Transfer RNA (tRNA) is a short molecule of about 80 nucleotides which carries a specific amino acid to the polypeptide chain at a ribosome. There is a different tRNA for each amino acid. Each one has a site for the amino acid to attach, and an anti-codon to match the codon on the mRNA. For example, codons UUU or UUC code for the amino acid phenylalanine.