A.
The nuclear envelope in all eukaryotic cells is perforated by nuclear pores that form the gates through which molecules enter or leave the nucleus. The nuclear pore complex forms a gate through which selected macromolecules and larger complexes enter or exit the nucleus.
B.
A nuclear pore is a large, elaborate structure composed of a complex of about 30 different proteins, each present in multiple copies.
C.
Many of the proteins that line the nuclear pore contain extensive, unstructured regions in which the polypeptide chains are largely disordered.
D.
These disordered segments form a soft, tangled meshwork—like a kelp forest—that fills the center of the channel, preventing the passage of large molecules but allowing small, water-soluble molecules to pass freely and nonselectively between the nucleus and the cytosol.