The key difference between CRISPR and CRISPR Cas9 is that CRISPR (clustered regularly interspaced short palindromic repeat) is a naturally occurring prokaryotic immune defence mechanism while CRISPR cas9 is an RNA-guided Cas9 nuclease which is a part of the CRISPR adaptive immune system.
CRISPR is an antiviral defence immune system found in bacteria and archaea. It is a family of DNA sequences found in the bacterial genome. It consists of spacer sequences which are derived from DNA fragments of bacteriophages that had previously infected them. In subsequent viral infections, bacteria use these sequences to destroy viral DNA. CRISPR sequences have short palindromic repeats and spacer sequences. Moreover, CRISPR repeat spacer sequences often have associated genes which code for Cas proteins. Cas9 is an RNA guided endonuclease. Together with CRISPR sequences, Cas9 protein acts as an adaptive immune system in bacteria against bacteriophages.
