The nucleolus is a eukaryotic cellular organelle located in the nucleus.
Structure of the nucleolus
The nucleolus is a region of the nucleus which can be seen under light microscopy (using basic dyes) and electron microscopy (it is dense to electrons). The nucleolus is considered to be an organelle but is not, however, surrounded by a lipid membrane and is not therefore physically separated from the nucleus.
It is usually single but several copies may be present depending on its activity and it may disappear during cell division.
Function of the nucleolus
The nucleolus is where ribosomal RNA (rRNA) is transcribed by the enzyme RNA polymerase 1. These rRNA (18S, 5.8S and 28S) are involved in the production of the ribosomes in combination with ribosomal proteins through a process which is also facilitated by factors contained in the nucleolus
Diagram of a typical animal cell with its organelles: 1. Nucleolus ; 2. Nucleus ; 3. Ribosome ; 4. Vesicle ; 5. Rough (or granular) endoplasmic reticulum , also called ergastoplasm ; 6. Golgi apparatus; 7. Cytoskeleton ; 8. Smooth endoplasmic reticulum ; 9. Mitochondrium ; 10. Vacuole ; 11. Cytosol ; 12. Lysosome ; 13. Centriole. © MesserWoland and Szczepan1990, Wikimedia, CC by-sa 3.0