Termination of Translation

Once an mRNA is translated, the ribosome needs to dissociate from the RNA and release the newly made polypeptide chain. 

Translation is terminated when a stop codon, UAA, UAG, or UGA, is encountered. There are no complementary tRNAs that correspond to stop codons.

Instead, when a stop codon is positioned on the A site of the ribosome, it is recognized by proteins called release factors, RF1 or RF2.

This binding forces the enzyme peptidyl transferase in the ribosome to catalyze the addition of a water molecule instead of an amino acid to the peptidyl-tRNA.

As a result, the P-site amino acid detaches from its tRNA, releasing the newly made polypeptide into the cytoplasm. 

Next, a third release factor, RF3, bound to GDP joins the ribosome.

On the ribosome, RF3 replaces GDP with GTP. This exchange brings about a conformational change in RF3, which triggers the dissociation of RF1 and RF2 from the ribosome.

Then, RF3 catalyzes GTP hydrolysis, which allows the ribosomal subunits to dissociate from each other and from the mRNA.

The disassembled ribosomal subunits bound to an initiator tRNA, can now join a new mRNA for another round of translation.