What chemical group alternates with sugar in the DNA backbone?

What chemical group alternates with sugar in the DNA backbone? Phosphate alternates with sugar in the DNA backbone.

DNA is a chain of nucleotides, and each nucleotide contains a deoxyribose sugar, a phosphate group, and a nitrogenous base. The phosphate links the 3′ carbon of one sugar to the 5′ carbon of the next sugar through phosphodiester bonds. Repeating sugar and phosphate units form the outer framework of each strand.

The bases—adenine, thymine, cytosine, and guanine—are attached to the sugars rather than forming the backbone. They point toward the center of the double helix, where complementary bases pair: adenine with thymine and guanine with cytosine. This arrangement separates DNA’s structural framework from its information-carrying sequence.

Phosphate groups also give DNA much of its negative electrical charge. The 3′-to-5′ connections create directionality, so strands have distinct 5′ and 3′ ends and run antiparallel to each other. “Carbonate,” “nitrate,” and “sulfate” are chemically familiar groups, but none is the repeating group in normal DNA.