Atomistic (atom-scale) and coarse-grained (meso-scale) simulations of the structure and dynamics of poly-isoprene melts are compared. The local structure and chain packing is mainly determined by the atomistic details of the polymer architecture. The large-time dynamics encountered in NMR experiments can be explained by meso-scale simulations including stiffness. The connecting link between the two scales is the stiffness which, although being a local property, influences strongly even the long-timescale dynamics. The standard reptation scenario fails to explain the observed dynamics. We propose strong reptation as a modified reptation scenario in which the local Rouse motion is absent.