Transfer RNA the Anticodon and the Wobble

Although 61 codons specify the 20 different amino acids, there are not 61 tRNAs; instead the cell economizes. The codons for some amino acids differ only in the third position of the codon. Figure 4.8 on page 78 shows that when an amino acid is encoded by only two different triplets the third bases will be either C and U, or A and G. For example aspartate is coded by GAC and GAU and glutamine by CAA and CAG. The wobble hypothesis suggests that the pairing of the first two bases in the codon and anticodon follows the standard rules—G bonds with C and A bonds with U—but the base pairing in the third position is not as restricted and can wobble. If there is a pyrimidine, U or C, in the third position of the codon, it can fit with any purine, G or A, in the anticodon, and vice versa. Thus, only one tRNA molecule is required for two codon sequences. The anticodon of some tRNAs contains the unusual nucleoside inosine (I), whose base is the purine hypoxanthine (Fig. 2.13, page 34). Inosine can base pair with any of U, C, or A in the third position of the codon. Some

OH 3' end - amino acid A attachment site

OH 3' end - amino acid A attachment site

Figure 8.2. Transfer RNA (tRNA).

tRNA molecules can therefore base pair with as many as three different codons provided the first two bases of the codon are the same. For example, the tRNA for isoleucine has the anticodon UAI and can therefore base pair with any of AUU, AUC, or AUA.

The attachment of an amino acid to its correct tRNA molecule is illustrated in Figure 8.3. This process occurs in two stages, both catalyzed by the enzyme aminoacyl tRNA synthase. During the first reaction, the amino acid is joined, via its carboxyl group, to an adenosine monophosphate (AMP) and remains bound to the enzyme. All tRNA molecules have at their 3' end the nucleotide sequence CCA. In the second reaction aminoacyl tRNA synthase transfers the amino acid from AMP to the tRNA, forming an ester bond between its carboxyl group and either the 2'- or 3'-hydroxyl group of the ribose of the terminal adenosine (A) on the tRNA to form an aminoacyl tRNA. This step is often referred to as amino acid activation because the energy of the ester bond can be used in the formation of a lower energy peptide bond between two amino acids. A tRNA that is attached to an amino acid is known as a charged tRNA. There are at least 20 aminoacyl tRNA synthases, one for each amino acid and its specific tRNA.

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