An Endogenous Nuclease in Hamster, Mouse, and Human Spermatozoa Cleaves DNA into Loop‐Sized Fragments

Recent work from our laboratory provided evidence for the existence of a nuclease in hamster spermatozoa. This endogenous nuclease cleaves sperm chromatin at the bases of DNA loop domains into large fragments with an average size of roughly 50 kb. Here, we demonstrate that this sperm nuclease is present in the sperm nucleus and that it is activated by the presence of both calcium and magnesium much more efficiently than with either ion alone, resulting in DNA degradation in 30 minutes. We also show that similar nucleases are present in mouse and human spermatozoa. The human nuclease can be activated by freeze-thawing spermatozoa in noncryoprotective media. The activity of the sperm nuclease in all 3 species resembles that of a group of somatic cell DNAses that also require both calcium and magnesium and that digest the chromatin into loop-sized fragments during apoptosis.