  The statistical mechanics of DNA denaturation under fixed linking number is
qualitatively different from that of the unconstrained DNA. Quantitatively
different melting scenarios are reached from two alternative assumptions,
namely, that the denatured loops are formed in expense of 1) overtwist, 2)
supercoils. Recent work has shown that the supercoiling mechanism results in a
BEC-like picture where a macroscopic loop appears at Tc and grows steadily with
temperature, while the nature of the denatured phase for the overtwisting case
has not been studied. By extending an earlier result, we show here that a
macroscopic loop appears in the overtwisting scenario as well. We calculate its
size as a function of temperature and show that the fraction of the total sum
of microscopic loops decreases above Tc, with a cusp at the critical point.
