Translating the Genetic Message
Protein biosynthesis is a complex process requiring ribosomes, messenger RNA (mRNA), transfer RNA (tRNA), and a number of protein factors. The ribosome is the site of protein synthesis. The mRNA and tRNA, which are bound to the ribosome in the course of protein synthesis, are responsible for the correct order of amino acids in the growing protein chain.
Before an amino acid can be incorporated into a growing protein chain, it must first be activated, a process involving both tRNA and a specific enzyme of the class known as aminoacyl-tRNA synthetases. The amino acid is covalently bonded to the tRNA in the process, forming an aminoacyl-tRNA. The actual formation of the polypeptide chain occurs in three steps. In the first step, that encodes the start of polypeptide synthesis. In this complex, the Mrna and the ribosome are bound to each other. The next aminoacyl-tRNA forms a complex with the ribosome and with mRNA. The binding site for the second aminoacyl-tRNA is close to that for the first aminoacyl-tRNA. A peptide bond is formed between the amino acids in the second step, called chain elongation. The chain-elongation process repeats itself until the polypeptide chain is com- plete. Finally, in the third step, chain termination takes place. Each of these steps has many distinguishing features (Figure 12.1), and we shall look at each of them in detail.
Protein translation involves three types of RNA and many protein factors.
Amino acids are activated via enzymes called aminoacyl-tRNA synthetases.
The formation of a protein takes place in four steps: activation, initiation, elongation, and termination.