HindIII cuts the sugar -phosphate backbone of each strand at the point indicated by the arrow, generating fragments with short, single-stranded overhanging ends:

Such ends are called cohesive ends or sticky ends, because they are complementary to each other and can spontaneously pair to connect the fragments. Thus DNA fragments can be "glued" together: any two fragments cleaved by the same enzyme will have complementary ends and will pair (I Figure 18.2). When their cohesive ends have paired, two DNA fragments can be joined together permanently by the enzyme DNA ligase, which seals nicks between the sugar -phosphate groups of the fragments.

Not all restriction enzymes produce staggered cuts and sticky ends. PvuII cuts in the middle of its recognition site, producing blunt-ended fragments:

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