DNA molecules are quasi one-dimensional structures consisting of two strands in a double helix form. Each strand consists of four building blocks, Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). The diameter of the DNA molecule is approximately 2 nm. These nanoscale building blocks can be reproducibly engineered in arbitrary sequences of A, T, C and G, and chemists have for long studied sequence dependent charge transfer in DNA . The distinct energy levels and ionization potentials of the building blocks provides a framework to construct DNA sequences with interesting device properties. One can think of quantum wells and barriers constructed from DNA having resonances akin to double barrier resonant tunneling diodes and superlattices . Of great interest are also recent experiments which show the potential to detect diseases  and sequence DNA , by measuring the electrical conductance of a single DNA molecule. These studies demonstrate potential for an all-electrical method for disease detection. In this talk, we present our understanding of how electrons flow in DNA at room temperature. We will first discuss results from phase coherent calculations, which reveal that the conductance without decoherence is orders of magnitude smaller than the experimental values of conductance . The floppy nature of DNA molecules, electromagnetic fluctuations, and, the movement of water molecules and counter-ions in the case of wet DNA, creates an effective mechanism for decoherence, which facilitates electron flow . While modeling the nature of these fluctuations from ab initio methods is insightful, they have so far not been applied to explain experiments where electrons flow between two metal contacts - a very challenging problem. Rather than apply brute force ab initio molecular dynamics, we explore the applicability of a simple model for decoherence based on Buttiker probes, which when applied to the case of DNA helps understand the magnitude of conductance seen in experiments and capture the sequence dependence in some cases.
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