After post-transcriptional processing, mRNA leaves the cell nucleus and bonds with a ribosome, a structure composed of proteins and RNA (rRNA) that assists in the binding process between mRNA codons and tRNA anticodons. While ribosome can only work on one mRNA strand at a time, multiple ribosomes can bind to any mRNA strand to form polyribosomes. Eukaryotic and prokaryotic ribosomes differ slightly in their size and complexity, though their function is generally similar. Until we discuss the specific properties of eukaryote translation, we will talk only about prokaryote translation.
Ribosomes are composed of two subunits, one small and one large. Four binding sites are located on the ribosome, one for mRNA and three for tRNA. The three tRNA sites are labeled P, A, and E. The P site, called the peptidyl site, binds to the tRNA holding the growing polypeptide chain of amino acids. The A site (acceptor site), binds to the aminoacyl tRNA, which holds the new amino acid to be added to the polypeptide chain. The E site (exit site), serves as a threshold, the final transitory step before a tRNA now bereft of its amino acid is let go by the ribosome.
Once the small subunit associates with an mRNA molecule, the two subunits come together, creating a compactor that keeps the mRNA and tRNA in stable and proper orientation for protein synthesis.
If we look at the chemical structure of an amino acid, we see that one end contains a terminal nitrogen group while the other contains a carboxyl group.
When amino acids are transferred from the aminoacyl tRNA in the A site to the growing protein chain attached to the P site, they are transferred in a specific orientation so that the chain grows by adding amino acids to the carboxyl, not nitrogen, end of the chain. In this way, the protein chain grows in the nitrogen to carboxyl direction. This synthesized chain is called a polypeptide chain. Similarly, each amino acid added can be called a peptide; a building block of the larger polypeptide chain. Proteins are polypeptides.
