Fig. 8. A coiled-coil model for the folding and replication of the HSR. A long and homogeneous array of repeat sequences might be folded as a giant coil that is further coiled `coiled coil' (A-C). When drawing the unit structure of replication firing, we tentatively assumed that one 30-nm fiber (shown in thin lines) might be folded into a replicon cluster. The initiation of replication (red line) from the periphery of the coiled coil would appear as replication bands in the mitotic chromosome (A'-C'). Topologically, the peripheral region might be loose and the inner region might be packed tightly, offering an explanation why replication starts in the periphery. To replicate the inner region, the entire coil might be loosened at the last stage (C).