutations arise from endogenous processes without exposure to external agents. Depurination, the loss of a purine base, occurs thousands of times per cell per day and creates an apurinic site that can cause misincorporation during replication. Deamination converts cytosine to uracil, which is normally repaired but can lead to C→T transitions if not corrected. Oxidative damage from reactive oxygen species creates 8-oxoguanine, which pairs with adenine instead of cytosine, causing G→T transversions. Replication errors, including DNA polymerase misincorporation and strand slippage at repetitive sequences, are another major source. Induced mutations are caused by environmental mutagens: chemical mutagens such as alkylating agents (ethyl methanesulfonate) and intercalating agents (ethidium bromide), physical mutagens such as ionizing radiation that causes double-strand breaks and UV light that creates thymine dimers, and biological mutagens such as transposable elements and certain viruses.
DNA Repair Mechanisms
Cells possess multiple DNA repair pathways to correct damage before it becomes fixed as mutation. Base excision repair (BER) removes single damaged bases by the action of DNA glycosylases, followed by AP endonuclease cleavage, gap filling by DNA polymerase, and ligation. Nucleotide excision repair (NER) removes bulky DNA lesions such as pyrimidine dimers by cutting the damaged strand on both sides and excising an oligonucleotide of 24–32 bases. Mismatch repair (MMR) corrects replication errors where the wrong nucleotide has been incorporated, using MutS and MutL homologs to detect the mismatch and direct excision of the newly synthesized strand. Double-strand break repair occurs through two principal mechanisms: homologous recombination (HR) uses the sister chromatid as a template for error-free repair during S and G₂ phases, while non-homologous end joining (NHEJ) directly ligates broken ends and is error-prone, often introducing small indels.
Consequences of Mutations
In germline cells, mutations can be passed to offspring and cause inherited genetic disorders such as cystic fibrosis (frameshift), sickle cell anemia (missense E6V in β-globin), and Duchenne muscular dystrophy (deletions in dystrophin). In somatic cells, mutations accumulate throughout life and can lead to cancer when they affect oncogenes, tumor suppressor genes, and DNA repair genes. Neutral mutations have no discernible effect on fitness and accumulate at a relatively constant rate, providing the basis for molecular clocks used in evolutionary studies. Beneficial mutations, such as the CCR5-Δ32 deletion that confers resistance to HIV infection, are rare but can increase in frequency through natural selection.
Genetic Variation in Populations
Single nucleotide polymorphisms (SNPs) are the most common type of genetic variation, occurring approximately every 300 base pairs in the human genome, with many millions of SNPs identified across populations. Copy number variations (CNVs) involve deletions or duplications of DNA segments larger than 1 kb and account for a substantial fraction of inter-individual genetic variation. Structural variants, including inversions and translocations, rearrange larger chromosomal segments. The allele frequency of genetic variants differs between populations due to founder effects, genetic drift, and selective pressures, which is important to consider in genome-wide association studies and pharmacogenomics.
Mutations in Evolution
Mutations provide the raw material for natural selection. The rate of mutation varies across the genome, with higher rates in repetitive regions and CpG islands, and varies between organisms (RNA viruses have mutation rates orders of magnitude higher than eukaryotes). Adaptive evolution occurs when beneficial mutations increase in frequency, while purifying selection removes deleterious mutations. Gene duplication followed by mutation and divergence is a major source of new genes and functions, exemplified by the globin gene family and olfactory receptor genes.
