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I have read that DNA can be concentrated by addition of isopropanol.

What does "concentrated" mean? What does isopropanol do on a molecular level to concentrate DNA?

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What you are asking about is the precipitation of DNA (or any other nucleic acid) by isopropanol (or ethanol, which is more common). To do so, you add salt (usually slightly acidic sodium acetate) which makes sure that the phosphate backbone of the DNA is saturated with sodium ions to make it less soluble. Then you add the organic solvent, which precipitates the DNA from the solution by changing the polarity of the solution. This makes the ions form salts and the DNA precipites. The principle is explained in the Wikipedia article on ethanol precipitation. Finally the solution is centrifuged to collect the DNA at the bottom of your reaction tube and to be able to take off the supernatant.

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Small addition: Isopropanol having a bigger alkyl group makes the solution more organic in nature compared to ethanol. Why isopropanol and not n-propanol is perhaps because it is easy to manufacture isopropanol. n-propanol is better at precipitating smaller nucleic acids. –  WYSIWYG Mar 26 at 13:37
    
It is because isopropanol is less toxic in the handling than n-propanol. n-propanol can cause severe eye damage and is generally more toxic than ethanol or isopropanol. –  Chris Mar 26 at 16:47
    
Point taken. but it is also easy to manufasture isopropanol :P –  WYSIWYG Mar 27 at 4:14
    
Not to mention every molecular bio lab has isopropanol on hand. –  leonardo Mar 27 at 13:31
    
@Chris.. I am not sure why you say n-propanol is more toxic. It can be easily metabolized to propionic acid. This article says that isopropanol is more toxic in terms of longlasting effects. –  WYSIWYG Mar 27 at 13:46
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