Schematic drawing of the nucleus and the nuclear envelope
In interphase, the chromatin (blue) of the nucleus is surrounded by the nuclear envelope that consists of two membranes in which nuclear pore complexes (red) are embedded: The outer nuclear membrane is continuous with the membrane of the endoplasmic reticulum whereas the inner nuclear membrane has a unique protein composition.
In higher eukaryotes, the nuclear envelope is a highly dynamic structure during the cell cycle. Whereas yeast cells segregate the chromatin during mitosis with the nuclear envelope remaining intact – a so called closed mitosis – in animal cells, the nuclear envelope breaks down at the beginning of mitosis. This means that nuclear pore complexes are disassembled into mostly soluble proteins or protein sub-complexes, which are dispersed all over the cytoplasm. The membranes of the nuclear envelope are, at least in mammals, reabsorbed into the endoplasmic reticulum. At the end of mitosis two new nuclear envelopes reform around the separated chromatin, i.e. the two-membrane structure and nuclear pore complexes re-assemble. This process is tightly coordinated and regulated: Although nuclear pore complexes are assembled from mostly cytosolic components, for assembly they need the presence of membranes in which they are integrated. On the other hand, formation of the two membrane structure without nuclear pore complexes would be disastrous for the cell, as the communication and transport between cyto- and nucleoplasm would be blocked.