text: CRISPR-Cas, yo, it's a system that's adaptive
  Captures DNA fragments, keeps invaders captive
  Bacteriophages, plasmids, they don't stand a chance
  Integrating spacers into bacterial CRISPR arrays, advance
  
  In type I-E and II-A, Cas1-Cas2 takes control
  Driving the adaptation process, playing the role
  But type I-F systems got something unique
  Cas2 fused with helicase, nuclease, havoc they wreak
  
  With biochem, structure, biophysics in sight
  400-kDa Cas1-Cas2-3 complex shining bright
  From <i>Pectobacterium atrosepticum</i>, the scene
  Bound protospacer substrate DNA, it's mean
  
  Two Cas1 dimers assemble, Cas2 core holds tight
  Cas3 domains flank, groove where protospacer's right
  In vitro assay developed, sensitivity on the rise
  Cas1-Cas2-3 catalyzing spacer integration, no surprise
  
  Integrase domain of Cas1, necessary for the quest
  Helicase, nuclease activities of Cas3, take a rest
  Partially duplex protospacers with 3'-OH free
  Leader-proximal integration, host factor, let it be
  
  Capture and integration together, Cas1-Cas2-3 going strong
  Processing, integrating protospacers, proving them wrong
  Independent of Cas3's activity, they make it clear
  Insight into adaptation complexes, structure we revere