What is the primary structure of DNA?

The primary structure of DNA is indeed defined by a double helix composed of nucleotides. This structure consists of two long strands of nucleotides that coil around each other, forming the iconic helical shape. Each nucleotide in DNA is made up of three components: a phosphate group, a five-carbon sugar (deoxyribose), and a nitrogenous base (adenine, thymine, cytosine, or guanine).

The double helix configuration allows for the bases on one strand to pair with the complementary bases on the opposite strand, facilitating the storage and transmission of genetic information. This feature is critical for DNA replication and the accurate expression of genes.

In contrast, other structures mentioned in the options do not correctly represent DNA. A single strand of nucleotides would relate to RNA, while a linear sequence of amino acids represents the structure of proteins, and a sphere made up of lipids is typically associated with cellular membranes rather than genetic material. Thus, the double helix formed by nucleotides is the defining characteristic of DNA's primary structure.