The present invention involves a method for categorizing nucleic acid which comprises: producing a nucleic acid population by action of an endonuclease on double-stranded nucleic acid, such that each nucleic acid in the nucleic acid population has a double-stranded portion; contacting the nucleic acid population with one or more oligonucleotide sequences; and isolating nucleic acid which correctly hybridizes to an oligonucleotide sequence. In the method of the present invention, each oligonucleotide sequence has a pre-determined recognition sequence. Furthermore, the nucleic acid is categorized by its ability to correctly hybridize to oligonucleotide sequences having the recognition sequence, the recognition sequence being situated such that it recognizes a sequence in the double-stranded portion of the nucleic acid. The oligonucleotide sequence can comprise one or more different recognition sequences.