  A statistical method is developed to estimate the maximum amplitude of the
base pair fluctuations in a three dimensional mesoscopic model for nucleic
acids. The base pair thermal vibrations around the helix diameter are viewed as
a Brownian motion for a particle embedded in a stable helical structure. The
probability to return to the initial position is computed, as a function of
time, by integrating over the particle paths consistent with the physical
properties of the model potential. The zero time condition for the
first-passage probability defines the constraint to select the integral cutoff
for various macroscopic helical conformations, obtained by tuning the twist,
the bending and the slide motion between adjacent base pairs along the molecule
stack. Applying the method to a short homogeneous chain at room temperature, we
obtain meaningful estimates for the maximum fluctuations in the twist
conformation with $\sim 10.5$ base pairs per helix turn, typical of double
stranded DNA helices. Untwisting the double helix, the base pair fluctuations
broaden and the integral cutoff grows. The cutoff is found to increase also in
the presence of a sliding motion which shortens the helix contour length, a
situation peculiar of dsRNA molecules.
