Translation

Translation is the process by which the coded sequence of codons carried by an mRNA molecule is used to direct the synthesis of a specific polypeptide (protein) chain, with each codon specifying one particular amino acid to be added, in order, to the growing chain. Translation takes place at the ribosome, a large ribonucleoprotein complex built of rRNA and ribosomal proteins, organised into a small subunit and a large subunit that come together around the mRNA molecule during translation. The specific matching of each mRNA codon to its correct corresponding amino acid is carried out by transfer RNA (tRNA): each tRNA molecule carries, at one end, a specific three-base sequence called an anticodon, which base-pairs (in antiparallel, complementary fashion) with a specific mRNA codon, and, at its other end, carries the one particular amino acid that corresponds to that codon — an enzyme called aminoacyl-tRNA synthetase is responsible for correctly "charging" each tRNA molecule with its own matching amino acid before the tRNA is used at the ribosome.

Translation proceeds through three sequential stages, closely paralleling the stages of transcription. During initiation, the small ribosomal subunit binds to the mRNA molecule near its 5' end and then scans along the mRNA until it locates the START codon, AUG; a special initiator tRNA, carrying the amino acid methionine and bearing the complementary anticodon, then base-pairs with this START codon, after which the large ribosomal subunit joins to complete the assembled, functional ribosome, positioned and ready to begin synthesising the polypeptide chain.

During elongation, the ribosome moves systematically along the mRNA, one codon at a time, in the 5' to 3' direction. At each successive codon, a new aminoacyl-tRNA (a tRNA already charged with its matching amino acid) enters the ribosome and its anticodon base-pairs with that codon; a peptide bond is then formed between this newly arrived amino acid and the last amino acid of the growing polypeptide chain (a reaction catalysed by the ribosome's own catalytic, rRNA-based peptidyl transferase activity); the ribosome then translocates (moves) forward by exactly one codon along the mRNA, ejecting the now-empty, uncharged tRNA and positioning the next codon to be read. This codon-by-codon cycle — a new charged tRNA arriving, a peptide bond forming, the ribosome translocating forward by one codon — repeats continuously, progressively lengthening the polypeptide chain by one amino acid with each cycle, until the ribosome reaches a STOP codon (UAA, UAG or UGA) on the mRNA. …