In DNA structure, what overall net charge does DNA carry at physiological pH, and what feature of the molecule is primarily responsible for this charge?

• A. Net positive charge, primarily due to protonated nitrogenous bases
• B. Net negative charge, primarily due to the phosphate groups in the sugar-phosphate backbone
• C. Net neutral charge, primarily because deoxyribose sugars are uncharged and dominate the molecule
• D. No net charge, because positive charges on bases exactly cancel negative charges on phosphates

Answer: (B)

Verified step by step guidance

1. Recall that DNA is composed of a sugar-phosphate backbone and nitrogenous bases attached to the sugars.
2. Understand that the sugar (deoxyribose) itself is neutral and does not contribute to the overall charge of DNA.
3. Focus on the phosphate groups in the backbone, which contain negatively charged oxygen atoms at physiological pH (around 7.4).
4. Recognize that the nitrogenous bases can have some protonation states, but at physiological pH, they do not carry a net positive charge sufficient to neutralize the phosphates.
5. Conclude that the overall net charge of DNA at physiological pH is negative, primarily due to the negatively charged phosphate groups in the sugar-phosphate backbone.