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You are here: Experts > Science > Biology > Molecular Biology > molecular biology
Expert: Adam Ross - 11/1/2009
Question
what is the use of isopropyl alcohol and NaCl in salting out method of DNA extraction?
thanks for taking time to respond to my question.
Suvagata Roychowdhury
Calcutta School Of Tropical Medicine
Calcutta, India.
Answer
Suvagata,
There is a very nice wikipedia page on salting out and the use of NaCl. The salt take proteins but not DNA out of the mix so that you are left with just DNA. Protein solubility depends upon the particular amino acid side chains that are exposed on their surfaces that are in contact with an aqueous environment, proteins are generally considerably more hydrophobic than DNA. The uniform phosphate backbone of DNA that is in contact with its solvent creates a high ionic charge that provides for high solubility in most aqueous environments. Although proteins do have some level of ionic charge on their surfaces as well, there are also many other hydrophobic amino acid side chains that cause most proteins to eventually precipitate from a solution of high enough ionic strength, since "like dissolves like". Conversely, it is interesting to note that the high ionic charge of its phosphate backbone is also what allows DNA to be precipitated by the more hydrophobic environment of alcohol solvents. The basic chemistry premise of "like dissolves like" is worth remembering when faced with general questions of solubility.
The isopropanol is used as a wash to extract the DNA and there is also a wiki article that will hold more information than I can type here. But in short because DNA is completely soluble in water, but not in alcohol, like isopropanol, when isopropanol is added, its engaged more and more water molecule to interact, as a result, less water molecules are available to dissolve DNA, and DNA starts to precipitate out.
http://en.wikipedia.org/wiki/DNA_extraction
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