The structure of tRNA. (a) The cloverleaf form. Large dots on the backbone represent the nucleotide residues; blue lines indicate base pairs. Three nucleotides constitute the anticodon (at the bottom), and an amino acid is attached to the 3′-terminal amino acid arm. Unusual modified nucleotides are present in the D arm and TΨC arm. Py (pyrimidine) can be either U or C; Pu (purine) can be either A or G. The D arm often contains one or more dihydrouracil residues (D). (b) The three-dimensional structure folds into a twisted L shape. The positions of the anticodon, the 3′-terminal amino acid arm, and the D and TΨC arms are shown.