What are the two main DNA repair pathways after a CRISPR-Cas9 induced break, and how do they differ?

What are the two main DNA repair pathways after a CRISPR-Cas9 induced break, and how do they differ?

• A. Base excision repair fixes the damage without changing the sequence; mismatch repair corrects replication errors.
• B. Mismatch repair fixes replication errors by correcting base mismatches; base excision repair removes damaged bases.
• C. Nonhomologous end joining repairs with high fidelity using a template; homologous recombination uses a template for exact edits.
• D. Nonhomologous end joining often introduces indels; homologous directed repair uses a repair template to introduce precise edits.

Answer: (D)

When a CRISPR-Cas9 cut creates a double-strand break, the cell mainly uses two repair routes: nonhomologous end joining (NHEJ) and homology-directed repair (HDR). NHEJ is the quick, default fix that joins the broken ends without a template, which often introduces small insertions or deletions (indels) and can disrupt the gene. HDR, on the other hand, uses a supplied DNA template with matching sequences around the break to guide a precise edit, allowing targeted changes or insertions. The statement captures this difference: NHEJ often yields indels, while HDR uses a repair template to achieve precise edits. This contrast is why the option is correct. Other repair pathways mentioned are more about single-base damage or replication errors and aren’t the main routes after a CRISPR-induced double-strand break.

When a CRISPR-Cas9 cut creates a double-strand break, the cell mainly uses two repair routes: nonhomologous end joining (NHEJ) and homology-directed repair (HDR). NHEJ is the quick, default fix that joins the broken ends without a template, which often introduces small insertions or deletions (indels) and can disrupt the gene. HDR, on the other hand, uses a supplied DNA template with matching sequences around the break to guide a precise edit, allowing targeted changes or insertions. The statement captures this difference: NHEJ often yields indels, while HDR uses a repair template to achieve precise edits. This contrast is why the option is correct. Other repair pathways mentioned are more about single-base damage or replication errors and aren’t the main routes after a CRISPR-induced double-strand break.