  Although mechanical properties of DNA are well characterized at the kilo
base-pair range, a number of recent experiments have suggested that DNA is more
flexible at shorter length scales, which correspond to the regime that is
crucial for cellular processes such as DNA packaging and gene regulation. Here,
we perform a systematic study of the effective elastic properties of DNA at
different length scales by probing the conformation and fluctuations of DNA
from single base-pair level up to four helical turns, using trajectories from
atomistic simulation. We find evidence that supports cooperative softening of
the stretch modulus and identify the essential modes that give rise to this
effect. The bend correlation exhibits modulations that reflect the helical
periodicity, while it yields a reasonable value for the effective persistence
length, and the twist modulus undergoes a smooth crossover---from a relatively
smaller value at the single base-pair level to the bulk value---over half a
DNA-turn.

Biomolecules