CRISPR-Cas9 genome editing uses a guide RNA (sgRNA) to direct Cas9 nuclease to a specific genomic locus. A 3-nucleotide sequence immediately 3' to the target DNA on the non-template strand, required for Cas9 cleavage, is called the:
- A Protospacer adjacent motif (PAM) ✓
- B Seed sequence
- C Repeat variable diresidue (RVD)
- D Zinc finger binding domain
Explanation
The protospacer adjacent motif (PAM) is a short DNA sequence (5'-NGG-3' for Streptococcus pyogenes Cas9) located immediately 3' to the target site (protospacer) on the non-template strand. Cas9 requires the PAM for initial DNA binding and for distinguishing the target DNA from the sgRNA itself. Without a PAM, Cas9 will not cleave. This constraint limits CRISPR targeting sites and has spurred development of Cas9 variants (SpRY, SpCas9-NG) with relaxed PAM requirements. The seed sequence refers to the sgRNA bases closest to the PAM which are most critical for specificity. RVDs are TALENs components; zinc finger nucleases use zinc finger domains.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.