This disclosure provides a method comprising: a) clamping the top and bottom strands of a double stranded DNA molecule to produce a duplex in which the top and bottom strands are linked; b) denaturing the duplex to produce a denatured product; and c) renaturing the denatured product in the presence of a labeled oligonucleotide that is complementary to a sequence of nucleotides in the double stranded DNA molecule, thereby producing a D-loop-containing product. Kits for performing the method and products made by the method are also provided.