Translation

Translation is the process that decodes the sequence of nucleotide triplets, or codons, carried by a mature mRNA into the corresponding sequence of amino acids making up a polypeptide chain, and it takes place on the ribosome, a large, non-membranous ribonucleoprotein machine built from two subunits that only come together once translation is ready to begin. A ribosome provides one binding site for the mRNA itself and two distinct binding sites for tRNA molecules: the P-site (peptidyl-tRNA site), which holds the tRNA that is linked to the already-growing end of the polypeptide chain, and the A-site (aminoacyl-tRNA site), which receives each new incoming amino-acid-charged tRNA and is where the codon of the mRNA actually pairs with the anticodon of that tRNA. Translation proceeds in three stages. In initiation, translation begins specifically at the AUG start codon, which specifies the amino acid methionine; the initiator tRNA, carrying methionine, occupies the ribosome's P-site opposite this start codon. In elongation, because the P-site and A-site sit right next to each other, two tRNA molecules can simultaneously base-pair with two adjacent codons; a peptide bond then forms between the amino acid held in the P-site and the new amino acid held in the A-site, a reaction catalysed by the ribosome's own peptidyl transferase activity (the ribosome itself, in this catalytic role, functions as a ribozyme); once the bond forms, the tRNA in the P-site is released, and the ribosome moves, or translocates, exactly one codon further along the mRNA -- a step that requires the hydrolysis of GTP for energy -- shifting the newly formed peptidyl-tRNA from the A-site into the now-vacant P-site and freeing the A-site to receive the next incoming aminoacyl-tRNA; this cycle of pairing, peptide-bond formation and translocation repeats for every subsequent codon, extending the polypeptide chain one amino acid at a time as the ribosome moves along the mRNA in the 5' to 3' direction. In termination, when one of the three stop codons (UAA, UAG or UGA) enters the A-site, no normal tRNA recognises it; instead, a cytosolic protein called a release factor binds there, which triggers release of the completed polypeptide chain, detachment of the mRNA, and separation of the ribosome back into its two constituent subunits. Before an amino acid can take part in any of this, it must first be attache …