Cells sometimes misread their own genetic code on purpose
Translation turns a string of RNA letters into a protein, reading three letters per amino acid. Accuracy depends on enzymes that load each carrier molecule with the right amino acid. They occasionally slip, inserting the wrong building block, and under certain conditions cells actually loosen their standards because a little mistranslation can help them survive.
In translation, a ribosome outside the nucleus decodes messenger RNA into a chain of amino acids, a polypeptide that folds into a working protein, sometimes beginning to fold before it is even finished. The rule linking each three-letter codon to an amino acid is the genetic code. Amino acids are added one at a time to the carboxyl end of the growing chain, so the process is described as running from amine to carboxyl. In eukaryotes the ribosome's two parts are known as the 40S and 60S subunits, and they join before work begins. The stages are initiation, elongation and termination.
The couriers are transfer RNAs, short non-coding chains of 74 to 93 nucleotides. Each has an attachment point for its amino acid and an anticodon, a three-letter sequence that pairs with the matching codon. Species vary hugely in how many tRNA genes they carry, from 20 to 30 in some bacteria to thousands in complex eukaryotes.
Loading is handled by aminoacyl-tRNA synthetases, which bond each amino acid to the right tRNA through an ester link at its 3' end; a loaded tRNA is called charged. When a synthetase pairs a tRNA with the wrong amino acid, the mischarged carrier can place an incorrect residue into a protein. Most organisms tolerate a low background of these errors, and some environments push the rate up, occasionally to the cell's advantage.
Inside the ribosome, sites line up along the message. An incoming charged tRNA settles into the A site opposite its codon. A peptide bond then forms between its amino acid and the chain held by the tRNA in the neighbouring P/E site, transferring the whole growing chain onto the newcomer. The ribosome shifts along toward the message's 3' end, the emptied tRNA departs, and the A site opens for the next arrival, repeating the cycle one amino acid at a time.
Source: Translation (biology)
