What forms the backbone of a DNA strand?

The backbone of a DNA strand is formed by the sugar and phosphate groups. In the structure of DNA, each nucleotide consists of a sugar molecule, a phosphate group, and a nitrogenous base. The sugar-phosphate backbone is created through covalent bonds between the phosphate of one nucleotide and the sugar of the next nucleotide. This structure provides stability and support, allowing the nitrogenous bases to project outward, where they can form specific base pairs with complementary bases on an opposing strand of DNA.

While the other choices include components important to DNA's structure and function, they do not accurately describe the backbone. For instance, base pairs and hydrogen bonds are crucial for the double helix's integrity, contributing to the overall stability of the DNA molecule, but they do not form the backbone itself. Nucleotides are indeed the building blocks of DNA, and while phosphodiester bonds link the nucleotides together, it is specifically the sugar and phosphate groups that comprise the backbone. Amino acids and proteins are unrelated to DNA's structure; they are the building blocks of proteins and play different roles in cellular function.