# A team of researchers used CRISPR-Cas9 to edit the genome of a crop plant, disabling a gene that normally suppresses disease resistance. Explain how the CRISPR-Cas9 system locates and cuts the target gene at a specific position in the plant's genome.

> OCR A-Level Biology A (H420) — 6.3 Manipulating genomes · Explain · 4 marks

> CRISPR-Cas9 is a genome-editing tool adapted from a bacterial immune defence mechanism. It has been used in agriculture to modify crop plants by making precise cuts at specific locations in the genome. The system relies on two key components: a guide RNA and the Cas9 endonuclease protein.

## Mark scheme (4 marks)

1. The guide RNA (gRNA) is complementary in base sequence to the target DNA sequence
2. The guide RNA binds / hybridises to the target DNA strand by hydrogen bonding (complementary base pairing)
3. The guide RNA is bound to / forms a complex with the Cas9 endonuclease, directing it to the correct location in the genome
4. Cas9 acts as an endonuclease, cutting both strands of the DNA double helix at the target site, producing a double-strand break

## Key terms

- [CRISPR-Cas9](https://www.gradenine.co.uk/glossary/crispr-cas9)
- [genome](https://www.gradenine.co.uk/glossary/genome)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-team-of-researchers-used-crispr-cas9-5c8cf498) · Published by Druglandscape Ltd.