The Structure of Transfer RNA

A unique feature of tRNA is the occurrence of rare, modified bases. All RNAs have the four standard bases (adenine, cytosine, guanine, and uracil) specified by DNA, but tRNAs have additional bases, including ribothymine, pseudourasil (which is also occasionally present in snRNAs and rRNA), and dozens of others. The structures of some of these modified bases are shown in (i Figure 14.21).

If there are only four bases in DNA, and all RNA molecules are transcribed from DNA, how do tRNAs acquire these additional bases? Modified bases arise from chemical changes made to the four standard bases after transcription. These changes are carried out by special tRNA-modifying enzymes. For example, the addition of a methyl group to uracil creates the modified base ribothymine.

The structures of all tRNAs are similar, a feature critical to tRNA function. Most tRNAs contain between 74 and 95 nucleotides, some of which are complementary to each other and form intramolecular hydrogen bonds. As a result, each tRNA has a cloverleaf structure (i Figure 14.22). The cloverleaf has four major arms, each consisting of a stem and a loop. The stem is formed by the paring of comple mentary nucleotides, and the loop lies at the terminus of the stem, where there is no nucleotide pairing. If we start at the top and proceed clockwise around the tRNA shown at the right in Figure 14.22, the four major arms are the acceptor arm, the T^C arm, the anticodon arm, and the DHU arm.

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