Abstract: The present invention provides new methods, employing a nucleotide integrase, for cleaving single-stranded RNA substrates, single-stranded DNA substrates, and double- stranded DNA substrates at specific sites and for inserting a nucleic acid molecule into the cleaved substrate. One method uses a nucleotide integrase to cleave one strand of a double-stranded DNA substrate. The method comprises the steps of: providing a nucleotide integrase comprising a group II intron RNA having two hybridizing sequences that are capable of hybridizing with two intron RNA binding sequences on the one strand of the DNA substrate, and a group II-intron encoded protein which binds to a first sequence element of the substrate; and reacting the nucleotide integrase with the double-stranded DNA substrate under conditions that permit the nucleotide integrase to cleave the one strand of the DNA substrate and to insert the group II intron RNA into the cleavage site.