Functional aspects of mammalian sperm chromatin

During spermiogenesis, the histones in the round spermatid, which bind the DNA in a relatively open configuration leaving much of the DNA exposed, are replaced by the much more tightly binding protamines, which package the DNA into highly condensed toroids. This knowledge has led to models of sperm chromatin structure that characterize the DNA as highly condensed and biologically inert, a view that is substantiated by the lack of detectable mRNA transcription or DNA replication in mature sperm nuclei. However, this view of sperm chromatin structure has been challenged recently by new data from several laboratories, including our own. These data indicate that mammalian sperm nuclei can interact with exogenous DNA and undergo sperm-specific apoptosis. We propose a model for sperm chromatin structure that provides a mechanism by which the tightly compacted mammalian sperm chromatin may be capable of these activities.