Stop codon

Definition

A trinucleotide sequence in messenger RNA that signals the termination of protein translation rather than encoding an amino acid. When a ribosome encounters a stop codon at the A site, release factors bind and catalyze hydrolysis of the peptidyl-tRNA bond, releasing the completed polypeptide and dissociating the ribosomal subunits. In the standard genetic code, UAA, UAG, and UGA serve as stop codons; UGA can alternatively encode selenocysteine in specific contexts. Stop codon read-through—where translation continues past the signal—occurs in some viral and cellular proteins and can be experimentally induced in insects for transgene expression studies.

Etymology

From English 'stop' (halt, cease) + 'codon' (trinucleotide unit of genetic code), reflecting its function in terminating translation.

Example

In the fruit fly Drosophila melanogaster, the white gene contains a stop codon that, when read through via suppression of a nonsense mutation, restores eye pigmentation—a classic genetic tool used to study translation termination efficiency and codon context effects in insects.

Synonyms

• termination codon
• nonsense codon

Related Terms

• Codon
• start codon
• release factor
• nonsense-mediated decay
• read-through
• Genetic code
• Ribosome
• translation
• messenger RNA

Usage Notes

The term 'nonsense codon' is largely historical and declining in use; 'stop codon' is preferred in current literature. Context matters: UGA functions as a stop codon in most genes but encodes selenocysteine when followed by a SECIS element. In insects and other arthropods, stop codon suppression has been exploited to create transgenic strains with inducible protein expression systems. The efficiency of stop codon recognition varies among taxa and can affect heterologous gene expression in insect cell culture systems.