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Bacteria and archaea have evolved various defense and regulatory mechanisms allowing them to react to various stressful situations caused by the environment, such as a virus attack. The recently discovered versatile CRISPR-Cas functions as a prokaryotic immune system. This system confers resistance to exogenous genetic elements such as plasmids and phages by providing a form of acquired immunity. The CRISPR Cas system has two novel features that allows the host to specifically incorporate short sequences from invading genetic elements such as a virus or plasmid into a region of its genome that is distinguished by clustered regularly interspaced short palindromic repeats (CRISPRs). Next, these sequences are transcribed and precisely processed into small RNAs to guide a multifunctional protein complex (Cas proteins) to recognize and cleave incoming foreign genetic material. This CRISPR Cas system is thought to be an adaptive immunity system which uses a library of small noncoding RNAs as a powerful weapon against fast-evolving viruses and is also used as a regulatory system by the host cells.

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