Figure 9.

Nucleolar segregation with and without spindle formation. Changes in nuclear structure during normal mitotic progression and SIM are depicted. In G2, DNA (blue) is surrounded by the nuclear envelope (solid black circle) in which is embedded the duplicated spindle pole bodies (pink spheres). At entry into mitosis the NPCs undergo partial disassembly, permeabilizing the NE (dotted black line), and DNA condenses (darker blue). At anaphase, the NORs (yellow around black) are removed from the nucleolus (red, An-Fib; green, An-Bop1), and a similar event occurs during SIM. Modifications of the NE (gray dotted line) result in exclusion of the nucleolus to the cytoplasm and occur during both a normal telophase and a SIM. As nuclei progress into G1 the NPCs reassemble, nuclear transport is reestablished, and DNA decondenses. The “old” cytoplasmic nucleolus undergoes stepwise disassembly and the “new” nucleolus is generated in a stepwise manner. All of these events occur in both normal mitosis and SIM, indicating that they are independent of microtubules. However, as DNA segregation is dependent on microtubules, NORs do not segregate during a SIM resulting in generation of a single new nucleolus. Thus mitosis involves integrated spindle dependent and independent processes for nucleolar segregation that are able to function independent of each other.