Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-Associated (CRISPR/Cas) Systems: Discovery, Structure, Classification, and General Mechanism

The discovery and implementation of the CRISPR-Cas systems have been a well-acknowledged milestone in the history of genetic engineering (GE). Exhilarating research has led to a systematic progress in the understanding of the system from the identification of CRISPR as short regularly spaced repeats (SRSRs) in a bacterial cell, to the recognition of CRISPR-Cas as a Prokaryotic RNAi-based Adaptive Immune System and finally to the discovery of its programmability which gives it enormous potential for applicability. The programmability of this gene editing tool is attributed to a well-characterized locus which can be divided into a leader sequence, followed by a CRISPR array and a termination sequence. The genes for 66 CRISPR-associated proteins (Cas proteins), which are positioned near the CRISPR array, code for the effector module. The architecture of effector modules is the basis of classification of the CRISPR-Cas systems; 13 families of core Cas proteins have been identified. The mechanism of the CRISPR-Cas system is fluid and adaptive. The first step is adaptation which occurs when the cell encounters a foreign DNA for the first time and identifies it as a potential target, leading to insertion of specific sequences in the CRISPR array as spacers. When the system encounters this foreign DNA once again, an effector/surveillance complex of Cas proteins and mature CRISPR RNA (crRNA) identifies it, forming a double stranded structure, which in turn recruits a Cas endonuclease that degrades the target DNA. This CRISPR-directed RNA interference (RNAi) forms the basis of the gene editing mechanism.