INTRODUCTION

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.

ANIMATION SCRIPT

Translation occurs in three steps: initiation, elongation, and termination. Translation begins with the association of an mRNA, a small ribosomal subunit, and an amino acid–charged tRNA, coming together to form an initiation complex. Note that the mRNA start codon is AUG, and the first amino acid of this polypeptide and all other newly made peptides is methionine.

The large subunit of the ribosome joins the complex. The ribosome contains three sites for tRNAs. The A, or amino acid, site accepts new tRNAs bearing amino acids. The P, or polypeptide, site is where a tRNA adds its amino acids to the growing polypeptide chain. The E, or exit, site is where the tRNA resides before exiting the ribosome.

The ribosome now enters the elongation phase of translation. A tRNA with an anticodon complementary to the next codon of the mRNA enters the A site of the large subunit. The large subunit catalyzes two reactions: It breaks the bond between the tRNA in the P site and its amino acid, and it catalyzes the formation of a peptide bond between the amino acids from the A and P sites.

After peptide linkage, the empty P site tRNA shifts to the E site on the ribosome as the ribosome translocates the length of one codon, releasing the empty tRNA. The second tRNA, now bearing a dipeptide, is in the P site. The next charged tRNA enters the open A site, and the elongation cycle repeats.

Translation is terminated when a stop codon—in this case UAA—enters the A site. Stop codons do not encode amino acids or bind tRNA molecules. Rather, they bind a protein release factor, which allows hydrolysis of the bond between the polypeptide chain and the tRNA in the P site. The various components of translation then dissociate from each other.

CONCLUSION

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.