A transfer RNA carries a specific amino acid and binds to a specific mRNA codon

There is at least one specific tRNA molecule for each of the 20 amino acids. Each tRNA has three functions that are fulfilled by its structure and base sequence (Figure 14.10):

  1. tRNAs bind to particular amino acids. Each tRNA binds to a specific enzyme that attaches it to only 1 of the 20 amino acids. The covalent attachment to an amino acid takes place at the 3′ end of the tRNA. When it is carrying an amino acid, the tRNA is said to be “charged.”

  2. tRNAs bind to mRNA. At about the midpoint on the tRNA polynucleotide chain there is a triplet of bases called the anticodon, which is complementary to the mRNA codon for the particular amino acid that the tRNA carries. For example, the mRNA codon for arginine is 5′-CGG-3′, and the complementary tRNA anticodon is 3′-GCC-5′. Like the two strands of DNA, the codon and anticodon bind together via noncovalent hydrogen bonds.

  3. tRNAs interact with ribosomes. The ribosome has several sites on its surface that just fit the three-dimensional structure of a tRNA molecule. Interaction between the ribosome and the tRNA is noncovalent.

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Figure 14.10 Transfer RNA The stem and loop structure of a tRNA molecule is well suited to its functions: binding to amino acids, associating with mRNA molecules, and interacting with ribosomes.

Recall that 61 different codons encode the 20 amino acids in proteins (see Figure 14.5). Does this mean that the cell must produce 61 different tRNA species, each with a different anticodon? No. The cell gets by with about two-thirds of that number of tRNA species because the specificity for the base at the 3′ end of the codon (and the 5′ end of the anticodon) is not always strictly observed. This phenomenon is called wobble, and it is possible because in some cases unusual or modified nucleotide bases occur in the 5′ position of the anticodon. One such unusual base is inosine (I), which can pair with A, C, and U. For example, the presence of inosine in the tRNA with the anticodon 3′-CGI-5′ allows it to recognize and bind to three of the alanine codons: GCA, GCC, and GCU. Wobble occurs in some matches but not in others; of most importance, it does not allow the genetic code to be ambiguous. That is, each mRNA codon binds to just one tRNA species, carrying a specific amino acid.