(N/A) Nucleic acids are polymeric macromolecules essential for the origin of life. The secondary structure of a nucleic acid refers to the spatial arrangement of the polynucleotide chain. $DNA$ and $RNA$ are the primary nucleic acids,and they differ significantly in their secondary structures. For example,the secondary structure of $DNA$ consists of two complementary polydeoxyribonucleotide strands that coil around a common axis to form a double-helical structure. This double-helical structure is stabilized by hydrogen bonds between complementary nitrogenous bases and hydrophobic interactions between stacked base pairs.