  We use the "magnetic tweezers" technique to reveal the structural transitions
that DNA undergoes in the force-torsion space. In particular, we focus on
regions corresponding to negative supercoiling. These regions are characterized
by the formation of so-called denaturation bubbles, which have an essential
role in the replication and transcription of DNA. We experimentally map the
region of the force-torsion space where the denaturation takes place. We
observe that large fluctuations in DNA extension occur at one of the boundaries
of this region, i.e., when the formation of denaturation bubbles and of
plectonemes are competing. To describe the experiments, we introduce a suitable
extension of the classical model. The model correctly describes the position of
the denaturation regions, the transition boundaries, and the measured values of
the DNA extension fluctuations.
