CRISPR allows bacteria to recall previous viral infections. When a virus attacks for the first time, the bacterium retains a fragment of the viral DNA in its CRISPR region. If the same virus attacks a second time, the stored sequence is transcribed into a guide RNA that directs CRISPR-associated proteins to the matching viral genetic material and helps destroy it. In the gene-editing system, the Cas9 enzyme and another RNA molecule attach to the guide RNA, which the finds the corresponding viral DNA. Cas9 then cuts the viral DNA and prevents the virus from reproducing. Doudna and Charpentier showed that the guide RNA could be changed to direct Cas9 to a precise DNA sequence.
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