How does a type II restriction enzyme generate blunt ends

Type II restriction enzymes are the familiar ones used for everyday molecular biology applications such as gene cloning and DNA fragmentation and analysis. These enzymes cleave DNA at fixed positions with respect to their recognition sequence, creating reproducible fragments and distinct gel electrophoresis patterns. Over 3,500 Type II enzymes have been discovered and characterized, recognizing some 350 different DNA sequences. Thousands more 'putative' Type II enzymes have been identified by analysis of sequenced bacterial and archaeal genomes, but remain uncharacterized.