# A research team used the CRISPR-Cas9 system to knock out a gene encoding a cell surface receptor in human embryonic kidney cells. After treatment, they confirmed that the target gene was no longer expressed. Explain how the CRISPR-Cas9 system is able to locate and disrupt a specific gene within the genome of these cells.

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

> CRISPR-Cas9 is a genome-editing tool derived from a bacterial immune defence mechanism. It consists of a guide RNA (gRNA) component and the Cas9 endonuclease enzyme. Scientists design the gRNA to match a target DNA sequence of interest before introducing the complex into cells.

## Mark scheme (4 marks)

1. The guide RNA (gRNA) is complementary in base sequence to the target DNA sequence within the specific gene
2. The gRNA binds to the target DNA sequence by complementary base pairing, guiding the Cas9 enzyme to the correct locus
3. Cas9 acts as an endonuclease that cuts both strands of the DNA double helix at the target site, producing a double-strand break
4. The double-strand break is repaired by non-homologous end joining (NHEJ), an error-prone process that introduces insertions or deletions (indels), disrupting the reading frame and preventing expression of the gene

## 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-research-team-used-the-crispr-cas9-6d1aa549) · Published by Druglandscape Ltd.