Why Is Cold Alcohol Used in DNA Tests?

Cold alcohol is used to separate DNA out of water-based solutions. This allows the DNA to be purified for subsequent genetic testing. Adding alcohol to a solution containing DNA is a simple way to obtain the pure DNA required, and colder temperatures slow down enzymes that can break down DNA, giving better extraction results.

Break Down Cell Walls

• Cells walls have to be destroyed to reach the DNA within.

  Extraction procedures to obtain pure DNA have to get rid of all the molecular and chemical components of the tissue from which the DNA is being extracted. First steps involve lysing or destroying the cell walls. This can be done with a variety of chemical agents that are caustic to cell membranes but do not harm the DNA. Cells can also be sonicated, homogenized, or ground up to destroy membranes.

Removing Lipids

• Once the cell walls have been destroyed, a detergent is added to get rid of the fats and oils that make up the cell membranes. Detergents cause the fats and oils to dissolve into the solution.

Remove Proteins

• Proteins and enzymes can be digested by adding a protease to the solution. Proteases break down proteins into small peptides and amino acids.

Precipitate DNA

• Adding cold alcohol to a solution will cause DNA to precipitate and aggregate. The DNA can be collected by centrifuging the sample and pouring off the liquid layer. The DNA should exist as a small pellet in the bottom of the centrifuge tube.

Purify DNA

• The DNA can be washed by re-suspending it in a cold alcohol solution and re-centrifuging it several times to obtain a very pure DNA sample. Typical alcohols used to precipitate DNA include ethanol and isopropanol. This process leaves a very pure sample for stringent DNA testing.