CRISPR

The CRISPR-Cas9 system is a DNA editing tool that stands for, Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR associated protein 9.

First observed in bacteria, CRISPR-Cas9 is a means of defense against viruses. As foreign viral DNA enters a bacterium, it's processed into smaller fragments, which may be inserted into a region of the bacterial genome called a CRISPR Locus.

When the region is transcribed, the product associates with smaller RNAs called tracrRNAs which may help to orient both the Cas9 protein and RNAse to the molecule. The latter of which cleaves the transcript.

The end result is several complexes each consisting of a Cas9 protein, tracrRNA and a crRNA derived from DNA in the Locus. The CRISPR RNA in these structures recognizes and guides Cas9 to viral DNA which is then cleaved and destroyed.

Scientists harness CRISPR-Cas9 by synthesizing individual RNA molecules that mimic tracrRNA and CRISPR RNA which can target a gene of interest. For example when two such guide RNAs are introduced into cells with Cas9 and both target the same gene, a sequence can be excised.

Once this target region is removed the cut ends are reconnected and the effects on the