When the ribosome reaches a stop codon, no aminoacyl tRNA binds to the empty A site. This isn't always the end of the story for the new protein. Often it will undergo post-translational modifications . Have some of its amino acids altered. For example, a tyrosine residue might become phosphorylated.
Many proteins have carbohydrates covalently attached to asparagine residues.
Discussion about where and how these processes occur can be found in the Cell Biology chapter of the hypertext.