---
title: "Uracil — AP Biology Definition & Exam Guide"
description: "Uracil is the RNA-only nitrogenous base that replaces thymine and pairs with adenine. Learn how it shows up in base pairing, transcription, and AP Bio Unit 6."
canonical: "https://fiveable.me/ap-bio/key-terms/uracil"
type: "key-term"
subject: "AP Biology"
---

# Uracil — AP Biology Definition & Exam Guide

## Definition

Uracil is a nitrogenous base found only in RNA. It replaces thymine from DNA and pairs with adenine.

## Related Study Guides

- [6.1 DNA and RNA Structure ](/ap-bio/unit-6/dna-rna-structure/study-guide/29u5wVp2a9rSDLfLGBlD)

## Review

### Related Terms

- [Ribonucleic Acid (RNA)](/ap-bio/key-terms/ribonucleic-acid-rna): A molecule similar to DNA but contains ribose as its sugar and uses uracil instead of thymine.
- [Transcription](/ap-bio/key-terms/transcription): The process where genetic information in DNA is rewritten into RNA using base pairing rules, including adenine pairing with uracil.
- [Messenger RNA (mRNA)](/ap-bio/key-terms/messenger-rna-mrna): A type of RNA that carries instructions from DNA for controlling the synthesis of proteins.