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Schematic diagram of DNA and RNA spacers acquisition by CRISPR-Cas systems. (a) Small segments of invasive DNA are assimilated into CRISPR arrays by Cas1 and Cas2 in a canonical spacer acquisition process that allows adaptive immunity in a wide variety of bacteria and archaea. (b) In the type Ⅵ CRISPR systems, an RT fused to Cas1 enables the acquisition of spacer sequences directly from RNA. This process can mediate adaptive immunity against RNA-based parasites [40].
