Removal of Unwanted RNA

Special enzymes remove contaminating RNA from a DNA sample. The enzyme ribonuclease degrades RNA into short oligonucleotides but leaves the giant DNA macromolecule unchanged. A mixture of DNA and RNA is first incubated with the ribonuclease at the optimal temperature for enzyme activity. Next, an equal volume of alcohol is added. The alcohol precipitates large macromolecules, including long chains of DNA, out of solution. However, the small RNA fragments remain dissolved. Note that alcohol treatment is not very specific and will precipitate most large carbohydrates and many proteins as well as intact macromolecules of both DNA and RNA. Thus, alcohol precipitation can only be used after these components have been removed from the DNA by centrifugation and phenol extraction. Next the DNA is sedimented to the bottom of the tube by centrifugation and the supernatant solution containing ribonuclease An enzyme that degrades RNA

FIGURE 21.02 Removal of RNA by Ribonuclease

A mixture of RNA and DNA is incubated with ribonuclease, which digests all the RNA into small fragments and leaves the DNA unaltered. An equal volume of alcohol is added, and the larger pieces of DNA are precipitated out of solution. The solution is centrifuged, and the large insoluble pieces of DNA form a small pellet at the bottom of the tube. The RNA fragments remain in solution.

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