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Origin and Development of CRISPR-Cas9 System
CRISPR loci are first identified in archaea and bacteria when they systematically drew attention from scientists with their biological function to fight phages and viruses (Hsu, Lander, Zhang, 2014). Structurally, a clustered set of Cas (CRISPR-associated) genes and a unique CRISPR array constitute the CRISPR loci. The CRISPR array was further comprised of short repetitive sequence interspaced by distinctive sequences (spacers) in correspondence with exogenous genetic bits (protospacer). The natural CRISPR systems in bacteria and archaea carried out their adaptive antiviral immunity by following a three-step mechanism, namely acquisition of spacers, crRNA biogenesis, and interference (Wright, Nuñez, Doudna, 2016).
The infection by undocumented DNA starts the acquisition of viral DNA. Upon the detection of the invasion of bacteriophages, bacteria defend themselves in a timely fashion by inserting bits of viral DNA, the protospacer, into their chromosome at the end of CRISPR locus (Wright, Nuñez, Doudna, 2016). To maintain the structure of CRISPR array, bacteria initiate the replication of a repetitive DNA sequence, the repeat (Barrangou et al., 2007).
Next, crRNA biogenesis takes place in two stages. First, the CRISPR array and the Cas gene are transcribed respectively into a single pre-crRNA and Cas proteins. In this process, different types of CRISPR systems are unique in their Cas proteins they encode. Specifically, type II

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