There are three types of CRISPR-Cas systems, type I, II and III.
Type I and III CRISPR/Cas systems share specialized Cas endo-nucleases that process the pre-crRNAs. After maturation, each crRNA assembles into a large multi-Cas protein complex that recognizes and cleaves nucleic acids complementary to the crRNA.
The type II CRISPR Cas system processes pre-crRNAs using a different mechanism in which a trans-activating crRNA (tracrRNA) complementary to the repeat sequences in pre-crRNAs triggers processing of the double-stranded RNA-specific ribonuclease RNAse II in the presence of the Cas9 protein. The Cas9 protein appears to be the responsible part for crRNA-guided silencing of foreign DNA. The system works by incorporating short exogenous DNA sequences from the invading pathogen into specific loci of the host genome. At the time of transcription, these sequences are processed into crRNAs via pre-crRNAs. The crRNAs act as a guide for the Cas9 nuclease machinery. The Cas9 nuclease cleaves and inactivates the foreign DNA. The system requires two more pieces to function. A tracrRNA that forms base-pairs with the crRNA provides the substrate for the host's ribonuclease RNase III. This system can identify DNA sequences that are complementary to the crRNA and degrade them.