Why does the ethanol used to isolate DNA from a kiwi, for example, have to be ice cold?

Answer

There are two reasons for this. First, nucleic acids will flocculate better at lower temperature (in ethanol, in which they are not soluble). As a result, they form larger complexes that pellet more easily after centrifugation. Lower temperature thus helps to precipitate DNA, which is important for its separation.

Secondly, when cells (of the kiwi, for example) burst open, enzymes are also released that can break down DNA. These enzymes are less active at lower temperatures. DNA therefore remains better intact at lower temperatures.