In phenol-chloroform extraction in which phase can DNA remain?

Correct answer: aqueous phase / Aqueous phase

The direct answer is D: DNA usually remains in the aqueous phase. In phenol-chloroform extraction, the mixture separates into an upper water-based aqueous phase, a lower organic phase containing phenol and chloroform, and sometimes an interface containing denatured proteins and debris. DNA is negatively charged and, under the usual buffered conditions, is soluble in the aqueous layer. Option D is correct. Option A is wrong because DNA is not normally restricted to the organic phase; that phase mainly receives lipids and many hydrophobic substances. Option B is wrong because the interface mainly contains precipitated or denatured protein, although DNA can be lost there if the sample is handled badly. Option C is wrong because DNA is not normally a solid protein pellet in this step. After carefully collecting the aqueous layer, DNA may later be precipitated with alcohol. Memory cue: water-loving nucleic acids stay in the water layer.