Translation

Translation is the process by which the ribosome polymerises a chain of amino acids, delivered one at a time as charged tRNAs, into a growing polypeptide chain, following the sequence of codons laid out on an mRNA molecule; this decoding job is carried out by ribosomes, which physically bind both the mRNA and successive charged tRNA molecules as translation proceeds. Structurally, a ribosome is built from a mixture of structural RNA molecules and roughly eighty different proteins, and when it is not actively engaged in translation it exists as two separate subunits, one large and one small, that are only brought together once translation is under way. Translation itself begins at the mRNA's 5' end, and as the assembled ribosome moves progressively along the mRNA, it adds one new amino acid to the growing chain each time it reads through one more codon, with each individual codon recognised, in turn, by the matching anticodon carried on the appropriate incoming tRNA -- so the final order and identity of amino acids in the finished polypeptide is dictated entirely by the order of bases in the mRNA. Prokaryotic ribosomes are described by their sedimentation coefficient as 70S particles, made up of a larger 50S subunit and a smaller 30S subunit ('S', the Svedberg unit, measures how fast a particle settles under centrifugation and is not simply additive). Eukaryotic ribosomes are somewhat larger overall, sedimenting at 80S and built from a 60S large subunit and a 40S small subunit. In bacteria, the 30S subunit itself contains 16S rRNA, while the 50S subunit contai …