[1] In bacteria and archaea, CRISPR-Cas (clustered, regularly interspaced short palindromic repeats/CRISPR-associated proteins) constitute an RNA-mediated defense system that protects against viruses and plasmids.
First, a copy of the invading nucleic acid is integrated into the CRISPR locus.
Type II CRISPR-Cas systems require a tracrRNA which plays a role in the maturation of crRNA.
[3] The tracrRNA is partially complementary to and base pairs with a pre-crRNA forming an RNA duplex.
This hybrid acts as a guide for the endonuclease Cas9, which cleaves the invading nucleic acid.