In Randall-Sundrum-type brane-world cosmologies, density perturbations generate a Weyl curvature in the bulk, which in turn back reacts on the brane via stress-energy perturbations. On large scales, the perturbation equations contain a closed system on the brane, which may be solved without solving for the bulk perturbations. Bulk effects produce a nonadiabatic mode, even when the matter perturbations are adiabatic, and alter the background dynamics. As a consequence, the standard evolution of large-scale fluctuations in general relativity is modified. The metric perturbation on large-scales is not constant during high-energy inflation. It is constant during the radiation era, except at most during the very beginning, if the energy is high enough.