What is the role of transfer RNA in protein synthesis?

What is the role of transfer RNA in protein synthesis?

The smallest of the three important kinds of RNA is tRNA. Different types of tRNA molecules can be found in every living cell because at least one tRNA bonds specifically to each of the amino acids that commonly occur in proteins.

Frequently there are several tRNA molecules for each amino acid. A tRNA is a single-stranded polynucleotide chain, between 73 and 94 nucleotide residues long, that generally has a molecular mass of about 25,000 Da. (Note that bio-chemists tend to call the unit of atomic mass the dalton, abbreviated Da.)

Intrachain hydrogen bonding occurs in tRNA, forming A–U and G–C base pairs similar to those that occur in DNA except for the substitution of uracil for thymine. The duplexes thus formed have the A-helical form, rather than the B-helical form, which is the predominant form in DNA. The molecule can be drawn as a cloverleaf structure, which can be considered the secondary structure of tRNA because it shows the hydrogen bonding between certain bases (Figure 9.22). 

The hydrogen-bonded portions of the molecule are called stems, and the non-hydrogen-bonded portions are loops. Some of these loops contain modified bases (Figure 9.23). During protein synthesis, both tRNA and mRNA are bound to the ribosome in a definite spatial arrange-ment that ultimately ensures the correct order of the amino acids in the grow-ing polypeptide chain.

A particular tertiary structure is necessary for tRNA to interact with the enzyme that covalently attaches the amino acid to the 2' or 3' end. To produce this tertiary structure, the tRNA folds into an L-shaped conformation that has been determined by X-ray diffraction (Figure 9.24).