How can CRISPR-Cas9 be used for gene editing?

• A. Cas9 edits RNA transcripts without altering DNA sequence.
• B. A guide RNA directs Cas9 to a specific DNA sequence to create a double-strand break; cellular repair (NHEJ or HDR) introduces targeted edits.
• C. A protein complex randomly integrates into the genome causing edits.
• D. The system recruits chromatin modifiers to suppress expression without altering DNA.

Answer: (B)

The main idea here is that CRISPR-Cas9 edits genes by targeting a specific DNA sequence and making a precise cut, with the cell’s repair mechanisms establishing the actual change. A guide RNA provides the exact DNA sequence the Cas9 enzyme should recognize, bringing Cas9 to that site. Cas9 then creates a double-strand break in the DNA. The cell repairs this break mainly through two pathways: non-homologous end joining, which often introduces small insertions or deletions that can disrupt the gene, and homology-directed repair, which uses a provided DNA template to insert a precise sequence change. This combination of targeted cutting and repair is what enables specific genome edits.

Other options don’t fit because Cas9’s primary action in this context is on DNA, not RNA transcripts. It doesn’t randomly integrate into the genome. And while CRISPR systems can be used to modulate gene expression without changing DNA, that falls under regulatory or epigenetic editing rather than the standard DNA-editing approach described here.