CRISPRCassysteem

CRISPR-Cas system is a collection of adaptive immune mechanisms found in bacteria and archaea that protect against invading genetic elements such as phages and plasmids. A CRISPR locus stores short DNA sequences, called spacers, derived from previous invaders, flanked by repetitive DNA. Associated Cas proteins execute the immune response. The system is guided by CRISPR RNA (crRNA) and, in many cases, a trans-activating crRNA (tracrRNA) that together form a guide to recognize matching genetic material.

The immune response occurs in three stages. Acquisition inserts a new spacer from an invading DNA into the CRISPR array. Expression and maturation transcribe the array and process it into crRNA among other RNAs, generating guides. Interference uses the guide RNA and a Cas protein to search for a complementary sequence adjacent to a short PAM motif and to cleave the target. Different Cas proteins have preferences for DNA or RNA targets and for specific PAM sequences, enabling diverse applications.

CRISPR-Cas systems are classified into two classes. Class 1 uses multi-subunit effector complexes, while Class 2 uses a single, large effector protein. Examples include Cas9 (Type II, DNA cutting), Cas12 (Type V, DNA cutting with distinct PAMs), and Cas13 (Type VI, RNA targeting). Since their adaptation for programmable genome editing, these systems have become foundational in molecular biology, biotechnology, and medicine. Beyond editing, Cas12- and Cas13-based platforms enable rapid diagnostics, and newer methods like base and prime editing expand the range of possible genetic modifications.

Limitations include off-target effects, delivery challenges in living organisms, and potential immune responses. Ethical and regulatory considerations, particularly for germline editing, shape ongoing research and clinical translation.