what is the difference between 100%ethanol and 70% ethanol - (Aug/21/2007 )

hi, guys, i am doing DNA purification, i have several questions regarding to my lab

1)what is the difference between 100%ethanol and 70% ethanol
seems like 100%ethanol is used to precipitate DNA while 70% ethanol is for wash salt but why and how does it work?

thanks!

DNA is very soluble in water so to precipitate the DNA out of solution you need to change the properties of the solution to one where DNA is less soluble. The precipitation protocol involves 3 steps to reduce the solubility of the solution to make the DNA come out of solution:

1. Ethanol is added to the solution because DNA is poorly soluble in ethanol. Normally 2 to 2.5 volumes of 100% ethanol are added, which gives a final concentration of around 70%.
2. Charged groups/atoms give molecules solubility. In DNA, the negatively charged phosphate backbone is a major contributer to its solubility. Adding salt (a.g. NaAc) to the solution shields the charges on the DNA phosphate backbone because the Na+ will bond with the phosphate group. This reduces the solubility of the DNA
3. Lowering the temperature further reduces the solubilty as everything is less soluble at lower temperatures.

The combination of these things reduces the solubility of the DNA to the point that it precipitates as a solid out of solution and can be pelleted.

After pelleting, the DNA is washed with 70% ethanol to remove some (or ideally all) of the salt from the pellet. If water was used as the wash then DNA would dissolve again and if 100% ethanol was used the salt would not wash off because sodium salts are poorly soluble in ethanol.

thanks for help, but i still do not understand that why DNA favor on water not ethanol, since both has OH bond,