The correct option is (a).
Explanation
The Cas9 (CRISPR-associated protein 9) is an enzyme that forms the core component of the CRISPR-Cas9 system, a technology adapted from the natural defense mechanisms of bacteria for genome editing.
Analysis of Options:
- Option (a) is Correct: Cas9 functions as molecular scissors. It is an endonuclease enzyme capable of cutting DNA strands at specific locations. When paired with a guide RNA (gRNA) that matches a target DNA sequence, Cas9 locates the specific site in the genome and creates a double-strand break. This break allows the cell's repair machinery to be hijacked to add, remove, or alter genetic material.
- Option (b) is Incorrect: While CRISPR-based technologies are being developed for diagnostic purposes (e.g., the FELUDA test), Cas9 is primarily defined by its catalytic function as a DNA-cutting enzyme used for editing, rather than as a passive biosensor.
- Option (c) is Incorrect: Cas9 is a protein, not a gene. Furthermore, while it is a tool used to create genetically modified plants (which may be pest-resistant), it is not the gene responsible for the resistance itself (such as the Cry genes used in Bt cotton).
- Option (d) is Incorrect: Cas9 is not a herbicidal substance. Herbicidal substances are chemical agents used to control weeds, whereas Cas9 is a biological tool used to modify the genetic code of organisms.
Key Takeaway: Cas9 is a DNA-cutting enzyme (endonuclease) central to the CRISPR gene-editing technology, widely referred to as "molecular scissors" for its ability to precisely target and cleave DNA sequences.