Aminoacyl-tRNA (also aa-tRNA or charged tRNA) is tRNA to which its cognated amino acid is chemically bonded (charged). The aa-tRNA, along with some elongation factors, deliver the amino acid to the ribosome for incorporation into the polypeptide chain that is being produced. A specific cognate amino acid is charged or aminoacylated to each tRNA by aminoacyl tRNA synthetase. This matching is crucial, since it ensures that only the particular amino acid matching the anticodon of the tRNA, and in turn matching the codon of the  mRNA, is used in protein synthesis.