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.
Ribosome Structure
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.
Figure %: A 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.
Growth of the Protein
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.
Figure %: Amino Acid
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.
