Cas3 didn’t cut the DNA just once; it chewed it up into hundreds of pieces. Once Cascade recruited Cas3 to the site of a CRISPR RNA–viral DNA match, Cas3 started shuttling along the phage genome at a rate of over three hundred base pairs per second, slicing up the DNA and leaving the lengthy phage genome as a jumble of scraps in its wake. If simpler nucleases were like pruning shears, Cas3 was like a pair of motorized hedge clippers. Its speed and efficiency were stunning.
