The cell nucleus contains the DNA of a given cell, i.e. its genetic information.

As shown in the figure, the cell nucleus is separated from the cytosol by a system of membraneous cisternae. These cisternae are crossed by the nuclear pores. They are connected to the endoplasmatic reticulum. The dark region within the cell nucleus from the figure refers to the nucleolus. Numerous copies for rRNA genes are located in this structure.