B. is mediated by structures embedded in the nuclear envelope called nuclear pore complexes.
A nuclear localization sequence is a sequence of amino acids that serves as a molecular "zip code" or tag to direct a protein to be imported into the cell nucleus. This sequence typically contains one or more signal sequences of positively charged lysine or arginine amino acids that are exposed on the surface of the protein. Once the protein reaches the nucleus, it can carry out its specific function, such as regulating gene expression or DNA replication.
Conversely, for proteins to be exported out of the nucleus, a different sequence called the nuclear export signal is used. This sequence functions similarly to the nuclear localization sequence, but instead targets the protein for export out of the nucleus and into the cytoplasm of the cell.
Both the nuclear localization and export signals are part of the larger nuclear core complex, which is embedded in the nuclear membrane. The nuclear pore complex is a structure composed of proteins that regulate the entry and exit of molecules and substances, such as proteins, RNA, and other molecules, into and out of the nucleus. This process is essential for the proper functioning of a cell, as it allows for communication between the nucleus and the rest of the cell and ensures that important molecules are transported to where they are needed.