Practice question

Bacterial CRISPR adaptive immunity comprises three functional phases. Adaptation involves spacer acquisition by Cas1-Cas2. Expression involves transcription of CRISPR array into pre-crRNA processed into mature crRNAs with tracrRNA assistance. Interference phase represents actual defense where effector complex loaded with crRNA surveys invading DNA for complementary protospacer adjacent to PAM and mediates degradation. In type II systems Cas9 itself cleaves target DNA three base pairs upstream of PAM, generating double strand breaks, thereby neutralizing bacteriophage replication during interference phase efficiently and rapidly.