NUCLEAR ENVELOPE

The nuclear envelope is a membrane that separates the nucleus
from the cytoplasm that is continuous with the endoplasmic
reticulum at a number of points. Over three thousand nuclear
pores penetrate the surface of the nuclear envelope.
These pores allow materials to enter and leave the nucleus,
and they give the nucleus direct contact with the endoplasmic
reticulum. Nuclear pores are not simply holes in
the nuclear envelope; each is composed of an ordered array of
globular and filamentous granules, probably proteins. These granules
form the nuclear pore complex, which governs the transport
of molecules into and out of the nucleus. The size of the pores prevents
DNA from leaving the nucleus but permits RNA to be
moved out.