DNA molecules are able to wrap carbon nanotubes (CNTs) in water solution, thus merging advantages of DNA chemistry with CNT physics as a natural way to design sensor devices. Using electrical means, a reversible semiconductor- metallic behavior has been found in CNT-DNA hybrid structures. Classical molecular dynamics simulations of the CNT-DNA wrapping process are performed. Then, structural conformations are analyzed by first-principles electronic structure methods; we correlate our results with previous experimental reports on carbon-DNA complexes, providing complementary information to understand better their electrical behavior.