Translation (also called protein biosynthesis or polypeptide synthesis) is the second process in gene expression. In translation, messenger RNA is used as a template to produce a specific polypeptide according to the rules specified by the genetic code.
The transfer RNA (tRNA) is a small RNA chain (74-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis.
Initiation of translation involves the small ribosomal subunit binding to the 'start' codon on the mRNA, which indicates where the mRNA starts coding for the protein. This codon is most commonly an AUG, but alternative start codons are common in prokaryotes. In eukaryotes and archaea, the amino acid encoded by the start codon is methionine. In bacteria, the protein starts instead with the modified amino acid N-formyl methionine (f-Met). In f-Met, the amino group has been blocked by a formyl group to form an amide, so this amino group cannot form a peptide bond –. This is not a problem because the f-Met is at the amino terminus of the protein. In prokaryotes the binding of the small subunit to the correct place on the mRNA is facilitated by base pairing to a series of bases known as the Shine-Dalgarno sequence, located 8-13 nucleotides before the start site.
The initiator tRNA, carrying Met or f-Met, base pairs to the start codon and sits in the P site of the ribosome. The large subunit then forms a complex with the small and elongation proceeds. A new activated tRNA enters the A site of the ribosome and base pairs with the mRNA. The enzyme peptidyl transferase forms a peptide bond between the adjacent amino acids. As this happens, the amino acid on the P site leaves its tRNA and joins the tRNA at the A site. The ribosome them moves in relation to the mRNA shifting the tRNA at the A site on to the P whilst releasing the empty tRNA, this process is known as translocation.
This procedure repeats until the ribosome encounters one of three possible stop codons, where termination occurs. This stalls protein growth, and release factors, proteins which mimic tRNA, enter the A site and release the protein in to the cytoplasm.
Synthesis of proteins can take place extremely quickly. This is aided by multiple ribosomes being able to attach themselves to one mRNA chain, thus allowing multiple proteins to be constructed at once. An mRNA chain with multiple ribosomes is called a polysome. Also, as prokaryotes have no nucleus, an mRNA can be translated while it is still being transcribed. This is not possible in eukaryotes as translation occurs in the cytoplasm, whereas transcription occurs in the nucleus.
