Before revision:  At physiological temperatures and superhelical densities, opening is strongly cooperative with average bubble sizes of  O (100-1000) bp. In general, their statistics and location are dominated by sequence heterogeneity.
After revision:  At physiological temperatures and superhelical densities, opening is strongly cooperative with average bubble sizes of  O 10^2-10^3 base pairs (bp), and orders of magnitude higher than in unconstrained DNA. In heterogeneous sequences, the average degree of base-pair opening is self-averaging, while bubble localization and statistics are dominated by sequence heterogeneity.