Why is 90% ethanol used in DNA extraction?
Answers
Answer:
It is well known that Ethanol has a lower dielectric constant than water, making it to promote ionic bond formations between the Na+ (from the salt) and the PO3- (from the DNA backbone), further, causing the DNA to precipitate.Then, how the ethanol wash of the DNA pellet removes its salts at the end step of DNA isolation Is there any effect of reaction environment on ethanol that it promotes salt addition at sometime and removal at other
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Answer:
First, let's determine whether or not it is needed. First off, you’ll find that there are dozens of procedures for DNA isolation, and that you can typically use either ethanol and/or isopropanol to produce crude extracts of DNA from samples of ground or macerated tissue. There is a slight advantage thought to be gained by using ethanol, however, since proteins will often co-precipitate when using isopropanol.
Second, let's talk about why you use 90% ethanol, rather than any other percentage. For starters, there is always going to be residual water in the sample you are extracting DNA from, so the exact particulars of a given percentage really don't matter- there's no advantage or particular disadvantage to using 91% or 89%. The DNA is usually suspended in a solution of salt and detergent first, so you are going to extract the DNA from water after water-insoluble components are removed or rendered insignificant.
So if a high concentration of ethanol is needed relative to any water present in order to maximize precipitation of DNA from solution, you'd want to start with the highest concentration ethanol to maximize your yield.
That said, why 90%, and not 100%? First off, 90% (also called 180 proof) is fairly cheap and can often be purchased at your corner liquor store, sold as “Everclear”. This is near the purity limit of ethanol: when distilling mixtures of ethanol and water, it is not possible to create a distillate purer than about 95%. You can make anhydrous ethanol, if you use additional purifying techniques, but as you go purer the price per gallon escalates.
As a practical matter for usage of readily available off the shelf reagents, 90% is widely available and sufficient.
Needless to say, if you have 100% on the shelf, that should be used instead. It would be absurd to first dilute it with water since you are going to be adding it TO water later. Knowing when substitutions like this are appropriate in a lab procedure is an important aspect of the practical performance of biotech research and is why biotech research can't be learned purely from books. You have to experience it.