---
title: "Stop Codon — AP Biology Definition & Exam Guide"
description: "A stop codon is a 3-nucleotide mRNA sequence (UAA, UAG, UGA) that ends translation. Learn how it controls protein length and shows up in AP Bio mutation questions."
canonical: "https://fiveable.me/ap-bio/key-terms/stop-codon"
type: "key-term"
subject: "AP Biology"
---

# Stop Codon — AP Biology Definition & Exam Guide

## Definition

A stop codon is a specific sequence of three nucleotides in the genetic code that signals the end of protein synthesis.

## Related Study Guides

- [6.4 Translation](/ap-bio/unit-6/translation/study-guide/U6N7DadIQajK0Z25lHSh)

## Review

### Related Terms

- [Codon](/ap-bio/key-terms/codon): A sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.
- [Translation](/ap-bio/key-terms/translation): The process by which mRNA is decoded and a protein is produced in the ribosome.
- Genetic Code: The set of rules by which information encoded within genetic material (DNA or mRNA sequences) is translated into proteins by living cells.