Figure 1.

CRISPR-Cas adaptive immunity. Upon injection of genetic material from a virus or a plasmid into the bacteria, part of the invading sequence is cleaved and incorporated into the CRISPR locus, forming a new spacer within the locus. The CRISPR array is transcribed into a precursor to crRNA molecules (pre-crRNA), which is then cleaved into mature crRNA, which form effector complexes with type-specific Cas proteins (brown). When a foreign sequence matches a CRISPR spacer, the matching crRNA binds to the invading strand, activating Cas proteins with nuclease activity which silence the invader.