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

Step 1: Understanding the Question:
The question asks about the net electrical charge carried by a DNA molecule at the pH found inside the body, roughly pH 7.4.

Step 2: Key Concept:
The DNA backbone is built from alternating sugar and phosphate groups joined by phosphodiester bonds. Each phosphate group in this backbone carries an ionizable hydroxyl group with a low pKa, well below physiological pH. At physiological pH these phosphate groups are ionized and each one carries a negative charge.

Step 3: Detailed Explanation:
Since every nucleotide along the DNA strand contributes one negatively charged phosphate group, a full DNA molecule carries a large net negative charge at physiological pH, this is why DNA moves toward the positive electrode (anode) during gel electrophoresis, a routine lab technique that depends on this negative charge.
Option 1, positively charged, is wrong, no group on the DNA backbone carries a positive charge at this pH.
Option 3, neutral, is wrong, the ionized phosphates give DNA a strong negative charge, not a neutral one.
Option 4, amphipathic, describes a molecule with both a water-loving and a water-repelling part in the same structure, like a phospholipid. DNA is uniformly hydrophilic along its charged backbone and does not have this dual character, so amphipathic does not correctly describe its charge property either.

Note on the answer key: The source paper marks option 4 (amphipathic) as correct, but this does not match standard biochemistry. The phosphate backbone of DNA is well established to carry a negative charge at physiological pH, which is also the basis of techniques like gel electrophoresis. This answer has been corrected to option 2 (negatively charged).

Step 4: Final Answer:
At physiological pH, DNA molecules are negatively charged, due to the ionized phosphate groups in the backbone.