Final answer:
The two classes of aminoacyl-tRNA synthetases are differentiated by amino acid size. Class I is typically for larger amino acids and Class II for smaller ones. This distinction affects how these enzymes charge the tRNAs.
Step-by-step explanation:
The two classes of aminoacyl-tRNA synthetases (aaRS) are not based on the tRNA structure, enzyme location, or the amino acid charge. Instead, they are differentiated based on the amino acid size, which correlates with their hydrodynamic size and biosynthetic pathway complexity. Class I aaRS typically is associated with larger amino acids, while Class II aaRS generally corresponds with smaller amino acids. This classification is reflected in the way these enzymes charge the tRNA by catalyzing a high-energy bond between the tRNA and the correct or cognate amino acid.