During the purification process for recombinant DNA technology, addition of chilled ethanol precipitates out :

**Correct answer:** DNA

During recombinant DNA purification, a common step is the addition of chilled ethanol to the aqueous solution containing nucleic acids. The principle behind this step is based on the solubility properties of DNA in the presence of alcohol and salts.

1. DNA is a highly charged polymer. In an aqueous buffer, the phosphate backbone of DNA is negatively charged, making the molecule very soluble in water because it interacts strongly with water molecules.

2. Ethanol reduces the dielectric constant of the solution. When ethanol (especially when chilled) is mixed with the aqueous buffer, the overall polarity of the solvent mixture drops. This weaker polarity lessens the ability of the solvent to shield the negative charges on the DNA.

3. Salt ions promote precipitation. In typical purification protocols, a monovalent cation such as Na⁺ or K⁺ is already present. These cations neutralize some of the negative charges on the DNA, further decreasing its solubility.

4. Resulting precipitation. With reduced solvation and charge shielding, the DNA molecules aggregate and become insoluble. They form a visible white pellet after centrifugation, while smaller, more soluble contaminants (proteins, polysaccharides, RNA) remain in the supernatant or are removed in subsequent washes.

Therefore, the addition of chilled ethanol selectively precipitates DNA out of the solution.