# Meiosis and Homologous Chromosome Crossing Over

> In prophase I of meiosis, homologous chromosomes pair up and swap segments. This crossing over mixes the alleles inherited from the two parents, so every egg or sperm carries a unique combination.

Web page: https://cellnaut.com/topics/meiosis-crossing-over

![Teaching illustration: Meiosis and Homologous Chromosome Crossing Over](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/meiosis-crossing-over-teaching.jpg)

## Labelled parts

1. **Nucleus**: Holds the chromosomes; its envelope breaks down by the end of prophase I.
2. **Nuclear envelope**: Breaks down as the cell prepares to build the spindle.
3. **Homologous chromosomes**: One from each parent, matching in size and genes, and they pair up.
4. **Sister chromatids**: Identical copies made by replication, still joined at the centromere.
5. **Tetrad**: A pair of homologous chromosomes, four chromatids in all, held together.
6. **Chiasma crossing point**: Where non-sister chromatids have crossed and swapped segments.
7. **Exchanged segment**: The DNA swapped between homologs creates new combinations of alleles.
8. **Centrosome**: Organizes the spindle microtubules.
9. **Spindle fibers**: Microtubules that will pull the homologs apart.

## Key points

- Homologous chromosomes are the matching pair, one from each parent.
- A tetrad contains four chromatids, and crossing over happens between non-sister chromatids.
- Crossing over, plus random assortment of chromosomes, creates genetic variation.

## Check yourself

**1. When does crossing over occur?**

- A. Prophase I of meiosis
- B. Anaphase II
- C. S phase
- D. Cytokinesis

**Answer:** A. Prophase I of meiosis - Homologs pair up in prophase I and exchange segments.

**2. Crossing over exchanges segments between which chromatids?**

- A. Two sister chromatids
- B. Non-sister chromatids of homologous chromosomes
- C. Chromatids of different species
- D. Chromatids in different cells

**Answer:** B. Non-sister chromatids of homologous chromosomes - Sister chromatids are identical, so swapping them would change nothing.

## Related lessons

- [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division)

## See it in 3D

- [Egg Cell](https://cellnaut.com/studio/egg)
- [Sperm Cell](https://cellnaut.com/studio/sperm)