Practice question

CRISPR immunity proceeds through adaptation, expression and interference stages. During adaptation, Cas1-Cas2 heterohexamer complex captures fragments of invading phage or plasmid DNA and integrates them as new spacers at leader end of CRISPR array. This spacer acquisition step records immunological memory for future defense. Cas1 is metal-dependent integrase cutting and ligating DNA, Cas2 provides essential structural scaffold. Interference later uses Cas9 guided by crRNA, while spacer incorporation entirely depends on Cas1-Cas2, making it essential for naive adaptation and primed acquisition pathways.