Ligase Joins Two DNA Segments. A DNA segment to be cloned is often obtained through the action of restriction endonucleases. Most restriction en- zymes cleave DNA to yield sticky ends (Section 3-4A). Therefore, as Janet Mertz and Ron Davis first demonstrated in 1972, a restriction fragment can be inserted into a cut made in a cloning vector by the same restriction enzyme (Fig. 3-24). The complementary ends of the two DNAs form base pairs (an- neal) and the sugar–phosphate backbones are covalently ligated, or spliced together, through the action of an enzyme named DNA ligase. (A ligase pro- duced by a bacteriophage can also join blunt-ended restriction fragments.) A great advantage of using a restriction enzyme to construct a recombinant DNA molecule is that the DNA insert can later be precisely excised from the cloned vector by cleaving it with the same restriction enzyme.