In CRISPR-Cas9 genome editing, how does Cas9 locate the correct site in DNA to cut?

• A. Cas9 scans for methylated cytosines and cuts DNA only at CpG islands.
• B. Cas9 uses a guide RNA (gRNA) that base-pairs with a complementary DNA sequence adjacent to a PAM motif, directing Cas9 to cut at that location.
• C. Cas9 cuts randomly throughout the genome, and the desired edit is selected by screening afterward.
• D. Cas9 recognizes specific amino acid sequences in histone tails to choose where to cut DNA.

Answer: (B)

Verified step by step guidance

1. Understand that Cas9 is an enzyme used in CRISPR genome editing to make precise cuts in DNA.
2. Recognize that Cas9 does not randomly cut DNA or rely on methylation patterns or histone modifications to find its target.
3. Learn that Cas9 is guided to the correct DNA sequence by a molecule called guide RNA (gRNA), which is designed to be complementary to the target DNA sequence.
4. Know that Cas9 requires the presence of a short DNA sequence called the PAM (Protospacer Adjacent Motif) next to the target site to successfully bind and cut the DNA.
5. Combine these concepts: Cas9 uses the gRNA to base-pair with the target DNA sequence adjacent to a PAM motif, ensuring precise cutting at the intended location.