In a standard DNA extraction protocol, which of the following is commonly added to precipitate DNA out of aqueous solution?

• A. Detergent such as SDS
• B. Cold ethanol (or isopropanol)
• C. EDTA
• D. Proteinase K

Answer: (B)

Verified step by step guidance

1. Understand the goal of the step in the DNA extraction protocol: to separate DNA from the aqueous solution by making it insoluble so it can be collected.
2. Recall that DNA is soluble in water but becomes less soluble in the presence of alcohols like ethanol or isopropanol, especially when these alcohols are cold.
3. Recognize that adding cold ethanol or isopropanol causes DNA to precipitate because the alcohol reduces the dielectric constant of the solution, decreasing DNA solubility and allowing it to aggregate.
4. Note that detergents like SDS are used earlier in the protocol to lyse cells and solubilize membranes, not to precipitate DNA.
5. Understand that EDTA chelates divalent cations to protect DNA from degradation, and Proteinase K digests proteins, but neither directly causes DNA precipitation.