Figure 1.

Three major structural elements of sperm chromatin.

(A) During spermiogenesis, histones are replaced by protamines, condensing the DNA into tightly packaged toroids. Each protamine toroid is a loop domain (part A of this figure is modified from a similar figure published in Ward, 1993). (B) Protamine toroids may be organized by stacking side to side. Recent evidence suggests that some large tracts of DNA retain histone (green solenoid). These may be entire loop domains that are not condensed by protamines. (C) The DNA strands that link the protamine toroids are nuclease sensitive, and may be bound to histones, as well. MAR, matrix attachment region