Determination of DNA cooperativity factor.

The paper presents measurements of the difference in the melting temperature of a colE1 DNA region when it is located inside the DNA helix and at its end. A direct comparison of calculations based on the rigorous theory of helix-coil transition with experimental data for .2 M Na+ (the conditions for fully reversible melting) yielded the value of 2.5-5x10(-5) for the cooperatively factor sigma. We discuss the reversibility of DNA melting and the possibility of applying the "all-or-nothing" concept to the melting of DNA regions