Question

DNA precipitation out of a mixture of biomolecules can be achieved by treatment with

• A. Isopropanol
• B. Chilled ethanol
• C. Chilled chlroform
• D. Methanol at room temperature

Answer 1

The correct option is B

Chilled ethanol

The correct options are A and B.

Explanation for correct option:

1. DNA is polar because of its phosphate group that make it soluble in water-solvents.
2. Ethanol and isopropanol are comparatively less polar than water.
3. The precipitation is based the electrostatic interactions between cations of salt and phosphates of DNA backbone.
4. On addition of ethanol or isopropanol having low dielectric constant to DNA solution, the medium dielectric constant decreases.
5. Such a modification of the dielectric properties of the solvent changes the fraction of DNA phosphate charge that is neutralized by counter-ions and the Coulomb interaction between the cations and the negatively charged groups increase.
6. When the ionic bonds are strong enough, nucleic acid no longer dissolve in solution and precipitate out of solution.
7. In addition, ethanol or isopropanol induces conformational changes to the nucleic acid structure in a way that the repulsive forces between the negatively charged phosphates within a nucleic acid polymer are so less that inter-helical interactions are possible, allowing the nucleic acids to aggregate.

Explanation for incorrect options:

Option C:

1. Chilled chloroform can only be used for the precipitation of DNA along with phenol.
2. The main function of chloroform is to protect genomic DNA during a disaster.
3. Chloroform increases the efficiency of phenol to denature proteins.
4. Here, chloroform allows proper separation of the organic phase and the aqueous phase and preserves the DNA in the aqueous phase.

Option D:

Methanol alone cannot be used for the precipitation of DNA but along with ninety-five percent alcohol.

Final answer: DNA precipitation out of a mixture of biomolecules can be achieved by treatment with isopropanol and chilled ethanol.