CRISPR-Cas9 gene editing relies on a two-component system — a Cas9 nuclease that creates a double-stranded DNA break and a guide RNA (gRNA) that directs Cas9 to a sequence-specific genomic target — with the cell's own DNA repair machinery then determining the editing outcome. Two repair pathways govern the result: non-homologous end joining (NHEJ), an error-prone pathway that joins broken ends without a template and produces variable insertions/deletions (indels) that can disrupt the reading frame for a gene knockout, and homology-directed repair (HDR), a template-based pathway that uses a donor DNA molecule with homology arms flanking a desired sequence to achieve precise, designed edits such as knock-ins, point mutations, or reporter-gene tagging. This places CRISPR-Cas9 within molecular biology's genome-editing subfield, where the choice between NHEJ- and HDR-based strategies determines whether an edit is a non-templated (indel-based) knockout or a templated, selectable knockout via homologous recombination.
CRISPR-Cas9 gene editing relies on a two-component system — a Cas9 nuclease that creates a double-stranded DNA break and a guide RNA (gRNA) that directs Cas9 to a sequence-specific genomic target — w…