CRISPR is an abbreviation of Clustered Regularly Interspaced Short Palindromic Repeats. It is a family of DNA sequences in bacteria and archaea . The sequences contain fragments of DNA from viruses that have attacked the prokaryote. These fragments are used by the prokaryote to detect and destroy DNA from similar viruses during subsequent attacks. These sequences play a key role in a prokaryotic defense system, and form the basis of the CRISPR-Cas9 technology .
The CRISPR-Cas system is a prokaryotic immune system that confers resistance to foreign genetic elements such as those present within plasmids and phages that provides a form of acquired immunity [3,4,5]. Upon the infection, new foreign DNA sequences are captured and integrated into the host CRISPR locus as new spacers. The CRISPR locus is transcribed and processed to generate mature CRISPR RNAs (crRNAs), each encoding a unique spacer sequence. Each crRNA associates with Cas effector proteins that use crRNAs as guides to silence foreign genetic elements that match the crRNA sequence. Some Cas (CRISPR-associated) proteins recognize and cut exogenous DNA. Other Cas proteins cut foreign RNA . CRISPR systems are found in approximately 50% of sequenced bacterial genomes and nearly 90% of sequenced archaea .
Fig 1. The immune process of CRISPR system. From Doudna's Lab
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