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All the DNA extraction protocols I have seen involve adding salts to the extraction buffer. What is the purpose of the salts? What happens if they aren't included?

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The role of the salt is to neutralize the charge of the DNA's sugar phosphate backbone. This makes the DNA less hydrophilic (less soluble in water).

Ethanol has a lower dielectric constant than water so it's used to promote ionic bonds between the Na+ (from the salt) and the PO3- (from the DNA backbone) causing the DNA to precipitate.

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    $\begingroup$ since precipitation is one of the methods to purify biomolecules, which separate them from the other components in the solution. $\endgroup$ – Ctina Feb 10 '12 at 19:59

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