By John Brennan
Updated Aug 30, 2022
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Scientists routinely break down DNA into its nucleotide components to analyze genetic traits or disease risk. A critical step in most DNA extraction workflows is the use of ethanol or isopropanol to isolate nucleic acids from cellular debris. Since cells contain proteins, lipids, and other contaminants, the goal is to produce a DNA solution that is as pure as possible.
Typical extraction protocols involve multiple stages: cell lysis, removal of membrane lipids, and separation of DNA from proteins, RNA, and other impurities. Two widely used methods are alkaline lysis (for bacterial plasmid DNA) and phenol–chloroform extraction. In both approaches, ethanol or isopropanol precipitation is employed as one of the final steps. After precipitation, the DNA pellet is resuspended in water or a low‑salt buffer.
Ethanol and isopropanol are fully miscible with water, but their dielectric constants are markedly lower (water = 78.5; ethanol = 24.3). DNA carries a negative charge, attracting cations such as Na⁺ or K⁺. Because ethanol is less efficient at shielding these ions, the DNA-cation complexes are more likely to aggregate and precipitate.
Beyond charge shielding, ethanol reduces the solubility of DNA by forming hydrogen bonds with water, thereby limiting the number of water molecules available to hydrate the nucleic acid. This dual effect causes DNA to co‑precipitate with positive ions, producing a solid pellet that is more concentrated than the original solution. Importantly, many co‑contaminants do not precipitate under the same conditions, so the overall purity of the DNA increases.
The ethanol wash also removes low‑molecular‑weight contaminants such as salts and detergents. The choice of salt can be critical when removing detergents like sodium dodecyl sulfate (SDS). For example, potassium dodecyl sulfate is insoluble and will precipitate, so using potassium acetate in alkaline lysis can eliminate SDS before ethanol or isopropanol is added. Ethanol can precipitate RNA as well, though typically a higher concentration of ethanol is required for optimal RNA recovery.
