The Biology of Native and Adapted CRISPR-Cas Systems

Abstract

age of invading DNA prevents expression of viral elements, which prevents successful infection of the bacterium. The type II CRISPR system of Streptococcus pyogenes requires only one effector protein, Cas9, which can be targeted to make a double-stranded break in DNA at a specific nucleotide sequence (Jinek et al., 2012). Modified CRISPR systems, the vast majority of which use the Cas9 protein, have become revolutionary tools for genetic modification for two main reasons: ease of use and high versatility. Previous methods to modify the genomes of organisms have also relied on the introduction of double-stranded breaks, but were difficult and expensive to design (Doudna & Charpentier, 2014). Examples of this include zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) (Doudna & Charpentier, 2014). CRISPR systems, however, require only the design of a guide RNA complementary to a target site. Recent developments have created numerous modified CRISPR systems, which use the targeted Cas9 protein for purposes beyond the standard double-stranded cleavage (Brocken et al., 2017; B. Chen et al., 2013; Cheng et al., 2013; Nishida et al., 2016; Qi et al., 2013). This review covers a brief history of CRISPR research, what is known about the biology of the native type II CRISPR system, and several of the numerous different CRISPR-based applications that have been developed in recent years. Adapted CRISPR systems have proven to be incredibly effective tools for biological and biomedical research due, in large part, to their versatility. Although Cas9 originally evolved to simply cleave invading viral elements in single-celled organisms, it has been used in adapted CRISPR systems to make targeted genetic and epigenetic alterations, image DNA elements, alter gene expression, and discover key genes inThe Biology of Native and Adapted CRISPRCas Systems