Nnnn Gaattc Nnnn Nnnn Cttaag Nnnn

Therefore, DNA restriction fragments having such "sticky" ends can be joined together to create new combinations of DNA sequence. If the fragments are derived from DNA molecules of different origin, novel recombinant forms of DNA are created.

£coRI leaves staggered 5'-termini. Other restriction enzymes, such as Pst I, which recognizes the sequence 5'-CTGCAG-3' and cleaves between A and G, produce cohesive staggered 3'-ends. Still others, such as BalI, act at the center of the twofold symmetry axis of their recognition site and generate blunt ends that are noncohesive. Bal I recognizes 5'-TGGCCA-3' and cuts between G and C.

Table 11.5

Restriction Endonucleases

About 1000 restriction enzymes have been characterized. They are named by italicized three-letter codes, the first a capital letter denoting the genus of the organism of origin, while the next two letters are an abbreviation of the particular species. Because prokaryotes often contain more than one restriction enzyme, the various representatives are assigned letter and number codes as they are identified. Thus, EcoRI is the initial restriction endonuclease isolated from Escherichia coli, strain R. With one exception (Ncil), all known type II restriction endonucleases generate fragments with 5'-PO4 and 3'-OH ends.

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