Step 1: Overall nature. Nucleic acids are polymers of nucleotides and are responsible for storing and passing on genetic information. The two members are DNA and RNA.
Step 2: Building blocks. On complete hydrolysis a nucleic acid gives three products: a pentose sugar, a set of heterocyclic nitrogen bases, and phosphoric acid. The sugar is 2-deoxyribose in DNA and ribose in RNA. The bases are the purines adenine and guanine and the pyrimidines cytosine, thymine (DNA) or uracil (RNA).
Step 3: How the units are assembled. First a base links to C-1 of the sugar to give a nucleoside; then phosphoric acid esterifies the C-5 hydroxyl of that sugar to give a nucleotide. These nucleotides are the true repeating units.
Step 4: Backbone. Nucleotides connect through 3',5'-phosphodiester bonds, producing an alternating sugar-phosphate backbone from which the bases hang. This ordered sequence of bases is the primary structure.
Step 5: Double helix. In DNA two strands twist about a common axis as a right-handed double helix. They lie antiparallel and are zipped together by hydrogen bonds obeying base pairing rules: A with T and G with C. This keeps the base composition complementary between the strands.
Step 6: RNA. RNA normally exists as a single strand, uses ribose, and carries uracil instead of thymine; it mainly helps in protein synthesis.
\[\boxed{\text{Sugar-phosphate backbone with paired bases; DNA is a double helix, RNA single-stranded}}\]