The Total Structure and Organization of Chromosomal Fibers in Eutherian Sperm Nuclei

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Abstract
The structural basis of the large anomalous circular dichroism spectrum observed for highly native chromosomal fibers released from the heads of equine sperm was investigated through physico-chemical and theoretical means. Linear dichroism measurements on the chromosomal fibers indicate an orientation of the DNA helix axis perpendicular to the fiber length. The fibers respond as cholesteric liquid crystals to reflectance spectroscopy with a characteristic wavelength of maximum reflectivity (310 nm). The circular dichroism spectrum calculated for a liquid crystal model of the ordering of DNA molecules is in excellent agreement with the spectrum observed for the chromosomal fibers. Dissolution of the fibers gives the same circular dichroism as that of the chromatin of eukaryote somatic cells. Collectively these facts allow formulation of a model for the total structure of equine sperm chromosomal fibers. DNA molecules maintained in the B-form secondary structure are compacted by coiling into a superhelical tertiary structure. These ordered molecules are aligned parallel to each other in planes, each plane slightly rotated as they stack upon each other, forming a quaternary cholesteric liquid crystal arrangement with a right-hand screw direction and a pitch of 170 .ANG.. A scheme for the cellular process of the structuring of eutherian sperm nuclei is discussed.