  The thermodynamical stability of DNA minicircles is investigated by means of
path integral techniques. Hydrogen bonds between base pairs on complementary
strands can be broken by thermal fluctuations and temporary fluctuational
openings along the double helix are essential to biological functions such as
transcription and replication of the genetic information. Helix unwinding and
bubble formation patterns are computed in circular sequences with variable
radius in order to analyze the interplay between molecule size and appearance
of helical disruptions. The latter are found in minicircles with $< 100$ base
pairs and appear as a strategy to soften the stress due to the bending and
torsion of the helix.
