How does CRISPR work?

1. DNA from the invading bacteriophage is incorporated into a region of the bacterial genome, called the CRISPR locus.
2. During the next bacteriophage invasion, DNA (A, T, G, C) in the CRISPR locus corresponding to the bacteriophage invader is transcribed into a short piece of RNA (A, U, G, C) called a CRISPR RNA (or crRNA).
3. crRNAs bind to a large protein (called an effector complex) and guides it to the bacteriophage DNA, the sequence of which complements the crRNA sequence. The effector complex and its crRNA bind to the invading bacteriophage DNA via Watson-Crick base pairing.
Note: Some CRISPR systems require a second RNA, called a tracrRNA, which helps process the crRNA. The tracrRNA is complementary to the pre-processed crRNAs. After cleavage by RNAse III, a ribonuclease, a crRNA/tracrRNA hybrid forms and guides the effector complex to degrade the invading bacteriophage DNA.
4. The invading bacteriophage DNA is cut and can be stored in the CRISPR locus for future invasions.

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