CRISPR-Cas: the effective immune systems in the prokaryotes

Approximately all sequenced archaeal and half of eubacterial genomes have some sort of adaptive immune system, which enables them to target and cleave invading foreign genetic elements by an RNAi-like pathway. CRISPR–Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated proteins) systems consist of the CRISPR loci with multiple copies of a short repeat sequence separated by variable sequences with similar size that are derived from invaders and cas genes encode proteins involved in RNA binding, endo- and exo-nucleases, helicases, and polymerases activities. There are three main types (I, II and III) of CRISPR/Cas systems. All systems function in three distinct stages: (1) adaptation, (2) crRNA biogenesis, and (3) interference. This review focuses on the features and mechanisms of the CRISPR-Cas systems and current finding about them.