The A form occurs under non-physiological conditions in partly dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, and in enzyme-DNA complexes.
A-formen fremkommer under ikke-fysiologiske forhold i delvist dehydrerede DNA-prøver, mens den i cellen kan produceres i hybridparringer af DNA- og RNA-strenge, såvel som i enzym-DNA-komplekser.