  Literature data on the properties of DNA open states are reviewed and
analyzed. These states are formed as a result of strong DNA fluctuations and
have a great impact on a number of biochemical processes; among them is charge
transfer in DNA, for example. A comparative analysis of experimental data on
the kinetics and thermodynamics of DNA open states for a wide temperature range
was carried out. Discrepancies between the results of various experiments have
been explained. Three types of DNA open states are recognized based on their
differences in thermodynamic properties and other characteristics. Besides, an
up-to-date definition of the term "open state" is given. A review is carried
out for simple mathematical models of DNA in most of which the state of one
pair is described by one or two variables. The main problems arising in
theoretical investigations of heterogeneous DNA in the framework of models of
this level are considered. The role of each group of models in interpretation
of experimental data is discussed. Special consideration is given to the
studies of the transfer and localization of the nucleotide pairs oscillations'
energy by mechanical models. These processes are shown to play a key role in
the dynamics of a heterogeneous duplex. Their theoretical interpretation is
proven to be very important for the development of modern molecular biology and
biophysics. The main features of the theoretical approaches are considered
which enabled describing various experimental data. Prospects of the models'
development are described, particular details of their optimization are
suggested, and possible ways of modernization of some experimental techniques
are discussed.
