This section covers the last major biomolecule class and closes the loop between organic chemistry and molecular biology. Nucleic acids — DNA and RNA — are polymers of nucleotides, and the whole structure is built from three organic pieces students already recognize: a sugar, a phosphate, and a nitrogenous base.

Build the monomer first. A nucleoside is a sugar + a base joined at the sugar's anomeric (C1′) carbon through a β-N-glycosidic bond — the same glycosidic linkage from the carbohydrate track, now connecting to nitrogen instead of oxygen. Add one or more phosphate groups at the sugar's 5′ carbon and it becomes a nucleotide. The naming convention matters: nucleoside = sugar + base; nucleotide = sugar + base + phosphate.

The sugar distinguishes the two nucleic acids. RNA uses ribose; DNA uses 2′-deoxyribose, which lacks the –OH at the 2′ carbon. That single missing hydroxyl makes DNA far more chemically stable (no 2′-OH to catalyze backbone cleavage), which is why the genome is stored as DNA and the working messages are RNA.

The bases come in two structural families straight out of aromatic/heterocyclic chemistry:

Purines — adenine (A) and guanine (G) — are the two-ring (fused bicyclic) bases.
Pyrimidines — cytosine (C), thymine (T), uracil (U) — are the one-ring bases. Thymine is DNA-only; uracil is RNA-only (uracil is simply thymine lacking a methyl group). All are aromatic and planar, which enables base stacking.

The backbone is a repeating sugar–phosphate–sugar–phosphate chain joined by phosphodiester bonds: each phosphate bridges the 3′ carbon of one sugar and the 5′ carbon of the next. This gives every strand directionality — a free 5′ phosphate end and a free 3′ hydroxyl end — and strands are read 5′ → 3′. The phosphate groups are deprotonated and negatively charged at physiological pH, making nucleic acids polyanions (the reason they migrate in electrophoresis and bind cationic proteins).

Finally, complementary base pairing holds the DNA double helix together through hydrogen bonds: A pairs with T (two H-bonds) and G pairs with C (three H-bonds). Because G–C has an extra hydrogen bond, GC-rich regions are more thermally stable. The two strands run antiparallel (one 5′→3′, the other 3′→5′), and the assembled molecule is the right-handed double helix. In RNA, A pairs with U instead of T, and RNA is typically single-stranded but folds on itself via internal pairing.

"The Code Molecules"