<P> During the G2 phase of interphase, the nuclear membrane increases its surface area and doubles its number of nuclear pore complexes . In eukaryotes, such as yeast, which undergo closed mitosis, the nuclear membrane stays intact during cell division . The spindle fibers either form within the membrane, or penetrate it without tearing it apart . In other eukaryotes (animals as well as plants), the nuclear membrane must break down during the prometaphase state of mitosis to allow the mitotic spindle fibers to access the chromosomes inside . The breakdown and reformation processes are not well understood . </P> <P> In mammals, the nuclear membrane can break down within minutes, following a set of steps during the early stages of mitosis . First, M - Cdk's phosphorylate nucleoporin polypeptides and they are selectively removed from the nuclear pore complexes . After that, the rest of the nuclear pore complexes break apart simultaneously . Biochemical evidence suggests that the nuclear pore complexes disassemble into stable pieces rather than disintegrating into small polypeptide fragments . M - Cdk's also phosphorylate elements of the nuclear lamina (the framework that supports the envelope) leading to the dis - assembly of the lamina and hence the envelope membranes into small vesicles . Electron and fluorescence microscopy has given strong evidence that the nuclear membrane is absorbed by the endoplasmic reticulum--nuclear proteins not normally found in the endoplasmic reticulum show up during mitosis . </P> <P> Exactly how the nuclear membrane reforms during telophase of mitosis is debated . Two theories exist--</P> <Ul> <Li> Vesicle fusion--where vesicles of nuclear membrane fuse together to rebuild the nuclear membrane </Li> <Li> Reshaping of the endoplasmic reticulum--where the parts of the endoplasmic reticulum containing the absorbed nuclear membrane envelop the nuclear space, reforming a closed membrane . </Li> </Ul>

When does the nuclear membrane form around chromosomes