Nucleic acid precipitation-Molecular Biology

Hi!

Would anyone out there be able to tell as to why alchohols like isopropanol or ethanol are used to precipitate nucleic acids,and also,why is the precipitation done at low temperatures

ramesh

-ramesh^-

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Precipitation is probably easier at lower temperatures because of less Brownian motion of the molecules?

-Cthulhu^-

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I think DNA is not soluble in alcohol because this kind of substance cannot shield the high charge of phosphate groups of the backbone.Water,a molecule that is more polar and has good capacity of shielding charged or polar groups,is more efficient to do that.When you increased the alcohol concentration in water,the DNA becomes progressively less shielded and eventually,precipitate.A good point to note here is that by adding salt,you accentuate a lot this effect (salt ions sequester water molecules wich become less available to solubilize DNA).

For the temperature,it is simply because it reduce the solubility (in general)of dissolved substances.So,it helps to precipitate things (DNA,and unfortunately,salts...).Someone else has given a more in-depht point of view for this explanation.

-Jof^-

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Regarding temperature dependence of solubility it depends on whether it is endo^- or egzotemic process.In the former case,solubility incrises with temperature.

Regards

-Szymek^-

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ok i remember something about...

DNA is soluble in water because the water molecules intercalate into the phosphate backbone of the DNA and thus maintain it in a soluble state.When you add salt and alcohol you are essentially removing the water molecules and teh DNA then precipitates.

-ros^-

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The differences in the Dielectric constant of alcohol and water are responsible for the precipitation of nucleic acids.

-niks80-

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I'll quote some information I printed off a while back:

"Ethanol's task is a little more complex than "removing" the water.For a precipitation,you're interested in forming ion pairs between the polyanion (DNA)and the cation (Na⁺,Mg²⁺,spermidine,protamine,etc).In dilute aqueous solution,DNA and counterions like Na⁺ and Mg²⁺ are more or less in the free ion form rather than the ion pair form (that is,they are surrounded by one or more layers of water molecules).

Water has a high dielectric constant (e),which from Coulomb's Law tells us that the electrostatic force (F)between two ions of opposite charge is very low in water:

F = (Q1 * Q2)/(e * r^2)

where Q is the charge on each ion and r is the distance between them.Adding organic solvent *decreaseundefined the dielectric constant of the solution.As e goes down,F goes up and *BANundefined,anion and cation form an ion pair and promptly swoon out of solution."

-lillywhite^-

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Also the ethanol takes away the hydradtion shell around the DNA,add a cation like Na⁺ or K⁺ and you not only take away the hydration shell,but also stablize the phosphate groups by making them interact with an opposing charge instead of water.

-nmstew^-

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