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We consider the Poland-Scheraga model of DNA thermal denaturation, where the unzipping of DNA double chain is viewed as a phase transition [1]. We apply polymer field theory to conform that this sort of transition is in fact a first-order one. The order of phase transition is governed by the loop exponent c, which may be calculated from other polymer entropy exponents. We show that DNA chain heterogeneity increases the strength of the first order transition. For dimensionality d = 3 the exponents were obtained from ε = 4 − d expansions up to the fourth order, and also for d = 2 in the case of the so-called quantum gravity the solution is exact. Additionally, presence of obstacles in the space around the molecule (crowded environment) further reinforces the first-order transition. [ABSTRACT FROM AUTHOR]