CRISPR/Cas system is the acquired immune system of almost all archaea and some bacteria against virus or phage infection. Due to the programmable property of the system, CRISPR/Cas system is utilized as means of genome editing. Besides, this system may become potential weapon to treat HIV infection, genetic disorders, and other diseases.
CRISPRs means clustered regularly interspaced short palindromic repeats; they are short repetitions of DNA sequences. Between these sequences, there are spacer DNA, which is acquired by previous exposures to viruses. Besides, CRISPRs are associated with cas genes, which encode cas proteins.
The mechanism of CRISPR/Cas system is not fully understood now; this introduction is based on proposed mechanism. Initially, when a virus or phage invade the bacteria or archaea, Cas protein will recognize the sequence of the virus and create new spacer sequence. The newly recognized spacer will incooperate into the CRISPR array. Later, when repeated exposure to the same virus, the bacteria or archaea will create a CAS crRNA complex, targeting viral DNA and inactivating it. There is a tracrRNA cooperating with naturally occurring CRISPR-Cas system as well. But in engineered CRISPR-Cas system, a fusion of crRNA and tracrRNA will be used instead.
RNA-based CRISPR-Cas system provides more flexible means of genome editing than protein-based zinc finger nucleases (ZFNs) and transcription-activator-like effector nucleases (TALENs). There have been many instances of applications of CRISPR-Cas system on different species. Maybe CRISPR-Cas system will become standard gene therapy for human; nevertheless, more trials are needed.