DNA isolation is a routine procedure to collect DNA for subsequent molecular or forensic analysis. There are three basic and one optional steps in a DNA extraction:
Breaking the cells open, commonly referred to as cell disruption or cell lysis, to expose the DNA within. This is commonly achieved by grinding or sonicating the sample.
Removing membrane lipids by adding a detergent.
Removing proteins by adding a protease (optional but almost always done).
Precipitating the DNA with an alcohol — usually ice-cold ethanol or isopropanol. Since DNA is insoluble in these alcohols, it will aggregate together, giving a pellet upon centrifugation. This step also removes alcohol-soluble salt.
Refinements of the technique include adding a chelating agent to sequester divalent cations such as Mg2+ and Ca2+, which prevents enzymes like DNAse from degrading the DNA.
Cellular and histone proteins bound to the DNA can be removed either by adding a protease or by having precipitated the proteins with sodium or ammonium acetate, or extracted them with a phenol-chloroform mixture prior to the DNA-precipitation.
If desired, the DNA can be resolubilized in a slightly alkaline buffer or in ultra-pure water.
