Cell nucleus
Cell nuclei contain most of the cell's genetic material, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome and are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Nuclear pore
As the NPC controls access to the genome, it is essential that it exists in large amounts in stages of the cell cycle where plenty of transcription is necessary. For example, cycling mammalian and yeast cells double the amount of NPC in the nucleus between the G1 and G2 phase of the cell cycle, and oocytes accumulate large numbers of NPCs to prepare for the rapid mitosis that exists in the early stages of development. Interphase cells must also keep up a level of NPC generation to keep the levels of NPC in the cell constant as some may get damaged. Some cells can even increase the NPC numbers due to increased transcriptional demand.