The analysis of the elementary processes of fragmentation (broken sub-boundaries and a misorientation band formation) is performed by the discrete dislocation computer simulation method. It is shown that a fragmentation driving force is a decrease of the system energy due to the dislocations redistribution in the elastic field of primary mesodefects (disclinations, dipoles of disclinations et al.) accumulating on the grain boundaries and triple junctions during plastic deformation. The comparison of the results with the results obtained by continual approach is made. The misoriented structures formation inside shear bands during plastic deformation of mono--crystals is considered. It is shown that this process is similar to the polycrystal fragmentation, but in this case dislocation clusters accumulating near shear band interface act as the primary mesodefects. An analysis of conditions of low-angle sub-boundaries trasformation into medium-angle boundaries is performed.