What is CRISPR-Cas9 and how is it used in genetics?

• A. A method for cloning entire genomes without using enzymes.
• B. A natural repair system that duplicates DNA during replication.
• C. A genome editing system that uses a guide RNA to direct Cas9 to a specific DNA sequence to make precise cuts for editing.
• D. A sequencing technology that reads DNA without cutting.

Answer: (C)

CRISPR-Cas9 is a genome editing system that uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence to make precise cuts for editing. The guide RNA is designed to match the target DNA, bringing Cas9 to that spot where it acts as molecular scissors to create a double-strand break. The cell then repairs the break using its normal DNA repair processes, and scientists can steer the outcome by providing a template for precise changes (homology-directed repair) or rely on the cell’s repair to introduce small mutations that disrupt the gene (non-homologous end joining). Because the guide RNA can be customized to target almost any gene beside a short PAM sequence, this tool enables targeted gene knockout, insertion, or modification across many organisms. Originating from a bacterial immune system, CRISPR-Cas9 is now a cornerstone of genetics research for studying gene function and exploring potential therapies. It’s not a method for cloning entire genomes, not simply a natural replication process, and not a sequencing technology.