  Analysis of single-molecule micromanipulation experiments of DNA unzipping
process shows some features of the force-distance curve, namely two consequent
plateaus in the area of ${\sim}12-14pN$ dependent on nucleotide sequence
structure, as well as peaks appearance in the plateau area, to which it was not
paid essential attention earlier. Using atom-atom potential function method the
estimations of Watson-Crick base pairs opening energies are made. On the basis
of this results two possible scenarios of the DNA double helix unzipping
process are proposed. According to the first scenario DNA unzipping takes place
slowly and as equilibrium process, with small difference between two plateaus
on the unzipping curve. In this case firstly base pairs transit into the
'pre-opened' metastable state along the 'opening' pathway and then open along
the 'stretch' pathway. Our estimations show that an important factor for the
realization of this scenario is the existence of double-stranded DNA coil in
the unopened part of DNA. The second scenario is characterized by higher
opening force. In this scenario base pairs open directly along the 'stretch'
pathway as non-equilibrium process. The conditions of the first scenario
realization show that it can play a key role in the understanding of the DNA
unzipping in vivo during transcription and genetic information transfer
processes.
