Subunit Structure of Eutherian Sperm Chromatin

The β-mercaptoethanol induced decondensation of spermatozoon cell nuclei from several Eutherian species has been followed from the intact spermatazoon cell to the solubilized linear unit sperm chromosomal fiber using fluorescence and electron microscopy. Data from nuclease digestion studies in conjunction with electron microscopic evidence indicate that the gross structure of the unit Eutherian sperm chromosomal fiber consists of DNA folded around sperm specific histone multimers spaced regularly along the fiber generating a linear array of sperm nucleosomes connected by short stretches of uncomplexed DNA. The sperm nucleosomes, 80 Å in diameter are separated by 20 Å filaments of DNA. This structure is remarkably similar to the structure of somatic chromatin although the protein components of the two chromosomes are markedly different. It seems likely that chromosomal fibers, similar to those described herein, may be present in the male pronucleus following fertilization.