The CRISPR-Cas system is a natural immune system found in prokaryotes. When certain bacteria are invaded by viruses, they can store a small piece of a viral gene in their DNA in a specific pattern to create segments called CRISPR arrays. Upon encountering the virus again, the bacteria can identify and target the virus using the stored segment and cut the viral DNA to render it ineffective.

For experimental convenience and improved stability, Jennifer Doudna and Emmanuelle Charpentier (both awarded with the Nobel Prize in Chemistry in 2020) combined crRNA and tracrRNA into a single RNA molecule called single guide RNA (sgRNA). This modified CRISPR/Cas9 system has emerged as the leading tool for gene editing research. The CRISPR-Cas9 technology comprises two key components: the Cas9 protein, which is responsible for double-stranded DNA (dsDNA) cleavage, and the sgRNA, which serves as a guide. The principle of the CRISPR-Cas9 technology involves two fundamental processes: Cas9-targeted DNA cleavage, guided by sgRNA, and DNA repair.

Within cells Cas9 binds to the sgRNA and targets the specific DNA sequence, leading to the cleavage of the dsDNA. Subsequently, the cell's DNA repair mechanisms is activated. Two primary repair mechanisms for dsDNA breaks come into play: homology-directed repair (HDR) and non-homologous end joining (NHEJ). In HDR, the broken DNA sequence is repaired using an intact copy from the sister chromosome or an exogenous piece of homologous DNA as a template. On the other hand, NHEJ repairs the broken DNA in a random manner, potentially resulting in insertions, deletions, and frame-shift mutations.

Through careful design of appropriate sgRNA sequences, researchers can guide the CRISPR-Cas9 system to target specific genes, enabling precise gene editing and regulation. This technology offers immense potential in distinct fields, including studying gene function, treating genetic diseases, and enhancing crop quality.