Codon

Definition

A sequence of three consecutive nucleotides in DNA or RNA that specifies a particular amino acid during protein synthesis or signals the termination of translation. Codons are read by ribosomes in the 5' to 3' direction, with each codon pairing with a complementary anticodon on a transfer RNA molecule that delivers the corresponding amino acid. The genetic code comprises 64 codons: 61 specify the 20 proteinogenic amino acids (with redundancy producing synonymous codons), while three (UAA, UAG, UGA in mRNA) function as stop codons that release the completed polypeptide chain.

Etymology

From Latin codex (book, code), reflecting its role as a unit of the genetic code.

Example

The codon AUG serves dual roles: it encodes methionine and acts as the start codon that initiates translation in nearly all eukaryotes, including insects such as Drosophila melanogaster. Mutations that convert a sense codon to a stop codon (nonsense mutations) frequently underlie genetic disorders and are used in laboratory mutagenesis screens to disrupt gene function in arthropod model organisms.

Synonyms

• triplet
• coding triplet

Related Terms

• Anticodon
• Genetic code
• nonsense mutation
• synonymous codon
• start codon
• Stop codon
• open reading frame
• messenger RNA
• transfer RNA
• Ribosome

Usage Notes

Specialists distinguish between sense codons (encoding amino acids) and stop codons. Codon usage bias—preferential use of certain synonymous codons—varies among arthropod taxa and can affect translation efficiency, making it relevant for heterologous protein expression in insect cell culture. The term applies identically across all domains of life; mitochondrial genomes of insects and arachnids often employ variant genetic codes with altered codon assignments (e.g., AGA/AGG as stop codons rather than arginine in Drosophila mitochondria).