<P> Nuclear import depends on the importin binding its cargo in the cytoplasm and carrying it through the nuclear pore into the nucleus . Inside the nucleus, RanGTP acts to separate the cargo from the importin, allowing the importin to exit the nucleus and be reused . Nuclear export is similar, as the exportin binds the cargo inside the nucleus in a process facilitated by RanGTP, exits through the nuclear pore, and separates from its cargo in the cytoplasm . </P> <P> Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after post-transcriptional modification is complete . This quality - control mechanism is important due to these molecules' central role in protein translation . Mis - expression of a protein due to incomplete excision of exons or mis - incorporation of amino acids could have negative consequences for the cell; thus, incompletely modified RNA that reaches the cytoplasm is degraded rather than used in translation . </P> <P> During its lifetime, a nucleus may be broken down or destroyed, either in the process of cell division or as a consequence of apoptosis (the process of programmed cell death). During these events, the structural components of the nucleus--the envelope and lamina--can be systematically degraded . In most cells, the disassembly of the nuclear envelope marks the end of the prophase of mitosis . However, this disassembly of the nucleus is not a universal feature of mitosis and does not occur in all cells . Some unicellular eukaryotes (e.g., yeasts) undergo so - called closed mitosis, in which the nuclear envelope remains intact . In closed mitosis, the daughter chromosomes migrate to opposite poles of the nucleus, which then divides in two . The cells of higher eukaryotes, however, usually undergo open mitosis, which is characterized by breakdown of the nuclear envelope . The daughter chromosomes then migrate to opposite poles of the mitotic spindle, and new nuclei reassemble around them . </P> <P> At a certain point during the cell cycle in open mitosis, the cell divides to form two cells . In order for this process to be possible, each of the new daughter cells must have a full set of genes, a process requiring replication of the chromosomes as well as segregation of the separate sets . This occurs by the replicated chromosomes, the sister chromatids, attaching to microtubules, which in turn are attached to different centrosomes . The sister chromatids can then be pulled to separate locations in the cell . In many cells, the centrosome is located in the cytoplasm, outside the nucleus; the microtubules would be unable to attach to the chromatids in the presence of the nuclear envelope . Therefore, the early stages in the cell cycle, beginning in prophase and until around prometaphase, the nuclear membrane is dismantled . Likewise, during the same period, the nuclear lamina is also disassembled, a process regulated by phosphorylation of the lamins by protein kinases such as the CDC2 protein kinase . Towards the end of the cell cycle, the nuclear membrane is reformed, and around the same time, the nuclear lamina are reassembled by dephosphorylating the lamins . </P>

When had genes been located in the nucleus of the cell