Transfer RNA plays a vital role in protein synthesis by bringing amino acids to the mRNA template in the cytoplasm. Francis Crick, who had revealed the DNA structure and its properties, postulated the ‘Adapter Hypothesis’ theory in which he stated that the amino acid is carried to the mRNA template by an adapter molecule and it actually fits onto the RNA. Later Robert Holley discovered the adapter molecule - transfer RNA or tRNA – and its entire chemical structure. tRNA, the smallest of all RNA molecules, is made up of 75 nucleotides. It is synthesised from a DNA molecule, similar to the synthesis of mRNA. tRNA is soluble in 1M-NaCl and hence known as soluble RNA or sRNA.
Each tRNA has G at its five prime end and CCA at its three prime end. It consists of four arms or sites – ribosome-recognition site, anticodon site, enzyme-recognition site and amino acid-binding site. The ribosome-recognition site recognises specific ribosomes attached to the mRNA and brings amino acids to the ribosome. The anticodon site consists of three nitrogen bases called the anticodon, which pairs with the codon of the mRNA strand. However, the anticodon of tRNA is complementary to the codon of the mRNA strand.
The enzyme-recognition site is the recognition site for amino acid synthetase enzyme, which helps the amino acid bind at the three-prime end of the tRNA. There is a specific tRNA for each of the twenty amino acids and also a specific tRNA called initiator tRNA which initiates the process of protein synthesis. But there are no tRNAs for stop codons.