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Figure 1. 

The classification of CRISPR/Cas systems. Representative operons for each type are shown here. Gray represents Cas systems employing multi-subunit protein complexes to cut target DNA or RNA; blue represents Cas employing a single multidomain protein in complex for sequence cleavage.

Figure 2. 

Composition of CRISPR/Cas system. CRISPR/Cas9, the Cas9 system is composed of a Cas9 protein and a sgRNA. The yellow arrow represents DNA double-strand break (DSB) induced by RuvC and HNH domains. CBE, a base editor is a fusion of catalytically deactivated Cas9 domain and deaminase domain. CBE deaminates cytosine to generate uracil (C-U), then converts the uracil into thymine (U-T) by DNA repair response, eventually effecting a C-T (or G-A) substitution. ABE deaminates adenosine to generate inosine (A-I), which is read as guanosine (G) by polymerase enzymes, eventually effecting a A-G (or T-C) substitution through DNA repair or replication. PE, Prime editor consist of two simple operational components, a fusion protein made by the fusion nickase Cas9 protein (nCas9) and reverse transcriptase (RT) and a pegRNA contains sgRNA, primer binding site and reverse transcription template.

Figure 3. 

Application of CRISPR/Cas12 and CRISPR/Cas13 in nucleic acid detection. Cas12 collateral cleaves a reporter nucleic acid, which is composed of a single stranded DNA (ssDNA) bearing a fluorophore, and the fluorophore is released and leads to the emission of a fluorescent signal. Cas13 : crRNA complexes activate and cleave fluorescent RNA sensors for RNA target detection.