The tRNA or the “adapter” is 74 to 95 bases long ribonucleic acid. It has two crucial properties:
1) It represents a single amino acid; the amino acid is covalently linked to the tRNA and carried to the site of protein synthesis (ribosomes).
2) It contains the anticodon , a trinucletide sequence, which is complementary to the codon sequence (representing an amino acid) on the mRNA. The anticodon enables the tRNA to recognize the codon via complementary base pairing.
All tRNAs share certain common features. They have “unusual” bases. These bases are generated by modification of the 4 standard bases after the synthesis of the polynucleotide chain. The secondary structure of tRNA is explained by the “cloverleaf” model. It emphasizes the two major types of secondary structure, stems and loops, which are formed by complementary base pairing. The tRNA loop regions are rich in modified bases. The stem-loop structures are referred to as the arms of tRNA. Each tRNA molecule is made up of four major arms;
1) Acceptor arm
2) TψC arm
3) Anticodon arm
4) D arm
A fifth arm is the so-called extra arm; it is also known as extra or variable arm.