stop codon
/ STOP KOH-don /
Every sentence needs a full stop, or it would run on forever. A protein recipe is the same: the cell must know not only where to start reading but where to finish. The stop codon is that period at the end — the signal on the messenger RNA that tells the protein-building machine, 'the chain is complete; release it.'
There are three stop codons: UAA, UAG, and UGA. What makes them special is what they do not do — no normal transfer RNA carries an amino acid that matches them. So when the ribosome arrives at a stop codon, instead of an amino acid being delivered, proteins called release factors slot into the empty spot. They trigger the ribosome to cut the finished protein loose and let go of the messenger RNA. The three stop codons are sometimes called 'nonsense' codons, an old name from the days before their job was understood.
Stop codons matter because they define a protein's exact length; without a timely stop the ribosome would keep adding amino acids into the untranslated tail and beyond, producing a bloated, malfunctioning protein. They are also a frequent site of disease: a 'nonsense mutation' that turns an amino-acid codon into a premature stop chops the protein short, and many genetic disorders (and some cancers) trace back to exactly that. A common misreading is to picture a stop codon as itself coding for some 'stop amino acid' — it codes for no amino acid at all; it is the absence of a matching tRNA that ends the chain.
If a healthy gene reads ...UGG (tryptophan)... but a single base change turns that into UGA, the ribosome now hits a stop where it used to read tryptophan and releases a half-finished protein. This kind of premature stop underlies some cases of diseases like cystic fibrosis.
A single base change can plant a stop codon in the middle of a gene and truncate the protein.
A stop codon codes for no amino acid; it is read by release factors, not tRNAs. The mnemonic 'U Are Away / U Are Gone / U Go Away' for UAA/UAG/UGA is just a memory aid, not chemistry.