Ethanol precipitation

DNA is typically separated from other cell constituents in a two-phase solution of phenol and water.

[1] This relation is reflected in Coulomb's law, which can be used to calculate the force acting on two charges

At an atomic level, the reduction in the force acting on a charge results from water molecules forming a hydration shell around it.

This fact makes water a very good solvent for charged compounds like salts.

This means that adding ethanol to a solution disrupts the screening of charges by water.

As the mechanism suggests, the solution has to contain positive ions for precipitation to occur; usually Na+, NH4+ or Li+ plays this role .

[3][4] Therefore, good efficiency can be achieved at room temperature, but when possible degradation is taken into account, it is probably better to incubate DNA on wet ice.

Smaller fragments and lower concentrations will require longer times to achieve acceptable recovery.

In such cases use of carriers like tRNA, glycogen or linear polyacrylamide can greatly improve recovery.

Again smaller fragments and higher dilutions require longer and faster centrifugation.

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.

[3][4] After centrifugation the supernatant solution is removed, leaving a pellet of crude DNA.

Finally, the pellet is air-dried and the DNA is resuspended in water or other desired buffer.

It is important not to over-dry the pellet as it may lead to denaturation of DNA and make it harder to resuspend.