The nuclear envelope (NE)(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The NE serves as the physical barrier, separating the contents of the nucleus (DNA in particular) from the cytosol (cytoplasm).
Numerous nuclear pores are inserted in the nuclear envelope and facilitate and regulate the exchange of materials (proteins such as transcription factors, and RNA) between the nucleus and the cytoplasm.
Each of the two membranes is composed of a lipid bilayer. The outer membrane is continuous with the rough endoplasmic reticulum while the inner nuclear membrane is the primary residence of several inner nuclear membrane proteins. The outer and inner nuclear membrane are fused at the site of nuclear pore complex insertion.
The inner nuclear membrane is connected to the nuclear lamina, a network of intermediate filaments composed of various lamins (A, B1, B2, & C). The lamina acts as a site of attachment for chromosomes and provides structural stability to the nucleus. The lamins have been associated with various genetic disorders collectively termed laminopathies.
The space between the two membranes that make up the nuclear envelope is called the perinuclear space (also called the perinuclear cisterna, NE Lumen), and is usually about 20 - 40 nm wide.
The nuclear envelope has been postulated to play a role in the organization and transcriptional activity of chromatin.
During prophase in mitosis, the chromatids begin condensing to form chromosomes, and the nuclear envelope breaks down and is retracted into the mitotic endoplasmic reticulum. At metaphase, the nuclear envelope has been completely disassembled and absorbed by the ER allowing the chromosomes to be pulled apart by spindle fibers attached to each chromosome at the kinetochore. Other eukaryotes such as yeast undergo closed mitosis, where the chromosomes segregate within the nuclear envelope, which then buds as the two daughter cells divide.