DNA Translation

A single base substitution in the anticodon loop of one of the tRNAs for glycine (tRNAgly) can hypothetically generate a mutant tRNA that is able to suppress translational termination by causing insertion of a glycine residue at certain termination codons.  Mutations of this sort are rarely encountered in nature, due in part to the existence of a key step in the chain of events that helps ensure translational fidelity.  What is this step and how might it prevent suppressor tRNAs from being able to incorporate amino acids at stop codons?