# CRISPR-Cas9 Gene Editing

> Cas9 is a protein that cuts DNA, and a guide RNA tells it where. When the guide pairs with a matching sequence, Cas9 cuts both strands, and the cell's own repair machinery then fixes the break, which can switch off a gene or let a new sequence be added.

Web page: https://cellnaut.com/topics/crispr-cas9-gene-editing

![Teaching illustration: CRISPR-Cas9 Gene Editing](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/crispr-cas9-gene-editing-teaching.jpg)

## Labelled parts

1. **Cas9 protein**: A nuclease that cuts both strands of DNA.
2. **Target DNA**: The sequence that is to be edited.
3. **Guide RNA**: A short RNA that directs Cas9 to a matching DNA sequence.
4. **RNA-DNA pairing region**: Base pairing between guide and target provides the specificity.
5. **Cleavage site**: Cas9 cuts here, near a short PAM sequence.
6. **Broken DNA ends**: The cell repairs the break, which can disable a gene or add new DNA.
7. **Metal ion cofactor**: Magnesium ions help the nuclease domains cut.
8. **Double-stranded DNA**: The full helix, cut across both strands.

## Key points

- The guide RNA gives specificity: about 20 bases pair with the target DNA.
- Cas9 cuts next to a short sequence called a PAM, using magnesium ions in its nuclease domains.
- The outcome depends on how the cell repairs the break.

## Check yourself

**1. What guides Cas9 to the right place in the genome?**

- A. A ribosome
- B. The guide RNA
- C. A mitochondrion
- D. A lysosome

**Answer:** B. The guide RNA - The guide RNA base-pairs with the target sequence.

**2. What happens after Cas9 cuts the DNA?**

- A. The cell's repair machinery fixes the break
- B. The cell always dies
- C. The DNA turns into RNA
- D. Nothing, the cut seals itself instantly with no effect

**Answer:** A. The cell's repair machinery fixes the break - How the break is repaired decides whether a gene is disabled or edited.

## Related lessons

- [From DNA to Protein](https://cellnaut.com/learn/dna-to-protein)

## See it in 3D

- [Stem Cell](https://cellnaut.com/studio/stem)