In a standard DNA extraction protocol, what is the primary purpose of adding cold alcohol (ethanol or isopropanol) to the aqueous lysate?

• A. To lyse cell membranes by dissolving phospholipids and releasing DNA into solution
• B. To precipitate DNA out of solution so it can be collected as a visible pellet or strands
• C. To denature and digest proteins by breaking peptide bonds
• D. To cleave DNA into smaller fragments for easier amplification

Answer: (B)

Verified step by step guidance

1. Understand the context: In a DNA extraction protocol, after lysing cells and releasing DNA into an aqueous solution, the goal is to isolate the DNA from other cellular components.
2. Recall the properties of DNA: DNA is soluble in water but insoluble in alcohol, especially when the alcohol is cold and mixed with salts present in the lysate.
3. Recognize the role of cold alcohol: Adding cold ethanol or isopropanol decreases the solubility of DNA, causing it to aggregate and come out of the aqueous solution.
4. Identify the outcome: This aggregation allows DNA to be seen as a visible pellet or strands, which can then be physically collected by centrifugation or spooling.
5. Distinguish from other options: Cold alcohol does not lyse cells, denature proteins, or cleave DNA; its primary function is to precipitate DNA for isolation.