DNA in human and stallion spermatozoa forms local hexagonal packing with twist and many defects

In human and other mammal sperm nuclei, DNA is packed in a highly condensed state, the structure of which remains unsolved. Cryoelectron microscopy of vitrified sections provides a first direct view of the local arrangement o f the nucleoprotamine filament. DNA aligns in parallel in layers and its or ientation rotates along a single-twist direction as in a cholesteric liquid crystal. The structure contains numerous defects, which introduce locally double-twist configurations. Destruction of the SS bonds with dithiotrehito l relaxes the twist and favors the extension of the hexagonal close packing of the filaments, though keeping constant their interfilament distance. (C ) 2001 Academic Press.