Nuclear structure and the transport of molecules
The structure of the nucleus is maintained by a family of intermediate filament proteins known as lamins, which form a network of filaments on the inner surface of the nuclear membrane. This network is called the nuclear lamina and is linked both to the nuclear membrane and to the chromatin; the nuclear lamina is thought to help to organise the chromatin.
When viewed at high magnification, it can be seen that the membrane that surrounds the nucleus is actually a double membrane, or nuclear envelope, as it is sometimes called. Within the envelope are gaps called nuclear pores (Figure 11b and Figure 14) which create channels of about 9 nm diameter. Small water-soluble molecules diffuse freely through the nuclear pores, but the movement of proteins into and out of the nuclear pore is regulated by a complex arrangement of proteins known as the nuclear pore complex. The nuclear pore complex allows RNA molecules and ribosomes to pass out of the nucleus, and allows selected proteins to enter the nucleus, but prevents passage of most other proteins.
What type of proteins would you predict to be transported into the nucleus from the cytoplasm?
Histones, which are needed for the packing of DNA; the enzymes and other proteins needed for replication and transcription of DNA; the proteins needed for ribosome assembly; and the proteins that make up the nuclear lamina.
The nucleus, then, is crucial as the site of DNA replication, transcription of DNA into RNA and ribosome assembly. You have seen that the ribosome subunits and mRNA molecules leave the nucleus through the nuclear pores. The next section describes what happens to them in the cytosol.