Figure 7.4.DNA fragments produced with a singleEcoRI restriction enzyme give rise to compatible
protruding termini that can anneal in either orientation, bringing together the 5^0 phosphate and the 3^0
hydroxyl residues on each strand. This allows DNA ligase to catalyze the formation of phosphodiester
bonds, joining the two molecules together.
Figure 7.5.DNA fragments produced with two restriction enzymes,EcoRI andSacI, give rise to
fragments with protruding termini that can anneal in only one orientation with respect to one
another, forcing the two molecules to combine in one direction only.
7.2. DNA VECTORS 165