codons

Codons are the basic units of the genetic code in messenger RNA. Each codon consists of three nucleotides, and the sequence is read 5' to 3' by the ribosome during translation. In DNA, the corresponding triplets on the coding strand use thymine (T) instead of uracil (U).

The standard genetic code contains 64 codons: 61 specify amino acids and 3 signal the end of translation (UAA, UGA, UAG). The start codon, AUG, marks the beginning of translation and also encodes methionine in most contexts; in bacteria it often initiates with formyl-methionine.

Because there are 64 codons but only 20 amino acids, most amino acids are encoded by more than one codon—a property called degeneracy. The third position of the codon often tolerates variability, a flexibility known as the wobble phenomenon, allowing a single tRNA to recognize multiple codons.

Although the genetic code is nearly universal, there are notable exceptions in mitochondria, chloroplasts, and some single-celled organisms, where certain codons correspond to different amino acids or stop signals. The code's universality underpins many biotechnological techniques, including codon optimization—adjusting codon usage in a gene to match a host organism's tRNA abundance to improve expression.

Codons, together with anticodons on tRNAs, enable the translation of genetic information into a polypeptide sequence. Errors in codon reading, such as frameshifts, can alter the resulting protein, highlighting the precise coordination required for accurate gene expression.