<Table> <Tr> <Td> The eukaryotic cell nucleus . Visible in this diagram are the ribosome - studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus . Within the cell nucleus is a viscous liquid called nucleoplasm, similar to the cytoplasm found outside the nucleus . </Td> <Td> A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments . </Td> </Tr> </Table> <Tr> <Td> The eukaryotic cell nucleus . Visible in this diagram are the ribosome - studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus . Within the cell nucleus is a viscous liquid called nucleoplasm, similar to the cytoplasm found outside the nucleus . </Td> <Td> A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments . </Td> </Tr> <P> The nuclear envelope, otherwise known as nuclear membrane, consists of two cellular membranes, an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometres (nm). The nuclear envelope completely encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm . The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes . The space between the membranes is called the perinuclear space and is continuous with the RER lumen . </P> <P> Nuclear pores, which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleoporins . The pores are about 125 million daltons in molecular weight and consist of around 50 (in yeast) to several hundred proteins (in vertebrates). The pores are 100 nm in total diameter; however, the gap through which molecules freely diffuse is only about 9 nm wide, due to the presence of regulatory systems within the center of the pore . This size selectively allows the passage of small water - soluble molecules while preventing larger molecules, such as nucleic acids and larger proteins, from inappropriately entering or exiting the nucleus . These large molecules must be actively transported into the nucleus instead . The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope, each of which contains an eightfold - symmetric ring - shaped structure at a position where the inner and outer membranes fuse . Attached to the ring is a structure called the nuclear basket that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins . </P>

What is the outer layer of the nucleus called