Figure 1

The CRISPR-Cas adaptive immune system in prokaryotes. 1. Acquisition: In this stage, the invading DNA is fragmented and a new protospacer is selected and integrated into the CRISPR array as a new spacer. 2. Expression: During this second stage, the pre-crRNA produced by the CRISPR array is cleaved into mature crRNAs by RNase III. These mature crRNAs, along with tracrRNAs and Cas proteins, assemble to form ribonucleoprotein (RNP) complexes. 3. Interference: In the final stage, the crRNP uses a small guide RNA (sgRNA) (composed of crRNA and tracrRNA) to identify invading DNA, and Cas proteins cut the foreign DNA, removing the foreign genetic material.