---
title: "Start Codon — AP Biology Definition & Exam Guide"
description: "The start codon (AUG) signals where translation begins on an mRNA and codes for methionine. Learn how it anchors initiation for AP Bio Unit 6."
canonical: "https://fiveable.me/ap-bio/key-terms/start-codon"
type: "key-term"
subject: "AP Biology"
---

# Start Codon — AP Biology Definition & Exam Guide

## Definition

The start codon is a specific sequence of three nucleotides in mRNA that signals the start of translation and codes for methionine in eukaryotes or formylmethionine in prokaryotes.

## 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 nucleotides which together form a unit of genetic code in DNA or RNA molecule.
- [Translation](/ap-bio/key-terms/translation): The process where genetic information coded in messenger RNA directs the formation of a specific protein at a ribosome in cytoplasm.
- Methionine: An essential amino acid that often acts as a starter amino acid in protein synthesis because it corresponds with the 'start' codon on mRNA.