translation termination
/ tranz-LAY-shun TUR-mih-NAY-shun /
Every manufacturing run has to end cleanly: finish the product, release it, shut the machine down, and free it to start again. In protein-making, that final phase is termination. It is what happens when the ribosome, stepping codon by codon, finally reaches the end of the message and has to let go of the completed protein.
Termination is triggered when the ribosome's A site lands on a stop codon (UAA, UAG, or UGA). No transfer RNA matches a stop codon, so instead of an amino acid, a protein called a release factor enters the A site. The release factor prompts the ribosome to add water rather than another amino acid, which breaks the bond holding the finished protein to the last tRNA and frees the new protein. Then the ribosome's two subunits, the messenger RNA, and the spent tRNAs all separate, recycled by helper factors so the parts can be used for the next round.
Termination matters because a protein must be released at exactly the right point to have the right length and shape; failing to stop on time produces extended, often useless or harmful products. It also closes the loop so the ribosome can be reused — cells make proteins by the millions, so efficient recycling is essential. A subtle point worth keeping: the release factor is a protein that mimics the shape of a tRNA well enough to fit the A site, which is how the machinery built to read tRNAs can be stopped by something that is not a tRNA at all.
Sometimes a messenger RNA is faulty and has no stop codon at all. Cells have a rescue system that recognizes a ribosome stuck at the bare end, tags the half-made protein for destruction, and frees the trapped ribosome — a quality-control backstop for failed termination.
Cells have backup systems to rescue ribosomes when normal termination fails.
Termination is read by release factor proteins, not by a 'stop tRNA' — there is no such thing. The release factor is shaped to mimic a tRNA, but it carries no amino acid.