  We analyse the effects of the environment (solvent quality, presence of
extended structures - crowded environment) that may have impact on the order of
the transition between denaturated and bounded DNA states and lead to changes
in the scaling laws that govern conformational properties of DNA strands. We
find that the effects studied significantly influence the strength of the first
order transition. To this end, we re-consider the Poland-Scheraga model and
apply a polymer field theory to calculate entropic exponents associated with
the denaturated loop distribution. For the $d = 3$ case, the corresponding
diverging $\epsilon = 4-d$ expansions are evaluated by restoring their
convergence via the resummation technique. For the space dimension $d = 2$, the
exponents are deduced from mapping the polymer model onto a two-dimensional
random lattice, i.e., in the presence of quantum gravity. We also show that the
first order transition is further strengthened by the presence of extended
impenetrable regions in a solvent that restrict the number of the macromolecule
configurations.
