Video narration
If DNA were a document, how would you change one word without cutting somewhere random? CRISPR-Cas9 finds that address using a guide RNA-a short RNA sequence designed to match the target DNA. When it pairs correctly, it brings Cas9, the cutting enzyme, to that spot. Cas9 also needs a nearby signal called PAM, so it does not cut every similar-looking sequence. Then comes the key point: Cas9 does not write your new gene. It cuts both DNA strands. The cell then repairs that break. Sometimes repair changes or disrupts the sequence; with a repair template, it can copy in a planned change. But this is not automatic or perfectly precise. Poor target design, missing PAMs, or off-target cuts can cause unintended results. So the answer is: CRISPR-Cas9 finds the DNA spot and cuts it; the cell's repair creates the edit.