In molecular biology, CRISPRs are (clustered regularly interspaced short palindromic repeats) are segments of prokaryotic DNA containing short repetitions of base sequences.

Each repetition is followed by short segments of "spacer DNA" from previous exposures to a bacterial virus or plasmid.  It is pronounced "crisper".

Diagram of the CRISPR prokaryotic viral defines mechanism CAS = CRISPR associated protein.
 image: James atoms / wikipedia

The CRISPR/Cas system is a prokaryotic immune system that confers resistance to foreign genetic elements such as plasmids and phages, and provides a form of acquired immunity. 

CRISPR spacers recognize and cut these exogenous genetic elements in a manner analogous to RNAi in eukaryotic organisms. CRISPRs are found in approximately 40% of sequenced bacteria genomes and 90% of sequenced archaea. 

The CRISPR/Cas system has been used for gene editing (adding, disrupting or changing the sequence of specific genes) and gene regulation in species throughout the tree of life. 

By delivering the Cas9 protein and appropriate guide RNAs into a cell, the organism's genome can be relatively cheaply cut at any desired location.

CRISPR/Cas9 is a powerful tool for genome engineering. The Cas9 complex with a sgRNA recognizes a specific sequence, the protospacer. This is only possible if this sequence is followed by a Protospacer Adjacent Motif (PAM). When Cas9 binds, a dsDNA break is generated. Now, Non-Homologous End Joining or Homology Directed Repair can occur, leading to mutations or gene changes, respectively. image :Esmée Dragt/

source: adapted from CRISPR. (2015, October 20). In Wikipedia, The Free Encyclopedia. Retrieved 20:29, October 22, 2015, from