A cell's DNA contains all of the information necessary to produce the hundreds or thousands of different proteins necessary for the cell's survival and function. In order to produce a protein, the cell must first transcribe the code contained in the DNA into a complementary mRNA code. The mRNA is then translated into a series of amino acids. The series of amino acids makes up all or part of the final protein.
This tutorial focuses on the second half of this process, called translation, or protein synthesis. Protein synthesis occurs on ribosomes, which serve as staging areas where mRNA and a series of amino acids—each carried on an adapter molecule called tRNA—come together. In addition to bringing these molecules together, ribosomes have catalytic activity—they facilitate the formation of peptide bonds between the amino acids in a growing polypeptide (protein) chain.
Protein synthesis occurs in three stages: initiation, elongation, and termination. In the initiation stage, the ribosomal subunits and an initiator tRNA assemble around the start codon—an AUG—in the mRNA. The initiator tRNA carries the amino acid methionine, which means that all newly made polypeptides begin with this amino acid. In many cases, however, the initiator methionine is removed by enzymes after translation.
During the elongation stage, the ribosome undergoes many cycles in which new tRNAs enter the ribosome carrying new amino acids. The ribosome catalyzes the formation of peptide bonds between the new amino acids and the growing polypeptide chain before moving to the next codon in the mRNA strand.
Termination occurs when the ribosome reaches one of three possible stop codons in the mRNA molecule. Stop codons are recognized by release factors, which allow the hydrolysis of the new polypeptide from the tRNA in the P site of the ribosome. After the polypeptide is released, the ribosomal subunits and the mRNA molecule also dissociate from each other, and each is then available for another round of protein synthesis.
Textbook Reference: Concept 10.4 Translation of the Genetic Code Is Mediated by tRNAs and Ribosomes