DNA carries which overall electrical charge under physiological conditions, and what structural feature is primarily responsible?

• A. Overall neutral charge because base pairing cancels all charges in the helix.
• B. Net negative charge due to the phosphate groups in the sugar-phosphate backbone.
• C. Net positive charge due to the deoxyribose sugars in the backbone.
• D. Net positive charge due to the amino groups on the nitrogenous bases.

Answer: (B)

Verified step by step guidance

1. Understand that DNA is a polymer made up of nucleotides, each consisting of a sugar, a phosphate group, and a nitrogenous base.
2. Recall that the sugar-phosphate backbone of DNA contains phosphate groups, which carry a negative charge due to their ionized oxygen atoms under physiological pH.
3. Recognize that the nitrogenous bases and sugars do not contribute a net positive charge that outweighs the negative charges from the phosphate groups.
4. Conclude that the overall electrical charge of DNA under physiological conditions is negative, primarily because of the negatively charged phosphate groups in the backbone.
5. Note that this net negative charge is important for DNA interactions with proteins and other molecules in the cell.