The average sperm nucleus is only 8 micrometers long, but contains 1 meter of DNA. We have previously described how the sperm cell is able to package this DNA into the small volume of the sperm head. The only DNA structure that exists in both somatic cells and sperm nuclei is the organization of DNA loop domains that are attached at their bases to the nuclear skeleton, the nuclear matrix. We compared the structure of one gene in sperm nuclei to that of spermatogenic and adult cells.

Individual loop domains of the 5S rRNA gene cluster were visualized by fluorescent in situ hybridization (FISH). Nuclei were isolated from hamster spermatozoa, spermatogonia, pachytene spermatocytes, round spermatids, and adult liver cells. They were then extracted to remove the protamines or histones, fixed, hybridized to a biotinylated 5S rDNA probe, then viewed by epifluorescence microscopy. The 5S rDNA in liver nuclei was organized into a single large loop domain (Fig. 1),