Giant non-universal critical index and fluctuations in DNA phase transition

Thermal unbinding of a double-stranded DNA molecule is a phase transition with a giant (over 100) critical index. The index is non-universal, it changes with concentrations and melting temperatures of base pairs. At slightly lower temperatures it switches to an essential singularity. A finite DNA completely unbinds below critical temperature. The shift from the critical temperature is stochastic. It is inversely proportional to the logarithm of the DNA length, and is observable even in a DNA solution, where the total number of DNA base pairs is, e.g., ∼1020. Its macroscopically large fluctuations are characteristic of DNA unbinding.