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Deoxyribonucleic acid (DNA) precipitation is a key step in the isolation and purification of genetic material in science. Generally, a sample of biological tissue contains DNA or RNA along with the rest of the organism body.To test the DNA, a scientist has to separate out the DNA from all the other substances. DNA precipitation refers specifically to a step that involves the separation of dissolved DNA from the liquid it is dissolved in. Common methods of DNA precipitation include addition of ethanol, ispropanol or glycogen to the liquid, which makes the DNA solidify in lumps and fall to the bottom of the liquid sample.
Initial steps in the purification of DNA from a sample can be as simple as crushing up leaves in a bowl to break down some of the structure. Then the mash can be broken down with chemicals or enzymes that leave the DNA intact. Commonly, geneticists use a centrifuge to help split up different components of a sample. This is a machine that spins a sample so that the heaviest component sinks to the bottom, and the lightest rises to the top.
By removing various unwanted components, the geneticist is commonly left with a clear liquid that contains the genetic material. He or she then needs to take out the DNA dissolved in that liquid, and discard the liquid and the other substances in the liquid. DNA precipitation is the way in which this is achieved. Most often, the scientist needs to add a chemical to the liquid perform DNA precipitation.
Ethanol or isopropanol, which are both forms of alcohol and fall into the solvent group of chemicals, are the most common chemicals used for DNA precipitation. Glycogen is another substance that can precipitate DNA, but it is less commonly used, apart from precipitating low concentrations of genetic material. When these chemicals mix with dissolved DNA, their chemistry allows them to alter the way DNA fits into its environment. Whereas before, the DNA mixed easily with the liquid, after the chemical addition, it stops bonding with the liquid and instead forms into a solid.
This solid is normally whitish and clumps together. As some of the solid is still in small particles, however, the scientist usually places the sample into a centrifuge to spin all the solids together into a pellet at the bottom of the sample tube. This is the purified form of the DNA originally present in the sample, which is useful for testing. Generally, the liquid the pellet is suspended in is removed from the tube, and the pellet may also be dried to allow the chemicals to evaporate off, in order to render the pellet as pure as possible.