Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
A special signal is required for the termination of polypeptide synthesis

Finally, the ribosome attaches the last amino acid, completely finishing the Synthesis of the polypeptide encoded by the mRNA. Polypeptide termination is signaled by one of three mRNA termination codons located immediately after the codon of the final amino acid. The termination triplets UAA, UAG, and UGA do not encode any amino acid. They are referred to as nonsense triplets. Originally, they were discovered while investigating single-nucleotide changes in certain codons corresponding to specific Amino Acids. This alteration led to The Emergence of nonsense mutants of E.cołi characterized by the premature termination of Polypeptide chain synthesis. With the help of such nonsense mutants—named amber, ochre, and opal—the UAA, UAG, and UGA triplets were ultimately identified as termination codons.

As soon as the ribosome reaches the termination codon, three termination factors (release factors)—Proteins R₁, R₂, and S—come into play. They induce: 1) the hydrolytic Cleavage of the polypeptide from the final tRNA and its release; 2) the dissociation of the now “empty” tRNA from the P-site; and 3) the dissociation of the 70S ribosome into 30S and 50S subunits, ready for the synthesis of a new polypeptide chain.