English: RNA is transcribed from only one of the two DNA strands, called the "sense" strand (the other being called the "anti-sense" strand). If DNA is isolated during transcription, there may be "melted out" loops consisting on one side of a length of RNA (shown in red) base-paired to its complementary DNA sequence on the "sense" strand, and on the other side of the now-unpaired "anti-sense" strand.
Tai Te Wu employed the potential difference in electrophoretic mobility, between sense strands containing D-loops and anti-sense strands containing no bound RNA, to effect a separation of the strands on agarose gels. See: Wu R. and T.T. Wu (1996). A novel intact circular dsDNA supercoil. Bull. Math. Biol, 58(6):1171-1185.