The nucleus of a cell contains most of the genes of a eukaryotic. It is about 5 μm in diameter and is composed of a nuclear envelope which encloses the nucleus, separating its contents from the cytoplasm. The nucleus of a cell contains DNA and proteins and communicates with the surrounding cytol via numerous nuclear pores. Because of its size, the nucleus is often visible under a light microscope. The nucleolus of a cell is located within the nucleus and produces ribosome which is critical in protein synthesis. In an electron microscope, it appears as a mass of densely stained granules and fibers adjoining part of the chromatin.