Figure 1. The nucleus in interphase.

The double phospholipid bilayer-bound nucleus is a specialized region of the endoplasmic reticulum (ER) that harbors the chromosomes of eukaryotic cells in interphase. The membranes of the inner (INM) and outer (ONM) nuclear envelope are continuous with one another and with the ER, but the INM is enriched with a specialized collection of INM proteins [15, 16] that are synthesized in the ER (as shown), transit to the ONM and then to the INM where they are retained by association with chromatin and/or other proteins at the nuclear periphery. The nuclear envelope (NE) is perforated by nuclear pore complexes (NPC), which surround the nuclear pores, aqueous channels that form a selectively-permeable barrier between the nucleoplasm and the cytoplasm. With few exceptions [36, 47], the exchange of material across the intact NE is restricted to the NPCs that allow the free diffusion of some small molecules and proteins and the selective Ran-GTPase dependent exchange of larger cargoes [13]. During interphase of the cell cycle (the time when cells are not in mitosis), the chromosomes are decondensed, the NE is intact, and some proteins of the INM (e.g. the nuclear lamina proteins of mammalian cells or telomere or heterochromatin-binding proteins of yeast), anchor specific chromosome domains, such as non-transcribed heterochromatin and telomeres, to the nuclear periphery.