kódon

In genetics, a codon is a sequence of three nucleotides in messenger RNA (mRNA) that specifies an amino acid or signals termination during protein synthesis. The genetic code uses 64 codons: 61 specify amino acids, and 3—UAA, UAG, and UGA—are stop signals. The term codon is used in English, while in some languages such as Czech and Slovak the equivalent term is kódon.

Codons are read in the 5' to 3' direction by the ribosome, with transfer RNAs (tRNAs) delivering the corresponding amino acids via anticodon–codon base pairing. The start codon AUG signals the beginning of translation and codes for methionine (formylmethionine in many bacteria). The translation process proceeds through initiation, elongation, and termination, producing a polypeptide chain that folds into a functional protein.

The code is nearly universal across organisms, with minor exceptions in mitochondria and certain microorganisms. Many amino acids are encoded by more than one codon (codon degeneracy), a feature that affects translation efficiency and codon usage bias between species. Mutations in codons can be silent (synonymous) or alter the amino acid (nonsynonymous). Frameshift mutations alter the reading frame and typically disrupt protein function.

The third base of a codon often participates in wobble base pairing, enabling a tRNA to recognize multiple codons for the same amino acid. Codon usage can be optimized in genetic engineering to improve protein expression. DNA uses thymine instead of uracil, but the conceptual codon framework remains the same when describing gene sequences.