  Nucleic acids are highly deformable helical molecules constantly stretched,
twisted and bent in their biological functioning. Single molecule experiments
have shown that double stranded (ds)-RNA and standard ds-DNA have opposite
twist-stretch patterns and stretching properties when overwound under a
constant applied load. The key structural features of the A-form RNA and B-form
DNA helices are here incorporated in a three-dimensional mesoscopic Hamiltonian
model which accounts for the radial, bending and twisting fluctuations of the
base pairs. Using path integral techniques which sum over the ensemble of the
base pair fluctuations, I compute the average helical repeat of the molecules
as a function of the load. The obtained twist-stretch relations and stretching
properties, for short A- and B-helical fragments, are consistent with the
opposite behaviors observed in kilo-base long molecules.
