Each set of three DNA or mRNA nucleotides is called a codon. The mRNA goes out of the nucleus into the cytoplasm to a ribosome (assuming eukaryote). A codon (3 nucleotides) of mRNA then hydrogen bonds with the anti-codon portion of the tRNA molecule, which has previously been loaded with an amino acid. so, 3 nucleotides code for each amino acid. However, more than one set of nucleotides can code for the same amino acid (called degeneracy), and also, the third nucleotide of a specific codon can usually change and still code for the same amino acid (called wobble).