Ethanol precipitation is a process of DNA extraction in which this material is brought out of a salty solution by the addition of ethanol. DNA is a double helix of two negatively charged phosphate-sugar backbones, along which nucleic acids are bonded, so positively charged ions such as calcium or sodium will be attracted to its backbone. Ionic salts and DNA both readily dissolve in water but not so easily in ethanol because they both require a polar solvent such as water to dissociate. Adding ethanol to a salty solution, therefore, causes the DNA to pellet and precipitate out of solution.
DNA is a polar molecule and, because "like dissolves like," polar molecules are readily dissolved in polar solvents. Although it easily dissolves in water, DNA does not dissolve in a less polar solvent such as ethanol. Ethanol still is a polar molecule, but it has a short, nonpolar carbon chain. This slight difference in polarity allows ethanol precipitation to occur based on the inability of DNA to dissolve as easily in ethanol as it does in water.
If an experimenter were to add ethanol directly to a solution of dissolved DNA in water, the molecule would not precipitate. This is because the DNA would still be surrounded by polar molecules of water and polar groups on ethanol that could function like a shield, separating the charged DNA from the uncharged carbon chains of ethanol. Salt has proved to be a vital factor in the process.
By adding salt to a solution before ethanol is added, DNA is freed from its reliance on water as a polarity shield. The positive elements of salt, which dissociate and are free-floating positively charged ions in water, are strongly attracted to the negative charge on the DNA. This salt attraction causes the DNA to become attracted to those ions rather than to the positive polarity of water molecules, allowing it to pull away from water easily when a less-polar solvent such as ethanol is added.
By allowing calcium or other positively charged ions to be attracted to DNA, then precipitating the salt out of solution by decreasing the water content by adding ethanol, scientists are provided with a method through which DNA can be brought out of solution. Many studies rely on DNA isolation and processing for genetic screens and protein expression, so many experimenters believe that ethanol precipitation has proved to be an invaluable scientific tool.