what is the function of salts in DNA extraction?
DNA can be purified for DNA sequencing or for restriction digestion by precipitation in an alcohol/water mixture in the presence of a high concentration of inorganic salt. DNA is recovered from the aqueous solution by addition of salt to final concentrations of 0.8M LiCl, 0.3-0.5M NaCl, NaOAc, or 2.5M NH4Ac and an appropriate volume of alcohol (30%-50% final percentage isopropanol; 60%-80% final percentage ethanol), storage for a brief period of time at -20°C or -70°C, followed by centrifugation. Subsequent desalting of the DNA pellet involves rinsing in 70% alcohol, recentrifugation and re-suspension in appropriate buffer.
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, to extract DNA finally..
