  The flexibility of short DNA fragments is studied by a Hamiltonian model
which treats the inter-strand and intra-strand forces at the level of the base
pair. The elastic response of a set of homogeneous helices to externally
applied forces is obtained by computing the average bending angles between
adjacent base pairs along the molecule axis. The ensemble averages are
performed over a room temperature equilibrium distribution of base pair
separations and bending fluctuations. The analysis of the end-to-end distances
and persistence lengths shows that even short sequences with less than $100$
base pairs maintain a significant bendability ascribed to thermal fluctuational
effects and kinks with large bending angles. The discrepancies between the
outcomes of the discrete model and those of the worm-like-chain model are
examined pointing out the inadequacy of the latter on short length scales.
