Isolated DNA can be cut at specific sites by enzymes called restriction endonucleases. Restriction enzymes are isolated from bacteria. They recognize and cleave the DNA at four to eight specific nucleotides in a palindromic sequence; a sequence that reads the same in the 5' to 3' direction on one strand, and in the 3' to 5' direction on the complementary strand. If the endonuclease cuts symmetrically between two bases opposite each other on complementary DNA strands, the resulting DNA fragments have blunt ends. If the endonuclease makes staggered cuts across the two DNA strands, leaving each fragment with an overhang of unpaired bases, then the ends of the DNA fragments are said to be sticky. The sticky end of a DNA fragment can pair with the single-stranded overhang of another piece of DNA cut by the same restriction enzyme. Different DNA fragments with sequence-specific ends can be joined to form new recombinant DNA constructs.