In CRISPR-Cas9 gene editing, which component provides the specificity for targeting a particular genomic sequence, and what short DNA motif adjacent to the target is required for Cas9 to cut?
- A Cas9 protein provides specificity; no additional motif required
- B Single guide RNA (sgRNA) provides specificity; PAM (protospacer adjacent motif, typically NGG for SpCas9) ✓
- C tracrRNA provides specificity; TATA box motif is required
- D Donor template DNA provides specificity; homology arms replace the PAM requirement
Explanation
In CRISPR-Cas9, the single guide RNA (sgRNA) contains a 20-nucleotide spacer sequence complementary to the target DNA strand, providing target specificity via Watson-Crick base pairing. Cas9 itself has no inherent DNA sequence specificity; it is recruited by the sgRNA. Critically, a protospacer adjacent motif (PAM) — 5'-NGG-3' for the commonly used Streptococcus pyogenes Cas9 (SpCas9) — must be present immediately 3' of the target sequence on the non-template strand. Cas9 recognizes the PAM first, then interrogates flanking sequences for sgRNA complementarity before cleaving both DNA strands.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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