Fully functional CRISPR/Cas enzymes will introduce a double-strand break (DSB) at a specific location based on a gRNA-defined target sequence. DSBs are preferentially repaired in the cell by non-homologous end joining (NHEJ), a mechanism which frequently causes insertions or deletions (indels) in the DNA. Indels often lead to frameshifts, creating loss of function alleles.
To introduce specific genomic changes, researchers use ssDNA or dsDNA repair templates with homology to the DNA flanking the DSB and a specific edit close to the gRNA PAM site. When a repair template is present, the cell may repair a DSB using homology-directed repair (HDR) instead of NHEJ. In most experimental systems, HDR occurs at a much lower efficiency than NHEJ.
https://www.addgene.org/crispr/yeast/There different types of CRIPSR/Cas kits: cut, based editing, nick, activate, and interference.
https://benchling.com/pub/ellis-crispr-tools
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