In the universal genetic code, 61 codons specify 20 amino acids. There are not 61 different tRNAs molecules, however, because some tRNAs can recognize more that one codon (especially when the difference is in the wobble position).

The generalized cloverleaf-like structure of a tRNA molecule (without its corresponding amino acid) is shown in Figure 27.6a. Variations on the theme are shown in Figures 27.6b, and 27.6c.

tRNAs are unique among RNA molecules in their high content of unusual and modified bases (Figure 27.7). The anticodon loop contains the three base sequence complementary to the mRNA codon that base pairs with it during translation. tRNA molecules have a 3' end that hangs over the 5' end. This region, called the acceptor stem, is where the amino acid is covalently attached. It always ends in the sequence 5'-CCA-3'. When the amino acid is attached, the amino acid is referred to as "charged." Other features of tRNAs include the D loop, the TC loop, and the variable loop. The three-dimensional structure of tRNAs is held together by the hydrogen bonds that result from intramolecular base pairing. In some cases, the base pairings are unusual (Figure 27.9).

1. Small RNA Database

2. The RNA World

3. RNA Modification Database

4. tRNA Sequence Database