The nuclear envelope provides a selective permeability barrier tween the nuclear compartment and the cytoplasm. This barrier ensures that only fully processed mRNAs are delivered to ribosomes for translation into protein. In addition, various chromosomal events, including DNA replication and expression of certain genes, are regulated, at least in part, by changes in the ability of factors to move from the cytoplasm into the nucleus.
The nuclear envelope is composed of two concentric lipid bilayers termed the inner and outer nuclear membranes. The outer nuclear membrane is continuous with the rough endoplasmic reticulum and shares its functions. For example, it has ribosomes attached to its outer surface. A fibrous nuclear lamina of intermediate filaments supports the inner nuclear membrane in higher eukaryotes. These and other proteins of the inner nuclear membrane mediate interactions of the envelope with chromatin. The inner and outer nuclear membranes are separated by a perinuclear space of about 30 nm that is continuous with the lumen of the endoplasmic reticulum. Nuclear pore complexes bridging both nuclear membranes provide the sole route for communication between the nucleus and cytoplasm during interphase.
Disassembly of the nuclear envelope is a critical aspect of mitosis in higher eukaryotes, as this releases the chromosomes so that they can be segregated to the daughter cells by the cytoplasmic mitotic spindle. Mitotic segregation of chromosomes to daughter cells takes place within the nucleus in some lower eukaryotes including yeasts.