CRISPR-Cas9 gene editing requires two components: the Cas9 endonuclease and a guide RNA (gRNA). After Cas9 creates a double-strand break (DSB), the cell repairs it via two competing pathways. Which pathway is preferred for precise gene correction, and what is required for this pathway to operate?
- A Non-homologous end joining (NHEJ); a repair template plasmid is not required
- B Base excision repair (BER); requires XRCC1 protein and DNA ligase III
- C Homology-directed repair (HDR); requires a donor template with homology arms flanking the desired edit ✓
- D Single-strand annealing (SSA); requires repeat sequences flanking the cut site
Explanation
After Cas9 creates a DSB, two pathways compete: NHEJ (error-prone, dominant in post-mitotic cells, generating indels) and HDR (precise, requires a donor repair template, active mainly in S/G2 phase of the cell cycle). For precise gene correction (e.g., correcting a point mutation in sickle cell disease), HDR is required, using a co-delivered single-stranded oligodeoxynucleotide (ssODN) or plasmid donor with homology arms matching sequences flanking the cut site. NHEJ is useful for gene knockout but not precise correction. HDR efficiency is lower in non-dividing cells, a major limitation for in vivo gene therapy.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.