  Despite numerous attempts, the understanding of the thermal denaturation of
DNA is still a challenge due to the lack of structural data at the transition
since standard experimental approaches to DNA melting are made in solution and
do not provide spatial information. We report a measurement using neutron
scattering from oriented DNA fibres to determine the size of the regions that
stay in the double-helix conformation as the melting temperature is approached
from below. A Bragg peak from the B-form of DNA has been observed as a function
of temperature and its width and integrated intensity have bean measured. These
results, complemented by a differential calorimetry study of the melting of B
DNA fibres as well as electrophoresis and optical observation data, are
analysed in terms of a one-dimensional mesoscopic model of DNA.
