  The equilibrium statistical properties of DNA denaturation bubbles are
examined in detail within the framework of the Peyrard-Bishop-Dauxois model.
Bubble formation in homogeneous DNA is found to depend crucially on the
presence of nonlinear base-stacking interactions. Small bubbles extending over
less than 10 base pairs are associated with much larger free energies of
formation per site than larger bubbles. As the critical temperature is
approached, the free energy associated with further bubble growth becomes
vanishingly small. An analysis of average displacement profiles of bubbles of
varying sizes at different temperatures reveals almost identical scaled shapes
in the absence of nonlinear stacking; nonlinear stacking leads to distinct
scaled shapes of large and small bubbles.
