Presence of some special NORs can account for the difference in the numbers of competent NORs obtained by the daughter cells in mitosis. Three NORs of different types are selected for comparison: a – single “asymmetrical”; b – single “symmetrical”; c- double, the most common type of NOR. Time course 00 min - metaphase. NORs a and b are seen as single dots, but c is represented by a double dot. Time course 01 min – beginning of anaphase. NORs b and c are divided into b1,2 and c1,2. NOR a remains single, is marked as a1. Time course 02 min - end of anaphase. NORs c1 and c2, as well as b1 and b2, are moving to the opposite poles of mitosis. NOR a1 has no counterpart. As a result, one of the two emerging daughter cells (top) acquires 9 competent NORs, the other (bottom) only 8. Scale bar: 5μm.