Primary, Secondary, Tertiary Structure of Nucleic Acids

1. The primary structure is the sequence of nucleoside monophosphates (usually written as the sequence of bases they contain).

2. The secondary structure refers to the shape a nucleic acid assumes as a result of the primary structure. B-DNA, A-DNA, and Z-DNA are forms of secondary structure (see here and here). B-DNA is the form that predominates in the aqueous environment of the cell.

3. Tertiary structure refers to large-scale folding in a linear polymer that is at a higher order than secondary structure. The tertiary structure is the specific three-dimensional shape into which an entire chain is folded.