What is the CRISPR-Cas system in prokaryotes?

CRISPR-Cas system is a naturally occurring genome editing system in prokaryotic organisms such as bacteria and archaea, where CRISPR-derived RNA and various CRISPR associated proteins (Cas) are employed to recognize, chop up and destroy the DNA of a foreign invader.

The bacteria or archaea first capture the DNA segments from the invading viruses, and add the foreign DNA spacers into the genome sequence, creating the CRISPR arrays. These CRISPR arrays are then transcribed and further converted to single spacer flanked region called short crRNA. Thereafter, the crRNA-foreign DNA complex is formed and cleaved by the endonuclease activity of Cas proteins, thus destroying the foreign invaders. In addition, the CRISPR arrays allow the prokaryotes to “remember” the viruses and disable them in future attacks, providing adaptive immunity to their antiviral defense system.

CRISPR-Cas systems are divided into three major types (type I, type II, and type III) and twelve subtypes, based on their genetic content and structural differences. The Cas9 used in CRISPR-Cas9 genome editing technique is a Type II CRISPR system.