Figure 2.

Comparison of DNA Packaging in Somatic and Sperm Cells. Somatic cell DNA (A) is coiled around histone octamers (B), then into a 30 nm filament that is often depicted as a solenoid (C). These 30 nm chromatin fibers are then attached at their bases to the sperm nuclear matrix to form loop domains (D). Loop domains can be examined experimentally by isolating nuclei and extracting the histones with 2 M NaCl (E). In sperm chromatin, the histones have been replaced by protamines that bind the DNA (F) in the major groove, completely neutralizing the negative charges of the DNA (G). This chromatin coils in to toroids (H and I). When sperm are extracted with salt after washing with SDS, the DNA loop domains are revealed (J). We have proposed that each protamine toroid represents one DNA loop domain. This figure was originally published in Biology of Reproduction [25] and a modified version published in Genetics of Human Male Fertility [33].