B. Nucleolus 1. Function. The nucleolus is usually located in the center of the nucleus. The number is related to the species from which the cell is derived. The size of the nucleolus can vary, depending on the level of activity. A prominent nucleolus means that the cell is actively synthesizing ribosomes .
Ribosomes are responsible for polypeptide synthesis.
Ribosomes are small, irregularly shaped particles, composed of two subunits. In the transmission electron microscope they appear as small black dots. Ribosomes are too small to see in the light microscope. However, because of their RNA content, large numbers of ribosomes can cause the cytoplasm of a cell to stain with basophilic dyes, a condition known as cytoplasmic basophilia . The large ribosomal subunit is composed of 3 different rRNA's and about 48 proteins.