The nucleolus is a dark-staining spherical body, but it is found within the nucleus, not in the mitochondria. New ribosomes are assembled in the nucleolus, specifically the subunits of the rRNA or the ribosomal RNA. It is the biggest structure within the nucleus and can be seen during interphase. Interestingly, studies have shown that the size of the nucleolus can determine the life span of the cell.
