After CRISPR-Cas9 creates a double-strand break, what is a likely outcome during cellular repair?

After CRISPR-Cas9 creates a double-strand break, what is a likely outcome during cellular repair?

• A. The genome remains unchanged in all cases
• B. Mutations at the target site or insertion of a new sequence
• C. Immediate restoration to original sequence with no changes
• D. The cell dies

Answer: (B)

When CRISPR-Cas9 cuts the DNA, the cell has to repair the break. The two main repair pathways are non-homologous end joining, which simply re-ligates the ends but is error-prone, and homology-directed repair, which uses a DNA template to guide repair. The error-prone path often introduces small insertions or deletions at the cut site, effectively mutating the sequence. If a donor DNA template is present, the cell can incorporate that new sequence during repair. So a likely outcome of cellular repair is mutations at the target site or insertion of a new sequence. The other possibilities are much less general: the genome remaining unchanged is unlikely after a break, and immediate restoration to the exact original sequence isn’t guaranteed unless precise HDR occurs with an appropriate template; cell death is not the typical immediate result of a single repair event.

When CRISPR-Cas9 cuts the DNA, the cell has to repair the break. The two main repair pathways are non-homologous end joining, which simply re-ligates the ends but is error-prone, and homology-directed repair, which uses a DNA template to guide repair. The error-prone path often introduces small insertions or deletions at the cut site, effectively mutating the sequence. If a donor DNA template is present, the cell can incorporate that new sequence during repair. So a likely outcome of cellular repair is mutations at the target site or insertion of a new sequence. The other possibilities are much less general: the genome remaining unchanged is unlikely after a break, and immediate restoration to the exact original sequence isn’t guaranteed unless precise HDR occurs with an appropriate template; cell death is not the typical immediate result of a single repair event.