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OK here it goes. There are sixty-four possible combinations for a codon (UUU, UUA, UUC, etc.). Three codons code for termination and they do not bind to a tRNA's. So there should be sixty-one tRNA's that recognize these codons and bring the correct amino acid to each codon during translation. However, there are only about fourty-five tRNA's.
It was found that some tRNA's can bind more than one codon via non-standard base-pairing between the last base (nucleotide) of a codon and the first base of a tRNA. However, the first two bases of a codon and the last two of a tRNA must follow the standard base-pairing. These non-standard base-pairings are G-U, I-U, I-A, and I-C.
"I" denotes the base "Inosine" that is present in the first location in some tRNA.
