What happens to the nucleus during cell division?

During mitosis, the nucleus disassembles and reassembles each time cells divide. After the initiation of mitosis, the chromosomes condense, the nucleolus dissipates, and the nuclear envelope breaks down. In the majority of cells, the disintegration of the nuclear envelope signals the end of prophase and beginning of prometaphase. This causes the release of most of the contents of the nucleus into the cytoplasm. During the disintegration of the nuclear envelope, the nuclear membranes are fragmented into vesicles, the nuclear pore complexes dissociate, and the nuclear lamina depolymerizes. Once this occurs, the chromosomes line up in the metaphase plate for the next steps of mitosis. 

This process of mitosis is controlled primarily through reversible phosphorylation and dephosphorylation of nuclear proteins induced by the Cdc2 protein kinase. At the end of mitosis, the entire process is reversed. The chromosomes decondense, and the nuclear envelope reforms around the separate sets of daughter chromosomes.