  Recent advances in the understanding of the melting behavior of
double-stranded DNA with statistical mechanics methods lead to improved
estimates of the weight factors for the dissociation events of the chains, in
particular for interior loop melting. So far, in the modeling of DNA melting,
the entropy of denaturated loops has been estimated from the number of
configurations of a closed self-avoiding walk. It is well understood now that a
loop embedded in a chain is characterized by a loop closure exponent c which is
higher than that of an isolated loop. Here we report an analysis of DNA melting
curves for sequences of a broad range of lengths (from 10 to 10^6 base pairs)
calculated with a program based on the algorithms underlying MELTSIM. Using the
embedded loop exponent we find that the cooperativity parameter is one order of
magnitude bigger than current estimates. We argue that in the melting region
the double helix persistence length is greatly reduced compared to its room
temperature value, so that the use of the embedded loop closure exponent for
real DNA sequences is justified.
