Nuclear Envelope

The nuclear envelope is the double membrane that surrounds the nucleus of a eukaryotic cell, separating the chromosomes from the rest of the cell. It is perforated by thousands of pores that control traffic of molecules in and out.

It has an inner and an outer membrane, separated by a narrow gap of 20 to 40 nanometers. The outer membrane is continuous with the endoplasmic reticulum, so the envelope is part of the endomembrane system. Beneath the inner membrane lies the nuclear lamina, a mesh of lamin proteins that gives the nucleus its shape and anchors chromatin. Mutations in lamin genes cause diseases of premature aging, such as progeria.

Each nuclear pore complex is a large structure of around thirty different proteins, repeated many times, with a total mass of over 100 million daltons in humans. Small molecules pass freely, but larger ones need a signal sequence and carrier proteins. Messenger RNA leaves through the pores after processing, and proteins such as transcription factors and histones enter. Molecular studies of the pore use methods such as cryo-electron microscopy.

In animal cells the envelope breaks down at the start of mitosis, so the spindle can reach the chromosomes, and re-forms around the two sets of chromosomes afterwards. Its origin is a central question in eukaryogenesis, and its presence defines eukaryotes in contrast with the nucleoid of bacteria.

• Nucleus — the organelle it encloses
• Endoplasmic Reticulum — the membrane it connects to
• Nucleoid — the equivalent region in bacteria