The primary structure of a DNA molecule is a double helix, consisting of two strands of nucleotides that wind around each other. Each nucleotide comprises a phosphate group, a sugar, and a nitrogenous base. The complementary base pairing (adenine-thymine and cytosine-guanine) ensures accurate replication and transcription, essential for genetic fidelity. This structure enables DNA to carry genetic information, control cellular functions, and play a critical role in heredity and evolution.