Lesson eLearning

To correct a G→A point mutation that creates a premature stop codon in exon 3 of the human DMD gene, which of the following CRISPR‑Cas9 strategies will most likely achieve precise correction in patient‑derived myoblasts?

hard MCQ 5 marks Class 12 · Biology · Molecular Basis of Inheritance
  1. Use a single guide RNA (sgRNA) targeting the mutant site, Cas9 nickase, and a single‑stranded DNA repair template containing the correct G; rely on non‑homologous end joining (NHEJ).
  2. Employ a catalytically dead Cas9 (dCas9) fused to a cytidine deaminase and deliver a guide RNA that directs deamination of the A back to G (base editing).
  3. Design an sgRNA that binds adjacent to the mutation, use wild‑type Cas9 to create a double‑strand break, and provide a double‑stranded DNA donor with the correct sequence; repair occurs via homology‑directed repair (HDR).
  4. Introduce Cas9 with a guide RNA that cuts elsewhere in the genome and a plasmid carrying the corrected gene; the cell incorporates the plasmid by random integration.
Answer
Design an sgRNA that binds adjacent to the mutation, use wild‑type Cas9 to create a double‑strand break, and provide a double‑stranded DNA donor with the correct sequence; repair occurs via homology‑directed repair (HDR).

Step-by-step solution

  1. Option 1 uses Cas9 nickase with NHEJ – NHEJ is error‑prone and cannot guarantee precise base replacement, so it is unsuitable.
  2. Option 2 describes base editing with a cytidine deaminase, which converts C→T, not A→G, thus cannot reverse the G→A mutation.
  3. Option 3 employs wild‑type Cas9 to cut near the mutation and supplies a homologous DNA donor; HDR uses the donor template to replace the mutant base with the correct G, giving precise correction.
  4. Option 4 relies on random integration of a plasmid, which is inefficient and does not target the specific point mutation.
  5. Therefore, Option 3 is the correct HDR‑based strategy.
Common mistake: Confusing NHEJ‑based repair with precise HDR correction

Concept tested: CRISPR‑Cas9 HDR gene correction

Published · How this solution was prepared and checked

← All Molecular Basis of Inheritance questions