The present invention is directed to a sequence dependent polynucleotide nanomechanical device based on a nucleic acid paranemic crossover (PX) molecule which is a four-stranded structure related to parallel double crossover molecules, except that every possible crossover takes place. Eliminating two crossovers leads to a topoisomer, termed JX.sub.2, in which one pair of ends are switched (rotated 180.degree.) while the other pair of ends remain the same. The device can be cycled between the two states, PX and JX.sub.2, by replacing single strands that set the state to be PX or JX.sub.2.