Practice question

Zinc finger nucleases and TALENs use FokI endonuclease domain which lacks catalytic activity as monomer and must dimerize to cleave DNA. Therefore they require paired binding sites in tail-to-tail orientation separated by spacer. Meganucleases act as single protein but still involve large protein-DNA recognition interface. CRISPR Cas9 avoids protein dimerization; a single Cas9 protein guided by single guide RNA recognizes target via RNA-DNA pairing and PAM, then introduces double strand break. This monomeric architecture simplifies multiplexing and delivery for genome editing.