Six years ago, a new sophisticated prokaryotic defense system, was identified in the structure complex CRISPR-Cas9 (CRISPR, clustered regularly interspaced short palindromic repeats; Cas, CRISPR associated nuclease). “Resistance is acquired by incorporating short stretches of invading DNA sequences in genomic CRISPR loci ” Brouns, S. J. J., et al. (2008). The CRISPR repeat sequence has been discovered in 1987 but the function remained unknown for the next 20 years. The CRISPR-Cas9 system is an adaptive, heritable defense system that prokaryotes (bacteria and archaea) use to protect themselves by degrading foreign invaders’ nucleic acids such as viruses. Recently, these RNA- guided Cas9 nucleases derived from CRISPR/Cas systems have shown promise in transforming our ability to edit mammalian genomes and developing new treatments.
I/ Structural characteristics
CRISPR consists of an RNA, partly constant with a palindrome, tied to a crRNA sequence DNA specific to a target, and a Cas9 protein (Figure 1). The short guide RNA directs Cas9 to a specific genomic sequence where it induces double-strand breaks that, when imperfectly repaired, yield mutations. Cas9 can also catalyze gene replacement through homologous recombination.