The protein synthesis in a cell is a consolidated, conveyor-like process including some steps in the nucleus (mRNA transcription and processing, ribosomal components production and assembly) continued in the cytoplasm where polypeptide chains are formed by ribosomes. Bilateral transportation of crude substances and final products between nuclear and cytoplasmic compartments is required for this synthesis. Besides, to accomplish their function, structural nuclear proteins (for example, those of the nuclear lamina), transcription-regulating factors, and some other molecules are produced in the cytoplasm to be transported into the nucleus. This is also true of some signaling molecules (growth factors, hormones, etc.) that exert their regulatory functions in the nuclear compartment by stimulating cell proliferation or protein synthesis

Nuclear pores #1, #2 are formed in the nuclear envelope with the assistance of the nuclear lamina to ensure transportation. The pores are constant channels of the nuclear envelope, which are limited by a fusion of the inner and outer nuclear membranes. There is no peripheral heterochromatin in this area.

The nuclear pores are not free to pass through. They are blocked by pretty thick plates. Any such plate is a complicated structure called nuclear pore complex. It provides a highly selective molecular transport between the nucleus and cytoplasm. Cytoplasmic parts of the nuclear pores are connected with the cytosol, and transcription products enter it as well as precursors of ribosomes. It seems that the process is demonstrated by the nuclear pore on the right. On the left, the image depicts some ribosomes #1, #2, which are attached to the cytoplasmic surface of the outer nuclear membrane.