What is the role of a PAM sequence in CRISPR-Cas9 gene editing?

• A. The PAM is a long RNA motif recognized by Cas9.
• B. The PAM sequence is a protein that guides Cas9 to the target.
• C. The PAM is a chemical modification of DNA.
• D. The PAM is a short DNA motif required for Cas9 recognition and cleavage immediately adjacent to the target sequence.

Answer: (D)

The key idea is that CRISPR-Cas9 needs a short DNA motif next to the target to recognize and cut. The PAM is a brief DNA sequence that must be immediately adjacent to the DNA region targeted by the guide RNA; Cas9 first checks for this motif, and only then does the RNA-guided pairing confirm the exact site and trigger a double-strand cut near the PAM. This specific requirement explains why the correct description is that the PAM is a short DNA motif needed for Cas9 recognition and cleavage right next to the target. It’s not an RNA motif, not a protein, and not a chemical DNA modification—the others don’t fit because they misidentify what the PAM is. In practice, the usual example is the NGG PAM for SpCas9, though other Cas9 variants recognize different PAMs.