Briefly describe the mechanism of CRISPR-Cas9 and its genome-editing capability.

• A. CRISPR-Cas9 uses a guide RNA to direct Cas9 nuclease to a specific DNA sequence, creating a double-strand break that can be repaired to introduce edits.
• B. RNA polymerase transcribes DNA into RNA
• C. Enzymes ligate DNA fragments without targeting
• D. PCR amplifies DNA sequences

Answer: (A)

CRISPR-Cas9 edits genomes by using a guide RNA to bring the Cas9 nuclease to a specific DNA sequence, where Cas9 creates a double-strand break. The cell then repairs that break, and researchers can exploit the repair process to introduce edits. The guide RNA is designed to be complementary to the target DNA and usually requires a PAM sequence next to the target for Cas9 binding. After the cut, the two main repair pathways are non-homologous end joining, which often produces small insertions or deletions, and homology-directed repair, which can use a supplied DNA template to install a precise change. This mechanism is what enables targeted genome editing. Other options describe processes like transcription by RNA polymerase, ligation of DNA fragments, or PCR amplification, which do not explain how CRISPR-Cas9 targets and edits a genome.