In mammals, the sperm nucleus contains a unique structure, the nuclear annulus . The DNA of the sperm nucleus appears to be anchored to this annulus. The DNA has been compacted by the replacement of the nuclear histones by sperm-specific protamines during spermatogenesis. This is much tighter than the normal solenoid loops typical of somatic cell nuclei. In somatic cells, DNA is compacted into solenoids with about 6 nucleosomes per turn, and they are attached to the nuclear matrix at intervals of about 60,000 base pairs.
Active genes tend to be associated with the nuclear matrix.
Each doughnut represents one DNA loop attached to the nuclear matrix. Formation of disulfide bonds in the nucleus and accessory structures of mammalian spermatozoa during maturation in the epididymis. Perreault, S. D., Wolff, R. A., and Zirkin, B. R. 1984. The role of disulfide bond reduction during mammalian sperm decondensation in vivo. Perreault, S. D., Barbee, R. R., and Slott, V. L. 1988.