Different Classes of CRISPR-Cas Systems

Genome editing refers to a set of scientific methods that make it possible to modify an organism's genetic elements. Recently emerged editing technique, CRISPR-Cas, which stands for clustered regularly interspaced short palindromic repeats and their associated proteins, is an adaptive immune system found in bacterial and archaeal prokaryotic systems. The structure of the effector Cas protein in the CRISPR system and its targeting mechanism led to its classification as Class I and II and then into six major types (I to VI). Type I, III, and IV come under Class I and comprise a multi-subunit crRNA–effector complex, and Class II having types II, V, and VI utilises only a single crRNA-effector module. The excessive diversity of Cas protein sequences and the architecture of the CRISPR-Cas loci are the consequences of the continuous arms race with mobile genetic elements (MGEs) in which Cas systems have evolved. Cas genes keep evolving so rapidly that they lead to minor variations in locus organisation, creating a scope for modifying nomenclature further into their subtypes in this CRISPR-Cas system. The type II Cas9 and type V Cas12a systems have attracted the most attention for genetic engineering among the CRISPR-Cas systems identified. This hapter focuses on the different classes of CRISPR-Cas systems, their types and subtypes, recent developments, as well as their potential applications.