Fig. 6

Working model of solenoidal supercoiling between homologous regions with differential accessibility. The model illustrates localized differences in chromatin accessibility at specific homologous loci in untreated cells (i.e. with DA, default state). Example of a homologous chromosomal region within 17p12 (black rectangle) ideogram is shown in the middle of the illustration. Immediately flanking each ideogram, chromatin loop size, its frequency, and distance between each helical turn (i.e. helical pitch) is kept the same for both homologs but inter-homolog accessibility within a localized loop is depicted to be variable. For homologous region A, this is illustrated as low-level compaction with widely spaced gray circles in contrast with high-level compaction in homologous region B. The outer most images show a partial cross section of each loop. In homolog A, the solenoid structure with greater accessibility has low longitudinal supercoiling vs. homolog B. For simplicity, additional levels of packing beyond the 300 nm loop fiber (indicated by multiple black arrows) are not shown. Chromatin loops are drawn in two-dimensions of a 3-D configuration found in vivo