MoBio Translation by tRNA Chapter 5

Translation is carried out by tRNA through the relationship between its anticodon and the associated amino acid. When a tRNA is brought to the ribosome by the pairing between its anticodon and the mRNA's codon, the amino acid attached at its 3' end will be added to the growing peptide. In bacteria, there are 30-40 tRNAs with different anticodons. In animal and plant cells, about 50 different tRNAs are found. However, there are 61 codons coded for amino acids. Suppose each codon can pair with only a unique anticodon, then 61 tRNAs would be needed.

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Figure 5-C-4. Pairing between tRNA's anticodon and mRNA's codon. The left figure defines the wobble position where base pairing does not obey the standard rule. The right tables show all possible base pairings at the wobble position. For example, guanine (G) can pair with both cytosine (C) and uracil (U) ; inosine (I) can pair with cytosine, adenine and uracil.

Wobble pairing

The reason why less than 61 tRNAs are required is because of the "wobble pairing" between anticodon and codon. As shown in the following figure, base pairing does not obey the standard Watson-Crick pairing at the wobble position. One base can pair with several other bases.

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Figure 5-C-5. The non-standard base pairing at the wobble position.

Aminoacyl-tRNA

The tRNA together with the amino acid attached to its 3' end is called an aminoacyl-tRNA. The attached amino acid is encoded by the codon which matches the tRNA's anticodon. Therefore, only a specific amino acid may be attached to the 3' end of a given tRNA. The process is catalyzed by a class of enzymes called aminoacyl-tRNA synthetases, which recognize the anticodon and its compatible amino acid. A cell has 20 different aminoacyl-tRNA synthetases, each can add only one of 20 amino acids to a compatible tRNA.

Let us use arginine as an example to explain the notation of aminoacyl-tRNA. "Arg-tRNA" denotes the tRNA with arginine attached while "tRNAArg" specifies the tRNA which contains the anticodon for the codon of arginine. "Arg-tRNAArg" represents the arginine-specific tRNA with arginine attached.