Question

At physiological pH, DNA molecules are:

• A. Positively charged.
• B. Negatively charged.
• C. Neutral.
• D. Amphipathic.

Hint:

The phosphate backbone of DNA is ionized at physiological pH, giving DNA a net negative charge.

Answer 1

The Correct Option is B

This question is about the charge state of DNA at the pH found inside living cells, close to pH 7.4. The backbone structure of DNA holds the answer.

1. Positively charged: The DNA backbone has no group that carries a positive charge at this pH, so this option does not fit.
2. Negatively charged: DNA is built from a repeating chain of sugar and phosphate units. Each phosphate group has an acidic hydroxyl with a low pKa, so at physiological pH it loses its proton and becomes negatively charged. With one such phosphate for every nucleotide, the whole strand carries a large net negative charge. This is the property that makes DNA move toward the positive electrode during gel electrophoresis, a technique used every day in molecular biology labs.
3. Neutral: This would only be true if the phosphates were not ionized, but at physiological pH they are, so DNA is far from neutral.
4. Amphipathic: This term describes molecules like phospholipids, which have one part that likes water and one part that avoids it. DNA's backbone is uniformly charged and water loving all along its length, it does not have this split character, so amphipathic is not the right description of its charge.

The source answer key for this paper lists amphipathic as correct, but that does not match standard biochemistry. The ionized phosphate backbone giving DNA a net negative charge is basic, well established chemistry, so the correct choice here is negatively charged, and this has been corrected accordingly.

Let's summarize:

• The phosphodiester backbone of DNA carries ionized phosphate groups at physiological pH.
• This gives DNA a strong net negative charge, the basis for techniques like gel electrophoresis.

So at physiological pH, DNA molecules are negatively charged.