# Explain how CRISPR-Cas9 can be used to correct a point mutation responsible for a genetic disorder in human somatic cells.

> IB DP Biology Higher Level (2023 syllabus) — D1.3 Mutations and gene editing (HL only) · Explain · 4 marks

> Sickle cell disease is caused by a single nucleotide substitution in the gene encoding the β-globin subunit of haemoglobin, resulting in the substitution of valine for glutamic acid at position 6 of the polypeptide. Researchers are investigating CRISPR-Cas9 as a therapeutic tool to correct this mutation directly in haematopoietic stem cells.

## Mark scheme (4 marks)

1. A guide RNA (gRNA) complementary to the target DNA sequence flanking the mutation is designed and directs Cas9 to the precise location in the genome.
2. Cas9 acts as an endonuclease, making a double-strand break (DSB) in the DNA at the target site.
3. A repair template (donor DNA) carrying the correct nucleotide sequence is introduced alongside the CRISPR components, and homology-directed repair (HDR) incorporates the corrected sequence at the break site.
4. The corrected somatic / haematopoietic stem cells can be reintroduced into the patient, restoring production of functional β-globin and reducing symptoms of the disorder.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-crispr-cas9-can-be-used-b73687dc) · Published by Druglandscape Ltd.