The CRISPR-Cas system serves as a bacterial defense mechanism against invading genetic elements such as viruses and plasmids, forming the foundation for its adaptation as a powerful genome-editing tool. Originally discovered in prokaryotes, this system has been repurposed to revolutionize genetic engineering across a wide range of organisms, including plants, animals, and humans. The core component, Cas9, is an endonuclease derived from Streptococcus pyogenes, capable of introducing site-specific double-stranded breaks (DSBs) in DNA. In laboratory applications, CRISPR-Cas9 is reprogrammed to modify the genomes of diverse organisms.
The system uses a synthetic single guide RNA (sgRNA) that directs Cas9 to precise genomic loci. The sgRNA contains a complementary sequence that guides Cas9 to a specific DNA target adjacent to a Protospacer Adjacent Motif (PAM) sequence, typically 5'-NGG-3' in the case of Cas9. Once binded, Cas9 induces a double-stranded break.
The DSB facilitates homologous recombination for gene insertion when a donor DNA template is provided, leading to the precise integration of new genetic material. In gene deletion, two sgRNAs guide Cas9 in excising a defined DNA fragment by generating DSBs at both ends of the target region. The excised region is removed, and the cell's repair machinery rejoins the flanking sequences through non-homologous end joining (NHEJ), often resulting in a knockout.
Beyond single-gene edits, CRISPR-Cas9 enables multiplexed genome editing, allowing simultaneous modification of multiple genes, which is advantageous in complex genetic studies and therapeutic applications. Notably, CRISPR-Cas9 has been employed to combat viral infections, such as HIV, by excising integrated viral DNA from host genomes. Other CRISPR-associated systems like Cys4 have also been developed to target and degrade free viral RNAs, such as HIV, broadening the scope of antiviral strategies.
The CRISPR-Cas system protects the bacteria from foreign genetic elements.
In laboratories, the system is programmed using Cas-9 to edit genes in plants, animals, and humans.
Cas9, an endonuclease from Streptococcus pyogenes, is delivered into the cells with a synthetic guide RNA- sgRNA.
The sgRNA guides Cas9 to the Protospacer Adjacent Motif sequence, allowing it to bind and cut the DNA at the target site.
For gene insertion, Cas9, guided by one sgRNA, cleaves the site, allowing homologous recombination to insert the new gene.
For gene deletion, two sgRNAs direct Cas9 to cut both ends of the target region.
The gene is excised, and the repair system joins the cut ends.
CRISPR-Cas9 has been designed to inactivate HIV by excising its DNA from infected cells. Engineered systems like Csy4 target free HIV RNA, though these methods are still experimental.
Besides targeting a single locus, CRISPR-Cas9 can edit multiple genes simultaneously—such as removing multiple retroviral copies.
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