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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

1. A guide RNA (gRNA) is designed to be complementary to the DNA sequence flanking the sickle‑cell mutation, directing the Cas9 nuclease to the exact site in the β‑globin gene.
2. Cas9, guided by the gRNA, cleaves both strands of the DNA, creating a double‑strand break (DSB) at the mutation site.
3. A donor DNA template containing the wild‑type nucleotide (glutamic acid codon) is supplied; homology‑directed repair (HDR) uses this template to replace the mutant sequence during DSB repair.
4. The corrected haematopoietic stem cells are then transplanted back into the patient, where they produce normal β‑globin, alleviating sickle‑cell symptoms.

Examiner tips

  • Use the exact terms: guide RNA, Cas9, double‑strand break, donor DNA, homology‑directed repair, corrected somatic cells.
  • Show the step‑by‑step flow: design gRNA → Cas9 cleavage → HDR with donor → cell reintroduction.
  • Keep each point concise and numbered to match the 4 marks.

Common mistakes

  • Confusing non‑homologous end joining (NHEJ) with HDR; the question asks for correction, so HDR must be mentioned.
  • Failing to specify that the donor DNA carries the correct wild‑type nucleotide.
  • Not linking the corrected cells back to the patient’s haematopoietic system.

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 in this question

CRISPR-Cas9 · point mutation · somatic cells

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