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Normal DNA comprises not only its primary structure built of a succession of nucleic bases, and not only its secondary structure which is built of the loose crosslinks between its two strands, but also a tertiary structure involving a specific and mobile coiling, the configuration of which is necessary for transcription and replication.

Winding of double strand DNA around the nucleosome	DNA supercoiling and nucleosomes in a chromosome

Two kinds of interaction have been described at this level:

• activation and blocking of topoisomerase II, an enzyme which allows rapid DNA unwinding via specific cuts and restitution of the tertiary structure.
• intercalation of plane chemical products between DNA twists, thus rendering the DNA molecule stiff and incapable of transcription or replication

Moreover, other mechanisms may interfere with certain molecules (for instance production of free radicals by anthracyclins).