The forming and melting of complementary base pairs in RNA duplexes are conformational transitions required to accomplish a plethora of biological functions. Yet the dynamic steps of these transitions have not been quantitatively characterized at the molecular level. In this work, the base opening process was first enforced by atomistic pulling simulations and then analyzed with a novel reweighting scheme, which allowed the free-energy profile along any suitable reaction coordinate, for example, solvation, to be reconstructed. The systematic application of such approach to different base-pair combinations provides a molecular motion picture of helix opening, which is validated by comparison with an extensive set of experimental observations and links them to the enzyme-dependent unwinding mechanism. The RNA intrinsic dynamics disclosed in this work could rationalize the directionality observed in RNA-processing molecular machineries.