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. 2021 Jul 8;17(7):e1009672. doi: 10.1371/journal.ppat.1009672

Fig 1. Mechanisms of action of CRISPR/Cas immunity.

Fig 1

Left: Class I CRISPR/Cas systems (modeled by Type I system) utilize a multi-subunit complex, termed “Cascade,” as the effector machinery. Right: Class II CRISPR/Cas systems (modeled by Type II system) use a single effector protein (such as Cas9) for interference. Both classes also consist of spacer (diamonds) and repeat (squares) arrays. Top: During adaptation, the Cas1–Cas2 complex takes a sequence of the invading DNA and integrates it into the CRISPR array as a novel spacer. Center: In the next stage, termed “expression,” the CRISPR array is transcribed into pre-crRNAs that are further processed into mature interfering crRNAs. Bottom: During the interference stage, the mature crRNAs guide the Cas proteins to their DNA target. Upon binding of the crRNA to their cognate DNA target, the Cas protein generates a double-stranded DNA break in the target. Created with BioRender.com. Cascade, CRISPR-associated complex for antiviral defense; CRISPR/Cas, clustered regularly interspaced short palindromic repeats/CRISPR-associated protein.