DNA is deoxyribonucleic acid, and its secondary structure is a double helix with two polynucleotide chains coiled in reverse parallel, with the following specific features. 1. Right-handed double helix structure, two polynucleotide chains are reverse parallel. 2. the sugar-phosphate backbone of the polynucleotide chain is located on the outside of this double helix, while the bases are located on the inside. 3. there is base complementarity between the two chains, connected by hydrogen bonds. 4. the base planes are nearly perpendicular to the longitudinal axis of the helix. 5. the axial distance between neighboring base pairs is 0.34 nm, the axial distance of each helix is 3.4 nm, and each helix loop contains 10 base pairs. The primary structure of DNA is single-stranded, while the tertiary structure is a super-helical structure, i.e., a double helix based on the structure of the re-helix, DNA carries the genetic information necessary for the synthesis of RNA and proteins.