Figure 5.

Putative mechanism for the formation of the tetracentric ring containing 6 copies of the pericentric region of 18q10-10q11.2 A. A supernumerary chromosome 18 from the father underwent transverse misdivision of the centromere or successive breaks at or adjacent to the centromere and in the long arm of chromosome 18. B. A small ring chromosome was generated through repair of the double-strand break of the chromosome by fusion of the two sister chromatids (U-type sister chromatin exchange). C. After DNA synthesis a subsequent sister chromatid exchange occurred. D. This lead to the formation of a dicentric ring chromosome containing 4 times the pericentric region of 18q10-10q11.2. E. By intrastrand homologous recombination and homologous end joining a monocentric ring chromosome and a bicentric ring chromosome (containing 3 times the pericentric region) were generated. F. Since most centromeres of the supernumerary ring were likely to be inactivated, the newly formed monocentric ring received one of the inactive centromeres, leading to loss of the monocentric ring in subsequent cell divisions. G. Sister chromatid exchange in the bicentric ring (H.) resulted in a tetracentric ring containing 6 copies of the pericentric region of 18q10-10q11.2.