Q. What is Cas9 protein that is often mentioned in news? (UPSC Prelims 2019)
Answer: A molecular scissors used in targeted gene editing
Notes: The correct answer is [A] A molecular scissors used in targeted gene editing. Cas9 (CRISPR-associated protein 9) is an enzyme that plays a critical role in the revolutionary gene-editing technology known as CRISPR-Cas9.

• Molecular Scissors (Statement A – Correct): Cas9 acts as a pair of "molecular scissors" that can cut the two strands of DNA at a specific location in the genome. This allows for the removal, addition, or replacement of specific DNA sequences with high precision.
• CRISPR System: The technology is derived from a naturally occurring genome-editing system in bacteria. Bacteria use CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) and the Cas9 protein to capture snippets of DNA from invading viruses and use them to create DNA segments that allow the Cas9 enzyme to recognize and "cut" the virus's DNA during future attacks.
• Mechanism: To use it in a lab, scientists create a small piece of RNA with a short "guide" sequence (gRNA) that attaches to a specific target sequence of DNA in a genome. The gRNA directs the Cas9 enzyme to the right spot to make the cut.
• Distinctions (Statements B, C, and D – Incorrect): While Cas9 helps in creating pest-resistant plants (like Bt crops) by modifying their genes, the protein itself is the tool used for editing, not the gene or the herbicidal substance. It is also distinct from biosensors, which are used for detection rather than genetic modification.

The developers of this technology, Jennifer Doudna and Emmanuelle Charpentier, were awarded the Nobel Prize in Chemistry in 2020 for their work on this "genetic tool."