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. Author manuscript; available in PMC: 2021 Jun 17.
Published in final edited form as: Prog Retin Eye Res. 2018 Mar 22;65:28–49. doi: 10.1016/j.preteyeres.2018.03.003

Figure 1. The CRISPR-Cas systems function in bacteria and archea as adaptive immune systems against foreign genetic material.

Figure 1.

The system is composed of a CRISPR array of alternating conserved repeats and target-specific spacers (protospacers) acquired from fragments of foreign genetic material. The bacterium creates a heritable memory of infection. Upon entry of a foreign invader (1 – infection), foreign DNA sequences are incorporated into the bacterial CRISPR locus (2 – acquisition) and subsequently the bacterium transcribes CRISPR RNAs (crRNAs) from the array which associate with Cas effector proteins to create a ribonucleoprotein surveillance complex (RSC) (3 – expression). The RSC recognizes a sequence directly downstream of the crRNA target sequence – the protospacer adjacent motif (PAM). Following guide binding, the Cas nuclease cleaves the target DNA leading to the clearance of the foreign invader (4 – interference) (Leenay and Beisel, 2017).