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, Sticky and blunt ends 5 - POM Restriction enzymes cleave dsDNA so as to produce a 3'-hydroxyl T group on one end and a 5'-phosphate group on the other. Some restriction endonucleases, such as Taql, form staggered cuts that produce sticky or cohesive ends (i.e., the resulting DNA fragments have single-stranded regions that are complementary to each other), as shown in Figure 34.3. Other restriction endonucleases, such as Haelll,, produce fragments that have blunt ends that are entirely double stranded and, therefore, do not form hydrogen bonds with each other. Using the enzyme DNA ligase (see p. 466), sticky ends of a DNA fragment of interest can be covalently joined with other DNA fragments that have sticky ends produced by cleavage with the same restriction endonuclease (Fig. 34.4). (Note: A ligase encoded by bacteriophage T4 can covalently join blunt- ended fragments.)