Figure 1.

CRISPR/Cas9 mediated acquired immunity in prokaryotes. During the acquisition phase (A), cellular invaders such as phage virus inject nucleic acid sequences into the host cell. After infection, novel DNA sequences from the cellular invaders are incorporated into the host CRIPSPR locus as spacers (colored circles) flanked by repeat sequences (gray diamonds). As a result, when the CRISPR locus is transcribed, the pre‐CRISPR RNAs (crRNAs) encode the newly acquired protospacer sequences. The pre‐crRNA is cleaved to produce individual crRNAs that will associate with Cas proteins. The Cas protein utilizes the crRNAs as guides to silence foreign DNA that matches the crRNA sequence (B, interference phase). As a result, the second time a bacteria encounters the same foreign DNA, the crRNA/Cas9 complex is able to identify and silence the DNA.