The so-called extra arm is the most variable part of tRNA molecule, and on the basis of nature of the extra arm, tRNAs have been divided into two classes:
1) Class 1 tRNAs: They have a smaller extra arm, which is 3-5 bases long. They represent approximately 75% of all tRNAs.
2) Class 2 tRNAs: The extra arm in class 2 tRNAs is 13-21 bases long.
In a tRNA occasional G.U, G.ψ, and A.ψ base pairs are found, besides the conventional ones (G.C and A.U). The secondary structure of tRNA folds into the L-shaped tertiary structur, and all tRNAs have the same tertiary structure.
A tRNA charged with the amino acid corresponding to its anticodon is referred to as aminoacyl-tRNA . An ester bond is formed between the carboxyl group of the amino acid and the 2′ or 3′ hydroxyl group of the ribose of the 3′ terminal base of the tRNA (an A) .
One tRNA is present for each amino acid. However, more than one tRNA also exist for some amino acids. A tRNA is always named by using the 3-letter abbreviation for the (corresponding) amino acid as a superscript. In case of more than one tRNA for the same amino acid, subscript numerals are used to distinguish them. Thus it is denoted as tRNAx and tRNA1x or tRNA2x where x=an amino acid. A charged tRNA carrying an amino acid (aminoacyl-tRNA) is indicated by a prefix identifying an amino acid: Ser-tRNA corresponds to tRNAser carrying serine amino acid or charged with serine.