Low temperatures protect the DNA by slowing down the activity of enzymes that could break it apart. A cell's DNA is usually protected from such enzymes (DNases) by the nuclear membrane which is disrupted by adding detergent. Cold ethanol helps the DNA to precipitate more quickly.
Besides, why does DNA precipitate when isopropanol is added?
Since DNA is insoluble in ethanol and isopropanol, the addition of alcohol, followed by centrifugation, will cause the DNA proteins to come out of the solution. When DNA concentration in the sample is heavy, the addition of ethanol will cause a white precipitate to form immediately.
Why do we use isopropyl alcohol to extract DNA?
It also helps to keep the proteins dissolved in the aqueous layer so they don't precipitate in the alcohol along with the DNA. Ethanol or isopropyl alcohol causes the DNA to precipitate. When DNA comes out of solution it tends to clump together, which makes it visible.