---
title: "Guanine — AP Biology Definition & Exam Guide"
description: "Guanine is a purine base in DNA and RNA that pairs with cytosine. Learn its double-ring structure, base-pairing rules, and how it shows up on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/guanine"
type: "key-term"
subject: "AP Biology"
---

# Guanine — AP Biology Definition & Exam Guide

## Definition

Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA. It pairs with cytosine.

## Related Study Guides

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

## Review

### Related Terms

- [Adenine](/ap-bio/key-terms/adenine): This is another nucleobase found in DNA and RNA that pairs with thymine in DNA and uracil in RNA.
- [Nucleotide](/ap-bio/key-terms/nucleotide): This is the basic building block of DNA and RNA consisting of a sugar, phosphate group, and a nitrogenous base (like guanine).
- Base Pairing: This refers to the phenomenon where specific nitrogenous bases (like guanine and cytosine) bind together to form the structure of DNA or RNA.