Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
As Crick’s adapter hypothesis proposed, the translation of mRNAinto proteins requires a molecule that links the information contained in mRNA codons with specific amino acids in proteins. That function is performed by tRNA. Two key events must take place to ensure that the protein made is the one specified by mRNA:
- tRNA must read mRNA correctly.
- tRNA must carry the amino acid that is correct for its reading of the mRNA.
Transfer RNAs carry specific amino acids and bind to specific codons
The codon in mRNA and the amino acid in a protein are related by way of an adapter — a specific tRNA with attached amino acid. For each of the 20 amino acids, there is at least one specific type (“species”) of tRNA molecule.
The tRNA molecule has three functions: It carries (is “charged with”) an amino acid, it associates with mRNA molecules, and it interacts with ribosomes. Its molecular structure relates clearly to all of these functions. AtRNAmolecule has about 75 to 80 nucleotides. It has a conformation (a three-dimensional shape) that is maintained by complementary base pairing (hydrogen bonding) within its own sequence The conformation of a tRNA molecule allows it to combine specifically with binding sites on ribosomes. At the 3′ end of every tRNA molecule is a site to which its specific amino acid binds covalently. At about the midpoint of tRNA is a group of three bases, called the anticodon, that constitutes the site of complementary base pairing (hydrogen bonding) with mRNA. Each tRNA species has a unique anticodon, which is complementary to the mRNA codon for that tRNAs amino acid. At contact, the codon and the anticodon are antiparallel to each other. As an example of this process, consider the amino acid arginine:
- The DNA coding region for arginine is 3′-GCC-5′, which is transcribed, by complementary base pairing, to the mRNA codon 5′-CGG-3′.
- That mRNA codon binds by complementary base pairing to a tRNA with the anticodon 3′-GCC-5′ which is charged with arginine.
Recall that 61 different codons encode the 20 amino acids in proteins. 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 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, called wobble, allows the alanine codons GCA, GCC, and GCU, for example, all to be recognized by the same tRNA. Wobble is allowed in some matches but not in others; of most importance, it does not allow the genetic code to be ambiguous!
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