CRISPR/CAS IS SO IMPORTANT IT GETS AN ENTIRE PAGE

CRISPR/CAS naturally functions as an adaptive immune response in many types of bacteria and most archaea to prevent reoccurring phage or plasmid infection. This immune system functions in 3 stages: adaptation, expression, and interference, with adaptation referring to infected or foreign DNA incorporation as “PROTOSPACERS” between short direct repeats of a CRISPR ARRAY LOCUS. These DNA spacers serve as a “memory” of preceding foreign DNA invasion and when EXPRESSED and processed into CRISPR RNAs (crRNAs) function to guide Cas proteins to cleave invading viruses or plasmids (interference).  Sequence specificity is determined by crRNA binding complementarity with target sequences. 

Among the 3 primary classes of CRISPR-Cas systems (classified by Cas type), type II (Cas9) has been most extensively studied and adapted for mammalian cell gene editing. Specifically, pre-crRNA maturation steps were omitted and a single guide RNA (gRNA) consisting of a crRNA and transactivating crRNA (tracrRNA) sequences, mediate delivery of Cas9 NUCLEASE activity with little targeting constraints. In particular, the first ~20bp of gRNAs complement target DNA sequences, merely requiring a 5’ protospacer adjacent motif (PAM) defined as 5’ NGG in the target (N being any DNA letter). 

As described in the HR and NHEJ section, like ZFs and TALENS, CRISPR/CAS can be delivered along with a DNA "REPAIR TEMPLATE," to facilitate incorporation of researcher-designed DNA into the break site. This enables replacement gene editing.

Below are a collection of videos that help to visualize this information and lead us into adapting these GENE-EDITING functions into EPIGENE-EDITING functions. Remember any HOMING system will do, but CRISPR/CAS is bar far the easiest to target.