The nuclear envelope surrounds and protects a eukaryotic cell's DNA and its surrounding nucleoplasm. The membrane is comprised of two layers with a narrow space between them. The surface of the envelope is covered with tiny holes that permit the free flow of small molecules, such as water and ATP, while regulating the passage of messenger RNA and various proteins.
The outer layer of the nuclear envelope is contiguous with the endoplasmic reticulum. This structure acts as a conveyor belt to transport amino acids along as they are assembled into proteins. The perforated structure of the nuclear membrane suggests that molecular transport across its barrier is important to the early stages of protein synthesis.
During cell division, the nuclear envelope undergoes a dramatic increase in its surface area and the number of pores in its membrane. Simple eukaryotes, such as yeast, divide without disrupting the envelope's surface integrity. In more complex eukaryotes, such as plants and animals, the nuclear envelope undergoes a severe disruption that leaves the chromosomes it contains exposed throughout the transition. After division, the membrane reassembles itself either by bringing together its disrupted fragments to reform the main structure or by regrowing from the attached endoplasmic reticulum.