What is CRISPR-Cas9 and how does it work?

CRISPR-Cas9 is a gene editing tool derived from bacterial adaptive immunity. It consists of Cas9, an endonuclease that cuts DNA, and a guide RNA (gRNA) that directs Cas9 to a specific genomic location through base pairing. The target site must be adjacent to a PAM sequence (NGG for SpCas9). When Cas9 creates a double-strand break, cells repair it via error-prone NHEJ (causing insertions/deletions for gene knockout) or precise HDR (for gene editing when donor template is provided). CRISPR revolutionized genetic engineering due to its simplicity, efficiency, and versatility.