Nonsense mutation occurs when a single nucleotide substitution converts a codon that normally specifies an amino acid into a termination codon (UAA, UAG, or UGA), causing translation to stop prematurely and yielding a truncated polypeptide.
Explanation
A nonsense mutation is a type of point mutation that alters the coding sequence of a gene by introducing a premature stop signal within an open reading frame. When ribosomes encounter the altered codon during translation, they disengage from the messenger RNA before the full protein is synthesized. This change can arise spontaneously through errors in DNA replication or be induced by mutagens. Since it truncates the polypeptide chain, the resulting protein is often nonfunctional or rapidly degraded. Many eukaryotic cells also recognize transcripts with nonsense codons and target them for nonsense mediated decay, a quality control process that reduces the accumulation of faulty proteins. The impact of this mutation depends on its position within the gene; early truncations typically abolish protein function, while mutations near the C terminus may have milder effects. Nonsense mutations are distinct from missense mutations, which swap one amino acid for another, and silent mutations, which leave the amino acid sequence unchanged.
Notable examples and effects
A classic example involves the beta globin gene, where a single substitution can change the codon for glutamine (CAA) to a stop codon (UAA), leading to beta thalassemia. In the dystrophin gene, nonsense mutations produce truncated proteins that cannot support muscle fibers, contributing to Duchenne muscular dystrophy. In cystic fibrosis, the G542X variant converts a glycine codon to a stop codon, causing a nonfunctional CFTR protein. Researchers have developed read through therapies using small molecules such as ataluren to allow ribosomes to bypass premature stop codons, which may help restore some protein function. Nonsense mutations highlight that the fidelity of genetic information is critical for proper protein synthesis and cellular function. Their study has led to a greater understanding of mRNA surveillance, the genetic basis of inherited diseases and the development of targeted therapies. Related Terms: Missense Mutation, Silent Mutation, Frameshift Mutation, Stop Codon, Nonsense Mediated Decay