What is Cas9 protein that is often mentioned in news?

The correct answer is A. Cas9 is an enzyme (a nuclease) from the bacterial CRISPR-Cas immune system, first studied in Streptococcus pyogenes. Guided by a short RNA that matches a target DNA sequence, it cuts both strands of DNA at a precise site. The cell then repairs the cut, which lets scientists disable, replace or insert genes. That is why it is called molecular scissors.

• B is wrong: Cas9 cuts DNA; it does not detect or report pathogens. CRISPR-based diagnostics exist, but they use other Cas enzymes and are a later development.
• C is wrong: Cas9 is a protein, not a gene. Pest resistance comes from genes such as Bt genes, which are a different thing.
• D is wrong: Herbicides are chemicals such as glyphosate. Cas9 is not a herbicide and is not made in crops for weed control.

Core concept: genome editing, and the difference between a gene, a protein and a tool. For Mains, ICAR released genome-edited rice varieties in 2022, so GS-III questions on biotechnology, biosafety and regulation of GM crops connect directly to this.

Trap: Candidates who see 'gene' in the news and assume Cas9 is a gene, or who confuse it with a diagnostic tool because CRISPR is now used in testing.