Fig. 1

Schematic of the mechanisms of different types of CRISPR systems. a The working principle of type II Cas9. In the presence of the PAM sequence (NGG), the targeting effect of sgRNA is used to guide Cas9 protein to cleave both the complementary and non-complementary strands, forming a blunt-ended nick. b The working principle of type V Cas12a protein. In the presence of the PAM sequence (NTTT), the targeting effect of crRNA is used to guide Cas12a protein to cleave both the complementary and non-complementary strands, forming a sticky-ended nick. c The working principle of type I Cas systems. In the presence of the PAM sequence, the targeting effect of crRNA is used to guide the Cas3 protein to cleave the non-complementary strand to form a large gap. d The working principle of type III Cas systems. In the absence of a PAM sequence, the targeting effect of sgRNA is used to guide Csm protein to cleave the non-complementary strand to form short nucleic acid fragments. The green transverse U represents sgRNA or crRNA, the nucleotide sequences marked in red represent the PAM sequence, and scissors represent the cleavage site of nucleases