translation termination and release factor

The ribosome has been adding amino acids codon by codon. Then it arrives at a stop codon — and there is no transfer RNA in the cell that fits it. The machine would stall. Termination is how the cell turns that dead end into a clean finish: it recognizes the stop signal, cuts the completed protein loose, and recycles the parts.

Termination works because of a protein, not a tRNA. When a stop codon (UAA, UAG or UGA) rolls into the A site, no charged tRNA can pair with it. Instead, a protein called a release factor — shaped, remarkably, to mimic a tRNA — slips into the A site and reads the stop codon directly. Its arrival triggers the ribosome's catalytic centre to do something slightly different: instead of joining the chain to a new amino acid, it adds a water molecule, hydrolysing the bond that tied the finished protein to the last tRNA. The completed protein floats free. Then other helper factors pry the ribosome apart into its two subunits and release the mRNA, so the pieces can be reused for the next round.

Termination is the punctuation that gives every protein a defined end. Its reliance on a release factor recognizing the stop codon is what makes nonsense mutations so consequential: create a premature stop codon and the release factor will cut the protein short, often destroying its function. Conversely, certain drugs (and natural 'readthrough' events) can coax the ribosome to slip past a premature stop, an idea being pursued as therapy for some genetic diseases. Because bacterial and human release factors differ, this final step is yet another point where the two translation systems part ways.