Transfer RNA (tRNA) : It is also called soluble or sRNA. There are over 100 types of tRNA’s. Transfer RNA constitutes about 15% of the total RNA. tRNA is the smallest RNA with 70-85 nucleotide’s and sedimentation coefficient of 4S. The nitrogen bases of several of its nucleotide’s get modified e.g., pseudouridine (φ), dihydrouridine (DHU), inosine (I). This causes coiling of the otherwise single- stranded tRNA into L-shaped form (three dimensional, Klug, 1974) or clover-like form (two dimensional, Holley, 1965). About half of the nucleotides are based paired to produce paired stems. Five regions are unpaired of single-stranded —AA-binding site, T φ C loop, DHU loop, extra arm and anticodon loop,

(i) Anticodon. It is made-up of three nitrogen bases for recognizing and attaching to the codon of tRNA.

(ii) AA-Binding site. It lies at the 3′ end opposite to the anticodon and has CCA—OH group (5′ ends bears G). Amino acid or AA-binding site and anticodon are the two recognition sites of tRNA. 

(iii) T φ C loop. It contains pseudouridine. The loop is the site for attaching to ribosomes. 

(iv) DHU loop. The loop contains dihydrouridine. It is binding site for aminoacyl synthetase enzyme, 

(v) Extra arm. It is a variable site arm or loop which lies between T φ C loop and anticodon. The exact role of extra arm is not known. 

Functions :

(i) tRNA is adapter molecule which is meant for transferring amino acids to ribosomes for synthesis of polypeptides. There are different tRNAs for different amino acids. Some amino acids can be picked up by 2-6 tRNA’s. tRNA’s carry specific amino acids at particular points during polypeptide synthesis as per codons of wRNA. Codons are recognised by anticodons of tRNA’s. Specific amino acids are recognized by particular activating or aminoacyl synthetase enzymes. 

(ii) They hold peptidyl chains over the mRNA’s. The initiator tRNA has the dual function of initiation of protein synthesis as well as bringing in of the first amino acids. There is, however, no tRNA for stop signals.