[The A-form of DNA: in search of the biological role]

Only recently first direct evidence for the involvement of the A-DNA in the biological processes and structures has been obtained. This was preceded by numerous studies during four decades, which revealed structural features of this second, next to the B-form, DNA conformation, the conditions for the B to A transition to occur in a tube, the requirements to nucleotide sequence. Due to these works the B-A cooperative transition in DNA is now understood as good as the DNA melting. The above physico-chemical studies forced one to think on the "biology" of the A-conformation and at the same time provided experimental possibilities for finding it in biological processes.