Charging of tRNAs by aminoacyl-tRNA synthetases. In step 1, the adenylylation step, the amino acid is linked to adenylate, forming aminoacyl-AMP. In step 2, the amino acid is transferred from the AMP to the tRNA in one of two pathways, catalyzed by a class I or class II aminoacyl-tRNA synthetase. The synthetases in each class are listed (denoted by the amino acid they transfer to tRNA).