In the nucleolus, ribosomal RNA (rRNA) is synthesized and assembled with proteins from the cytoplasm to form ribosomal subunits. In the nucleus is a region of densely stained fibers and granules adjoining chromatin, the nucleolus. A typical human cell has 46 chromosomes, but sex cells (eggs and sperm) have only 23 chromosomes. Each eukaryotic species has a characteristic number of chromosomes. However when the cell prepares to divide, the chromatin fibers coil up to be seen as separate structures, chromosomes. In a normal cell they appear as diffuse mass. Within the nucleus, the DNA and associated proteins are organized into fibrous material, chromatin. The nuclear side of the envelope is lined by the nuclear lamina, a network of intermediate filaments that maintain the shape of the nucleus. Where the double membranes are fused, a pore allows large macromolecules and particles to pass through. 
The nucleus is separated from the cytoplasm by a double membrane. Some genes are located in mitochondria and chloroplasts. The nucleus contains most of the genes in a eukaryotic cell. 
The nucleus contains a eukaryotic cell’s genetic library
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