FIGURE 5-20 Structure of tRNAs. (a) Although the exact nucleotide sequence varies among tRNAs, they all fold into four base-paired stems and three loops. The CCA sequence at the 3′ end is also found in all tRNAs. Attachment of an amino acid to the 3′ A yields an aminoacyl-tRNA. Some of the A, C, G, and U residues are modified post-transcriptionally in most tRNAs (see key). Dihydrouridine (D) is nearly always present in the D loop; likewise, ribothymidine (T) and pseudouridine (ψ) are almost always present in the TΨCG loop. Yeast alanine tRNA, represented here, also contains other modified bases. The triplet at the tip of the anticodon loop base-pairs with the corresponding codon in mRNA. See R. W. Holly et al., 1965, Science 147:1462. (b) Three-dimensional model of the generalized backbone of all tRNAs. Note the L shape of the molecule.

[Part (b) data from J. G. Arnez and D. Moras, 1997, Trends Biochem. Sci. 22:211, PDB ID 1vtq.]