CRISPRs (Clustered Regularly Interspaced Short Palindromic Repeats) are part of an adaptive defense mechanism in bacteria and archaea. Use of the CRISPR/Cas9 system for genome editing has been a major technological breakthrough, making genome modification in cells or organisms fast, more efficient, and much more robust than previous genome editing methods. Single guide RNAs (sgRNAs) or guide RNAs (gRNAs) direct and activate the Cas9 endonuclease at a specific genomic sequence. Cas9 then cleaves the target DNA, making it available for repair by the non-homologous end joining (NHEJ) system or for creating an insertion site for exogenous donor DNA by homologous recombination.
Types of CRISPR/Cas9 RNA species:
- crRNA (CRISPR-targeting RNA)— provides target specificity (20 bases) and an interaction domain with the tracrRNA
- tracrRNA (trans-activation crRNA)— acts as a scaffold between the crRNA and Cas9 endonuclease
- sgRNA (single guide RNA) or gRNA (guide RNA)—fusion of crRNA and tracrRNA sequences into a single RNA that includes sequences for DNA targeting specificity and Cas9 endonuclease binding