Non-cognate codon–anticodon interactions on the ribosome do occur, but do not normally result in misincorporation of amino acids into protein. This is because aminoacyl-tRNA binds the ribosomal A-site in an EF-1A–aminoacyl-tRNA–GTP complex, and hydrolysis of GTP must occur before the peptide bond can form. The non-cognate interaction appears to be unable to trigger this hydrolysis and dissociates from the ribosome.
More Detailed Explanation
The hydrogen bonding between the bases of the mRNA codon and the tRNA anticodon is the basis of the specificity of decoding. It is easy to see that a codon such as 5’-GGG-3’ would form nine hydrogen bonds with an anticodon 3’-CCC-5’, but none with an anticodon 3’-AAA-5’. However, as the question makes clear, the situation is not quite so simple if two of the codon bases form the correct hydrogen bonds. A more telling example is shown below (here with the same tRNA, but different codons), where the difference between a his-tRNA interacting with a His codon and a Gln codon is only the strength of a GU ‘wobble’ base pair, often reckoned to be that of a single hydrogen bond. Certainly there is a difference in energy of the two interactions, but not enough to prevent a significant extent of miscoding.
The answer to this conundrum turned out to be the solution to another long-standing puzzle — that of why GTP is required in the binding of aminoacyl-tRNA to the A-site of the ribosome, which is directed by EF-1A (originally known in bacteria as EF-Tu). The GTP is not required for peptide-bond formation — the energy for that is supplied by the ATP in the tRNA synthetase reaction. It transpires that the role of GTP is in maintaining the fidelity of translation. The hydrolysis of GTP only occurs when the correct tRNA is bound — presumably the conformation of the incorrect interaction does not allow this to occur — so the non-cognate tRNA will eventually dissociate from the ribosome.
It is interesting that the other elongation factor, EF-2 (EF-G), involved in translocation of the peptidyl-tRNA, binds to the same site on the ribosome in a complex with GTP, and that translocation and removal of the deacylated tRNA can only occur after hydrolysis of GTP. It is as if the requirement for GTP hydrolysis ensures that the system is in the correct conformation before the next step of the complex process proceeds.
Berg et al. provides a general account, freely available online. A more detailed structural consideration can be found in a review by Nilsson and Nissen, which requires library access.