Stop Codon - National Human Genome Research Institute

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Skip to main content ​Stop Codon updated: March 3, 2026

Definition

A stop codon is a sequence of three nucleotides (a trinucleotide) in DNA or messenger RNA (mRNA) that signals a halt to protein synthesis in the cell. There are 64 different trinucleotide codons: 61 specify amino acids and 3 are stop codons (i.e., UAA, UAG and UGA).

Narration

Stop codon. A stop codon is to protein synthesis what a period is to a sentence. Since a protein isn't infinitely long, biology created an incredibly clever way to say, “Stop! You've reached the end of this protein.”

Prabarna Ganguly, Ph.D.

Chief Communications Officer

Nucleus Genomics

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