<P> The same mechanism operates in the case of negatively charged phosphate groups on a DNA backbone: even though positive ions are present in solution, the relatively weak net electrostatic force prevents them from forming stable ionic bonds with phosphates and precipitating out of solution . </P> <P> Ethanol is much less polar than water, with a dielectric constant of 24.3 (at 25 ° C). This means that adding ethanol to solution disrupts the screening of charges by water . If enough ethanol is added, the electrical attraction between phosphate groups and any positive ions present in solution becomes strong enough to form stable ionic bonds and DNA precipitation . This usually happens when ethanol composes over 64% of the solution . As the mechanism suggests, the solution has to contain positive ions for precipitation to occur; usually Na, NH or Li plays this role . </P> <P> DNA is precipitated by first ensuring that the correct concentration of positive ions is present in solution (too much will result in a lot of salt co-precipitating with DNA, too little will result in incomplete DNA recovery) and then adding two to three volumes of at least 95% ethanol . Many protocols advise storing DNA at low temperature at this point, but there are also observation that it does not improve DNA recovery, and may even lower precipitation efficiency while using over-night incubation time . In most cases, good efficiency is achieved at room temperature but when possible degradation is taken into account it is probably best to incubate DNA on wet ice . Optimal incubation time depends on the length and concentration of DNA . Smaller fragments and lower concentrations will require longer times to achieve acceptable recovery . For very small lengths and low concentrations over-night incubation is recommended . In such cases use of carriers like tRNA, glycogen or linear polyacrylamide can greatly improve recovery . </P> <P> During incubation DNA and some salts will precipitate from solution, in the next step this precipitate is collected by centrifugation in a microcentrifuge tube at high speeds (~ 12,000 g). Time and speed of centrifugation has the biggest effect on DNA recovery rates . Again smaller fragments and higher dilutions require longer and faster centrifugation . Centrifugation can be done either at room temperature or in 4 ° C or 0 ° C. During centrifugation precipitated DNA has to move through ethanol solution to the bottom of the tube, lower temperatures increase viscosity of the solution and larger volumes make the distance longer, so both those factors lower efficiency of this process requiring longer centrifugation for the same effect . After centrifugation the supernatant solution is removed, leaving a pellet of crude DNA . Whether the pellet is visible depends on the amount of DNA and on its purity (dirtier pellets are easier to see) or the use of co-precipitants . </P>

What solution helps the dna to come out of solution or to precipitate
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