Recent advances in the design and synthesis of DNA with programmable 3D nanostructures have stimulated new efforts to study charge transport in DNA, and to explore DNA as building blocks of electronic devices. We examine basic charge transport, electromechanical and thermoelectric properties in single DNA molecules, and study the dependence of these properties on the DNA length and sequence. We also demonstrate a molecular switch by inserting a redox group in between DNA bases. Electrically tuning of the redox group between reduced and oxidized states leads to reversible switching of the molecule between high and low conductivity states. We further show that this strategy allows tracking of single chemical reaction events, and examining of the thermodynamics and kinetics of the reaction at the single-molecule level.