The nucleolus is the only structure in the nucleus that is detectable in a light microscope without staining. It is the site of rRNA synthesis. (rRNA or ribosomal RNA is an essential part of ribosomes, the structures in the Cytoplasma where proteins are made from mRNA templates.) The nucleolus contains many rDNA genes from which the rRNA is made. The human genome contains 10 chromosomes with arrays of rDNA genes (two copies of chromosomes 13, 14, 15, 21 and 22) but not all of these arrays are necessarily actively used in all cells.