Fibres of DNA assume the so called B- Conformation, when the counter ion is an alkali metal such as Na⁺ and the relative humidity is >92%. It is the most stable structure for a random sequence of DNA and is therefore the standard point of reference.

The primary structure of DNA is simple covalent structure and is actually the nucleotide sequence. Secondary structure on the other hand represents any regular, stable structure taken up by some or all of the nucleotides in a nucleic acid. The further coiling and complex folding of large chromosomes within eukaryotic chromatin and bacterial nucleic acids are generally considered tertiary structures.