Why is the DNA denaturation transition first order?

We study a model for the denaturation transition of DNA in which the molecu les are considered as being composed of a sequence of alternating bound seg ments and denaturated loops. We take into account the excluded-volume inter actions between denaturated loops and the rest of the chain by exploiting r ecent results on scaling properties of polymer networks of arbitrary topolo gy. The phase transition is found to be first order in d = 2 dimensions and above, in agreement with experiments and at variance with previous theoret ical results, in which only excluded-volume interactions within denaturated loops were taken into account. Our results agree with recent numerical sim ulations.