The nuclear envelope is a double, parallel membrane with an intervening space. It demarcates the nucleus from the cytoplasm. The inner membrane is continuous with a nuclear network of filaments which support structures such as chromatin; the filament network is termed the nuclear lamina. The outer membrane is continuous with endoplasmic reticulum and has ribosomes on its surface.
Nuclear pores are passageways for the transfer of molecules into and out of the nucleus. Each nucleus may have several thousand. They are marked by fusion of the nuclear membranes to give a tunnel-like structure. They are surrounded by a number of proteins. The proteins regulate transit through the pore; they are termed collectively the nuclear pore complex. Molecules greater than about 62 kD cannot traverse the pores but may be able to cross the envelope via active, transmembranous transport.
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