The bacterium then transcribes the CRISPR DNA and produces two specialized types of RNA: CRISPR RNA (crRNA), which matches the viral DNA, and tracer RNA (Step 3). Both of those RNA pieces then form a complex with CAS9, an endonuclease, or an enzyme that can cut DNA (Step 4). The guide crRNA, now part of the CAS complex, finds the matching sequence on the viral DNA (Step 5). The CAS protein-RNA complex unwinds and cuts the viral DNA (Step 6). Because the viral DNA is cut, it can no longer infect the bacterial cell.
When DNA is damaged, a cell can naturally repair the break. The genome editing process takes advantage of natural repair mechanisms. By introducing breakage into a formerly functional gene, CRISPR-Cas9 can disable genes. With a few small tweaks, the same system can be used to introduce functional copies of broken genes.