  The mechanical properties of DNA are typically described by elastic theories
with purely local couplings (on-site models). We discuss and analyze
coarse-grained (oxDNA) and all-atom simulations, which indicate that in DNA
distal sites are coupled. Hence, off-site models provide a more realistic
description of the mechanics of the double helix. We show that off-site
interactions are responsible for a length scale dependence of the elasticity,
and we develop an analytical framework to estimate bending and torsional
persistence lengths in models including these interactions. Our simulations
indicate that off-site couplings are particularly strong for certain degrees of
freedom, while they are very weak for others. If stiffness parameters obtained
from DNA data are used, the theory predicts large length scale dependent
effects for torsional fluctuations and a modest effect in bending fluctuations,
which is in agreement with experiments.
