The theory of X-ray diffraction from ideal, rigid
helices allowed Watson and Crick to unravel the
DNA structure, thereby elucidating functions
encoded in it. Yet, as we know now, the DNA
double helix is neither ideal nor rigid. Its structure
varies with the base pair sequence. Its flexibility
leads to thermal fluctuations and allows molecules
to adapt their structure to optimize their intermolecular
interactions. In addition to the double helix
symmetry revealed by Watson and Crick, classical
X-ray diffraction patterns of DNA contain information
about the flexibility, interactions and
sequence-related variations encoded within the
helical structure. To extract this information, we
have developed a new diffraction theory that
accounts for these effects. We show how double
helix non-ideality and fluctuations broaden the
diffraction peaks. Meridional intensity profiles of
the peaks at the first three helical layer lines reveal
information about structural adaptation and intermolecular
interactions. The meridional width of the
fifth layer line peaks is inversely proportional to
the helical coherence length that characterizes
sequence-related and thermal variations in the
double helix structure. Analysis of measured fiber
diffraction patterns based on this theory yields important
parameters that control DNA structure,
packing and function.