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In the genome, the four nucleotides—adenine (A), cytosine (C), guanine (G), and thymine (T) in DNA—serve as the alphabet that encodes every protein. A point mutation swaps one of these letters for another, a seemingly minor alteration that can have profound consequences.
When a single‑base change introduces a stop codon (UAA, UAG, or UGA) into a coding sequence, the ribosome encounters a signal that halts translation. The resulting polypeptide is truncated, missing critical functional domains. Because the ribosome cannot read beyond the premature stop, downstream mRNA is never translated.
Cells have a quality‑control system called nonsense‑mediated decay. If the mRNA contains an early stop codon, the NMD machinery flags it for rapid degradation. By eliminating defective transcripts, the cell prevents the production of truncated, potentially harmful proteins. However, this also means that the protein is simply not made at all.
Point mutations can also alter non‑coding regulatory elements—promoters, enhancers, or transcription‑factor binding sites. A single nucleotide change can weaken or abolish the binding of transcription factors, effectively turning a gene off and stopping protein synthesis entirely.
The effect of a nonsense mutation depends on where it occurs and which protein it disrupts. A mutation near the 5′ end of a gene can truncate most of the protein, whereas one near the 3′ end may remove only a small tail. Approximately 15–30 % of inherited human diseases, from cystic fibrosis to hemophilia, are attributable to nonsense mutations.
Understanding these mechanisms is essential for diagnosing genetic disorders, designing gene‑editing therapies, and developing targeted drugs that can bypass or correct premature stop codons.
