Digital ACT Reading Practice Question #1741

According to the passage, the role of Cas9 during bacterial defense is to: Passage Excerpt (Natural Science): Before CRISPR-Cas9, targeted gene editing required engineering complex zinc-finger nucleases or TALEN proteins for every DNA sequence—an arduous, costly process. CRISPR was originally discovered as an adaptive immune mechanism in bacteria fighting bacteriophages. When a bacterium survives infection, it captures snippets of viral DNA into its genome as "spacer" sequences. Upon reinfection, it transcribes spacers into guide RNAs. The Cas9 endonuclease complexes with this guide RNA, scouring the cell until it finds a complementary viral sequence adjacent to a Protospacer Adjacent Motif (PAM). Once bound, Cas9 acts as molecular scissors, cleaving both strands of viral DNA. By simply swapping the guide RNA's 20-nucleotide sequence, geneticists can direct Cas9 to cleave virtually any gene in any organism.