Devices and methods generate an ordered restriction map of genomic DNA extracted from whole cells. The devices have a fluidic microchannel that merges into a reaction nanochannel that merges into a detection nanochannel at an interface where the nanochannel diameter decreases in size by between 50% to 99%. Intact molecules of DNA are transported to the reaction nanochannel and then fragmented in the reaction nanochannel using restriction endonuclease enzymes. The reaction nanochannel is sized and configured so that the fragments stay in an original order until they are injected into the detection nanochannel. Signal at one or more locations along the detection nanochannel is detected to map fragments in the order they occur along a long DNA molecule.