  We have measured the temperature driven denaturing, or melting transition in
poly d(A)-poly d(T) DNA oligomers of various lengths in different buffer
conditions. Our findings are in clear disagreement with two state, reaction
kinetics model, and we find that the so-called zipper model, where denaturing
proceeds through opening of the duplex at the ends describes well the
temperature dependence of the average number of open base pairs. Analysis of
the length dependence of the transition parameters however suggest that bubble
formation is important and that the transition, in the thermodynamic limit, is
continuous, albeit close to first order.
