Extraction of DNA is often an early step in many diagnostic processes used to detect bacteria and viruses in the environment as well as diagnosing disease and genetic disorders. The DNA will be found at the interface between the two phases. 2  DNA is the precipitated by mixing with cold ethanol or isopropanol and then centrifuging. 3  The DNA is insoluble in the alcohol and will come out of solution, and the alcohol serves as a wash to remove the salt previously added.