Physical mechanisms that might assure the functioning of DNA as a molecular wire are considered on the basis of recent progress in understanding long-range charge transfer in this biologically important molecule. Our analysis shows that DNA behaves as an insulator at low bias, while beyond the threshold the current sharply increases. Such behaviour concurs with recent experimental observations and is explained by the decrease of the energy gap between the HOMO of guanine bases and the Fermi level of the contact with the voltage applied across the individual DNA molecule. We propose a model for the hole injection in DNA, which is based on the dynamic control of this process by internal motions of base pairs in the stack. The temperature dependence of the voltage gap obtained within this model is found to be in reasonable agreement with the available experimental data. For systems, where charge transfer is controlled by changes in the relative orientation of the donor and acceptor and where the equilibrium states are optimally overlapped, the model predicts the decrease of the tunneling transfer rate with temperature. We also demonstrate that depending on the structure of the stack, hole transport along DNA wires above the voltage threshold can proceed via two different mechanisms. In the case of duplex DNA oligomers with stacked adenine-thymine and guanine-cytosine pairs migration of injected holes can be viewed as a series of short-range hops between energetically appropriate guanine bases. By contrast, in double-stranded poly(guanine)-poly(cytosine) the band-like motion of holes through bases dominates.