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CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) along with Cas proteins form the CRISPR-Cas immune system in archaea and bacteria[1]. The bacterial CRISPR system has been adapted to edit genomes in a wide variety of species. The CRISPR-Cas cleaves DNA at a specific sequence allowing the modification of the gene[2].

CRISPR in Prokaryotes

There are three main steps to the CRISPR/Cas system in Prokaryotes. The first step, Insertion, allows Cas1 and Cas2 to detect the foreign DNA, cleave a protospacer, and attach it to the target DNA. The next step, crRNA processing, can be carried out through 3 unique mechanisms, although the primary pathway is Type II. In Type II, the sequence is transcribed and cleaved by Cas9 and RNase III. In the final step, Interference, additional trimming occurs at the 5’ end and mature crRNAs are created. The target sequences then intereact wwith the PAM sequence before the invading DNA is degraded[3].


  2. B Alberts, A Johnson, J Lewis, D Morgan, M Raff, K Roberts et al. (2015) Molecular Biology of the cell Sixth edition, page 497-498, Garland Science, New York, USA
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