Figure 1.

The CRISPR antiviral defense system of prokaryotes. A population of viruses may contain both wild type (black) and defective phages (gray; see text for further explanation). Upon infection of a defective bacteriophage as in the shown example (A), part of the viral genome is inserted inside one of the CRISPR loci of the bacterial genome (adaptation, B). In case of a second infection, even in case of a wild type phage (C) the CRISPR locus is transcribed (expression) and promotes viral genome degradation by site-specific, Cas-mediated cleavage (interference, D).