# A research team used CRISPR-Cas9 to knock out a gene encoding a surface receptor in human cell lines. After editing, some cells were found to contain the intended deletion, while others carried small insertions at the target site. Explain why the presence of insertions at the cut site would be expected, and suggest why these insertions may disrupt gene function even though the original gene sequence has not been deleted.

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

> CRISPR-Cas9 is a genome-editing tool that uses a guide RNA to direct the Cas9 endonuclease to a specific DNA sequence, where it introduces a double-strand break. The cell then repairs this break using one of two main pathways: non-homologous end joining (NHEJ) or homology-directed repair (HDR).

## Mark scheme (4 marks)

1. Insertions arise because NHEJ is error-prone / imprecise
2. Insertions cause a frameshift mutation in the coding sequence
3. The altered reading frame produces a different (non-functional) amino acid sequence / truncated polypeptide
4. The non-functional receptor means the protein cannot perform its normal role / cannot bind its ligand / correct tertiary structure is lost

## Key terms

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

## 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-crispr-cas9-to-203011e8) · Published by Druglandscape Ltd.