A nucleus is held together by a membrane called the nuclear envelope. The envelope is formed of phospholipids that create a lipid bilayer to separate the contents of the nucleus from the cytoplasm.
The lipid bilayer is formed of an inner and outer layer separated by 10 to 50 nanometers. The space between the two layers is called the perinuclear space. Additionally, the envelope gives the nucleus shape and helps control the flow of molecules in and out of the pores of the nucleus. The size of the pores allows the passage of small, water-soluble molecules while preventing larger molecules free passage into the nucleus. Typical mammal cells have roughly 3000 to 4000 pores in the envelope.
The function of the nucleus is to monitor and control gene expression and the replication of DNA during the cell cycle. When a cytoplasmic process needs isolation, key parts are removed to the nucleus.
Inside the nucleus is the nucleolus, a mass of RNA and proteins that contains the nucleolar organizer regions. These are chromosome parts that contain genes for ribosome synthesis. In contrast to the nucleus, the nucleolus does not have a membrane and is referred to as a suborganelle. The nucleus can contain a number of types of subnuclear structures.