# CRISPR-Cas9

A gene-editing tool taken from bacterial defence against viruses. A guide RNA leads the Cas9 enzyme to a matching DNA sequence, where it cuts both strands. The cell's repair then disables or changes the gene.

Biology (university) · Part 05: Gene regulation and biotechnology · Card 8 of 8 · Last updated 2026-10-19

## Analogy

Find and replace for DNA.

## In practice

In 2023 a CRISPR-based therapy for sickle cell disease was approved in some countries.

## Practice

### Flashcards

- **How does CRISPR-Cas9 find its target?** A guide RNA matches the target DNA sequence and leads Cas9 to it.

## Questions people ask

### How does CRISPR work as a bacterial defence?

Bacteria store short pieces of DNA from viruses that attacked them in the past, in regions called CRISPR. If the virus returns, the bacteria make guide RNAs from the stored pieces, and Cas9 uses them to find and cut the viral DNA. It is a kind of memory-based immune system.

### What are the ethical concerns about gene editing?

Editing body cells to treat disease is widely accepted, with careful oversight. Editing embryos, sperm or eggs is more controversial because changes would pass to future generations and the person cannot consent. Other concerns include off-target cuts, fair access, and use for enhancement rather than treatment.

## Related concepts

- [Restriction enzyme](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-4)
- [Recombinant DNA](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-5)
- [Polymerase chain reaction](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-6)
- [Gel electrophoresis](https://studyvisually.com/learn/science/biology-university/biology-university-part-5/biology-university-part-5-concept-7)

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