  The mechanism of threshold elongation (overstretching) of DNA macromolecules
under the action of external force is studied within the framework of
phenomenological approach. When considering the task it is taken into account
that double-stranded DNA is a polymorphic macromolecule with a set of
metastable states. Accordingly to proposed mechanism, DNA threshold elongation
can take place as a cooperative structural transition of macromolecule to
metastable form, which is stabilized by the external force. For the description
of DNA overstretching, the model included external (stretching) and internal
(conformation) displacement components is constructed. As assumed, both
components are coupled on the pathway of double helix transformations. It is
shown, that under force action DNA deformation proceeds in two stages. First
one is the restructuring of the double helix, which leads to formation of
conformational bistability in DNA macromolecule. On the second stage, the
conformational transition and the deformation induced by it consistently cover
the macromolecule as a threshold process. The comparison of calculated
characteristics of DNA overstretching process with experimental data shows good
agreement. Analysis of the results obtained, together with the available
literature data allow to conclude, that overstretching transition in the double
helix is dynamical process and can spread in DNA chain. In the same time, in
DNA with A$\cdot$T-rich content due to large dissipation the overstretching
process leads to force induced melting transition, and should have nearly
static character.
