Transfer RNA: Structure

 
Transfer RNA (tRNA) is an essential component of the protein synthesis reaction. There are at least twenty different kinds of tRNA in the cell¹ and each one serves as the carrier of a specific amino acid to the site of translation.

tRNA's are L-shaped molecules. The amino acid is attached to one end and the other end consists of three anticodon nucleotides. The anticodon pairs with a codon in messenger RNA (mRNA) ensuring that the correct amino acid is incorporated into the growing polypeptide chain.

The L-shaped tRNA is formed from a small single-stranded RNA molecule that folds into the proper conformation. Four different regions of double-stranded RNA are formed during the folding process.

The two ends of the molecule form the acceptor stem region where the amino acid is attached. The anticodon is an exposed single-stranded region in a loop at the end of the anticodon arm.

The two other stem/loop structures are named after the modified nucleotides that are found in those parts of the molecule. The D arm contains dihydrouridylate residues while the TΨC arm contains a ribothymidylate residue (T), a pseudouridylate residue (Ψ) and a cytidylate (C) residue in that order. All tRNA's have a similar TΨC sequence. The variable arm is variable, just as you would expect. In some tRNA's it is barely noticable while in others it is the largest arm.

tRNA's are usually drawn in the "cloverleaf" form (below) to emphasize the base-pairs in the secondary structure.

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1. Most genomes contain 40-80 different tRNA genes. While there are only 20 common amino acids, there are 61 different codons. Many codons are recognized by more than one different tRNA—the classic example is the codon AUG that can be recognized by methionyl-tRNA and initiator tRNA.