Cell nucleus
The nucleus is the largest cellular organelle in animal cells.
In mammalian cells, the average diameter of the nucleus is approximately 6 micrometres (µm), which occupies about 10% of the total cell volume. The viscous liquid within it is called nucleoplasm (or karyolymph), and is similar in composition to the cytosol found outside the nucleus. It appears as a dense, roughly spherical or irregular organelle. The composition by dry weight of the nucleus is approximately: DNA 9%, RNA 1%, Histone Protein 11%, Residual Protein 14%, Acidic Proteins 65%.
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.