Cleavage of DNA molecules by restriction endonucleases. When Type II restriction endonucleases cleave DNA, they leave either (a) sticky ends (with protruding single strands) or (b) blunt ends. The restriction fragments can be ligated to other DNAs, such as the plasmid cloning vector shown here. Ligation is facilitated by the annealing of complementary sticky ends, and it is less efficient for DNA fragments with blunt ends than for those with complementary sticky ends. DNA fragments with noncomplementary sticky ends (i.e., those created by different restriction enzymes) generally are not ligated.