Why is the DNA Denaturation Transition First Order?

Thermal denaturation or melting of double-stranded DNA is the process by which two strands unbind upon heating. The nature of the transition has been investigated for almost four decades. Experimentally the melting curve exhibits a multistep behaviour consisting of plateaus with sharp jumps. These jumps have been attributed to the unbinding of domains characterised by different frequencies of AT and GC pairs. Simple theoretical models studied for the last thirty years predict the transition to be continuous in two and three dimensions. In our work we consider analytically the effects of excluded volume interaction between unbound loops and the rest of the chain. The phase transition is now found to be first order in dimensions two and above.