Cleavage Pattern of Type II Restriction Endonucleases,Recognition Sequences,Restriction Enzyme,DNA Molecule,Construction of Recombinant DNAs

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	Cleavage Pattern of Type II Restriction Endonucleases - Most type II restriction endonucleases cleave the DNA molecules within their specific recognition sequences, but some produce cuts immediately outside the target sequence, e.g., NlaIII, Sau3A, etc. These cuts are either (1) staggered or (2) even, depending on the enzyme.
Most enzymes produce staggered cuts in which the two strands of a DNA double helix are cleaved at different locations; this generates protruding (3'- or 5'-) ends, i.e., one strand of the double helix extends some bases beyond the other: Due to the palindromic (symmetrical) nature of the target sites, the two protruding ends generated by such a cleavage by a given enzyme have complementary base sequence. As a result, they readily pair with each other; such ends are called cohesive or sticky ends. An important consequence of this fact is that when fragments generated by a single restriction enzyme from different DNAs are mixed, they join together due to their sticky ends. Therefore, this property of the restriction enzymes is of great value for the construction of recombinant DNAs.
Some restriction enzymes, on the other hand, cut both the strands of a DNA molecule at the same site so that the resulting termini or ends have blunt or flush ends in which the two strands end at the same point. The blunt cut ends also can be effectively utilized for construction of recombinant DNAs following one of several strategies.