Many biological macromolecules will not or cannot crystallize. However, an important group of fibrous macromolecules such as DNA or many of the components of the cytoskeleton form orientated fibers in which the axes of the long polymeric structures are parallel to each other.
Fibers show helical symmetry rather than the three-dimensional symmetry taken on by crystals.
By analysing the diffraction from orientated fibers one can deduce the helical symmetry of the molecule and in favourable cases one can deduce the structure. By comparing the calculated and observed diffraction patterns one eventually arrives at a better model. The micro-crystals are randomly arranged around this axis. All Bragg reflexions are registered at one time. However, particularly at high resolution the Bragg reflexions tend to fall on top of each other.