Translation ends when one of three stop codons, UAA, UAG, or UGA, enters the A site of the ribosome. There are no aminoacyl tRNA molecules that recognize these sequences. Instead, release factors bind to the P site, catalyzing the release of the completed polypeptide chain and separating the ribosome into its original small and large subunits.
Remember that only eukaryotes, and not prokaryotes, underwent post- transcriptional RNA modifications. These modifications are responsible for one difference between prokaryotic and eukaryotic translation. Whereas prokaryotic initiation, which we have just covered, begins with the ribosomal recognition of the ribosome binding site on the mRNA, eukaryotic initiation begins with the ribosomal recognition of the 5' cap. Eukaryotic mRNA need not contain a ribosome binding cap because the post-transcriptionally added 5' cap suffices for recognition. Once the eukaryote ribosome binds to the mRNA, further differences appear. Whereas in prokaryotes the initiator codon can be either GUG or AUG, in eukaryotes the codon must be AUG. Additionally, the tRNA responsible for recognizing the initiator codon is not the special tRNA, fMet, but rather the normal met-tRNA.