Figure 2.

A schematic view of chromatin remodeling in SPZ. During spermiogenesis, cells suffer chromatin conformational changes regulated by epigenetic marks, ultimately resulting in an extremely condensed chromatin that protects SPZ from external assault. In this last step of spermatogenesis DNA is methylated, histones suffer posttranslational modifications and are replaced by transition proteins (TPs), an intermediary state of packaging that soon disappears. TPs are then exchanged with more basic proteins, the protamines. DNA strands are tightly wrapped to protamines, forming supercoiled doughnut-like structures named toroids, the basic packaging units of sperm chromatin. Of note, a small subset of histones escapes this fate and is not replaced.