In addition to mRNA, two other types of RNA are needed for protein synthesis. These are ribosomal RNA (rRNA ) and transfer RNA (tRNA ). Ribosomal RNA combines with proteins to form ribosomes; the cellular structures where the synthesis of polypeptides occurs. Ribosomes consist of two subunits: one large and one small. tRNA molecules transport amino acids to the growing polypeptide chain. Each tRNA molecule has an amino acid attachment site for a particular amino acid and an anticodon (a sequence of three nucleotides that is complementary to a sequence of bases in the mRNA strand).
The enzyme aminoacyl-tRNA synthetase ensures that a given tRNA molecule picks up only its specific amino acid. Aminoacyl-tRNA synthetase contains sites that bind amino acids and tRNA, and energy is required to bring these raw materials together. To ensure high fidelity of protein translation, each tRNA has a corresponding aminoacyl-tRNA.
Just like transcription, translation takes place in three stages: initiation, elongation, and termination.
Initiation: In initiation, mRNA binds to the small subunit of a ribosome. An initiation codon, AUG, binds with an initiator tRNA molecule that bears the anticodon UAC and the amino acid methionine. Then a large ribosomal subunit attaches, making the initiation complex complete and allowing translation to begin.
There are three attachment sites on a ribosome. The E site is the exit site, the P site is the peptidyl-tRNA binding site, and the A site is aminoacyl-tRNA binding site.
Elongation : In the elongation stage, the polypeptide grows by addition of amino acids according to the sequence of bases in the mRNA molecule. This is accomplished through codon recognition, peptide bond formation, and translocation. A tRNA molecule carrying the appropriate amino acid binds to the A-site, and a peptide bond forms between the new amino acid and the end of the growing polypeptide. Then the complex shifts down the mRNA molecule: the P-site tRNA is bumped to the E-site, where it dissociates from the ribosome; the A-site tRNA moves into the P-site; and a new aminoacyl-tRNA attaches to the now open A-site.
Termination: Elongation continues until a mRNA stop codon reaches the A-site of the ribosome. Stop codons include UAA, UAG and UGA, and they do not code for an amino acid. Instead of a tRNA, a release factor protein binds to the stop codon and the newly synthesized polypeptide is liberated from the ribosome.
The newly synthesized polypeptide will undergo coiling and folding to form its secondary and tertiary structures, and it may combine with additional polypeptide chains to achieve quaternary structure (protein structure was discussed in the "Carbon and Life" tutorial).