Figure 6. Human sperm have endogenous nuclease that is activated more easily in frozen sperm. Samples from 2 donors were Percoll purified and mixed. This suspension was then separated into 2 aliquots. One aliquot (Fresh) was further separated into 5 samples and treated with or without Mg²⁺ and Ca²⁺, Triton X-100 (TX), or EDTA, as indicated (lanes 1-5). The other sample was frozen overnight at -20°C then thawed and treated the same way (lanes 6-10). Spermatozoa were then embedded in agarose, treated with sodium dodecyl sulfate (SDS) and proteinase K, and analyzed by pulse-field gel electrophoresis. M indicates molecular weight marker.