by 
bhagu
ANSWERS: 2
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you need to use an alcohol and ethanol is relatively non-toxic. the alcohol does two things, changes the ionic potential of the DNA and removes water molecules. this helps the DNA to precipitate
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Ethanol has a lower dielectric constant than water (it is less electronegative by a fair margin), so when mixed with a pure water/ DNA sample, it effectively lowers the dielectric constant of the solution. When monovalent cations are also added to the solution, the hydration shell that is keeping DNA dissolved (water molecules bonded to and surrounding the dissolved DNA) is replaced by ionic (salt bonds) and the DNA molecule is knocked out of solution (precipitates). This is how ethanol (working with cations) works to precipitate DNA.
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