Practice question

Specificity of CRISPR Cas9 system is dictated predominantly by guide RNA rather than protein itself, contrasting with zinc finger and TALEN protein-guided recognition. Cas9 from Streptococcus pyogenes contains recognition lobe and nuclease lobe with HNH and RuvC domains, plus PAM interacting domain at C-terminus that scans DNA for 5'-NGG-3' PAM via major groove interactions, required for binding. Spontaneous DNA unwinding initiates RNA-DNA heteroduplex formation with 20 nt spacer of crRNA embedded in single guide RNA fusion with tracrRNA scaffold. Complementarity drives conformational activation: full pairing especially in seed region nucleotides 1-8 proximal to PAM triggers HNH domain rotation cutting target strand complementary, RuvC cutting noncomplementary strand. Mismatches in PAM distal region tolerated leading to off-targets predictable by computational tools. Therefore reprogramming requires only swapping 20 nt spacer chemically synthesized within hours, whereas Cas9 protein unchanged. This RNA-guided programmable nature underlies ease and multiplex ability simply expressing multiple guides, enabling library screening, lineage tracing and epigenome editing fusing dCas9 to effectors where guide directs localization.