Toroid loop model for sperm chromatin. (A) We have proposed a model for sperm chromatin in which each protamine toroid is one DNA loop domain attached at their bases to a sperm nuclear matrix (Ward, 2010). The sperm chromatin fragmentation (SCF) causes DNA breaks at the toroid linker regions (TLRs). (B) A possible structure for how protamine toroid fibers might be compact. The model results in two channels that might provide proteins with access to the condensed sperm chromatin. Intra-toroidal channels might result from the center holes of toroids aligning. Inter-toroidal channels would result from the conjunction of cylindrical protamine toroid fibers compacting. We would expect TLRs to be located in inter-toroidal channels.