In eukaryotic cells, nuclear envelope separates the nucleus of replication and transcription of the DNA from the cytoplasm where protein biosynthesis occurs. The proper functioning of a cell depends on the efficient exchange of proteins and RNA between the two compartments phones. Nuclear-cytoplasmic transport takes place through complex protein structures, which are nuclear pore complexes. Transport proteins and the RNA is a selective process, depending on the energy signal and the specific nuclear and cytoplasmic receptors.