---
title: "Prophase I — AP Biology Definition & Exam Guide"
description: "Prophase I is the first stage of meiosis I where homologous chromosomes pair up and cross over. Learn why it drives genetic diversity and how AP Bio tests it."
canonical: "https://fiveable.me/ap-bio/key-terms/prophase-i"
type: "key-term"
subject: "AP Biology"
---

# Prophase I — AP Biology Definition & Exam Guide

## Definition

Prophase I is the first phase in meiosis I where chromosomes condense, homologous chromosomes pair up to form tetrads, and crossing over occurs.

## Related Study Guides

- [5.1 Meiosis](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z)

## Review

### Related Terms

- Tetrad: A tetrad is a four-part structure that forms during the prophase of meiosis and consists of two homologous chromosomes, each composed of two sister chromatids.
- Synapsis: Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation.
- Chiasmata: Chiasmata are the points where crossing over occurs between two nonsister chromatids in a tetrad during meiosis.