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The cell nucleus is the organelle of the eukaryotes responsible for maintaining the integrity of DNA and for controlling cellular activities such as metabolism, growth, and reproduction by regulating gene expression. It is a double-membraned organelle and this double membrane is referred to as nuclear envelope (also called nuclear membrane, plasmalemma, sor karyotheca). The nuclear envelope surrounds most of the eukaryotic cell’s genetic material as opposed to the genetic material of the prokaryotes that is not membrane bound. The nuclear envelope is a lipid bilayer. It has nuclear pores that control the movement of molecules between the nucleoplasm and the cytoplasm. It is impermeable to large molecules. Thus, it separates the contents of the nucleus from the cellular cytoplasm and allows entry of selected molecules.
Nuclear pores pertain to the numerous openings in the nuclear envelope. They are octagonal in shape owing to the nuclear pore complex that forms by the assembly of protein molecules called nucleoporins that span the nuclear envelope. The pore measures about 9 nm in diameter whereas the depth is about 200nm. There are at least 456 nucleoproteins that make up the nuclear pore complex. There are numerous nuclear pore complexes that form in the nuclear envelope, i.e. about two thousand in average. These pores allow the passage of molecules into and out of the nucleus. Examples of molecules that pass through these pores are ribonucleoproteins, DNA polymerases, lipids, signaling molecules, and carbohydrates.
Common biological reactions
Common biological reactions
The presence of the nuclear envelope prevents the easy passage of large molecules (e.g. proteins and RNA) into and out of the nucleus. Although the nuclear envelope is perforated with nuclear pores, large molecules would still need a nuclear transport mechanism in order to enter and exit the nucleus. Small molecules, such as ions, can pass through the nucleus with ease. However, cargo proteins and RNAs that need to be transported require importins and exportins to enter and exit the nucleus, respectively. On one hand, the cargo binds with the importin in the cytoplasm, and then moved into the nucleus through the nuclear pore. On the other hand, the cargo binds with the exportin inside the nucleus, and then moved outside the nucleus via the nuclear pore. Nuclear transport needs energy to proceed. Thus, GTPases (e.g. Ran enzyme) help by hydrolyzing GTP (guanosine triphosphate) so that energy would be released in the process. The energy released would be used to dissociate the cargo from the importins and to bind the cargo to the exportins.
The nuclear pores allow the entry and exit of select biomolecules. For instance, ribonucleoproteins leave the nucleus whereas certain proteins (e.g. DNA polymerases), carbohydrates, lipids, and signaling molecules enter the nucleus through the nuclear pores.
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