Nucleolus
The nucleolus is by far the most easily recognized substructure in the eu­karyotic nucleus, and can be seen by using a variety of dyes as well as by phase contrast microscopy. Indeed, in budding yeast, the single nucleolus takes up nearly half of the nucleus. Cells from other species often have mul­tiple nucleoli. The nucleolus is a ribosome factory, composed of deoxyri­bonucleic acid (DNA), ribonucleic acid (RNA), and protein. At the nucleolus, a long ribosomal RNA (rRNA) precursor molecule is transcribed from DNA, processed into three mature RNAs, and packaged together with specific proteins to make the large and small ribosomal subunits. Once the subunits have been assembled, they are transported out of the nucleolus to the cytoplasm for use in translation (protein synthesis).
Nucleoli are not static structures. They disassemble during mitosis and reform in early G1 phase. Nucleolar formation does not cause expression of rRNA genes. Rather, nucleoli are the result of rRNA transcription and processing.

A colored transmission electron micrograph of a mammalian tissue culture cell, showing the nucleus (red), nucleolus (orange), and cytoplasm (blue).
Viewed in the electron microscope, a nucleolus has two distinct parts: the fibrillar component and the granular component. The fibrillar compo­nent can be subdivided into two compartments: the dense fibrillar compo­nent and the fibrillar center. Fibrillar centers contain large amounts of RNA polymerase I, which transcribes rRNA. Transcription of rRNA genes is thought to occur at the interface between the dense fibrillar component and the fibrillar center. Later stages of ribosome assembly take place in the gran­ular component.
Human chromosomes contain five nucleolar organizer regions (called NORs), located on the short arms of the chromosomes 13, 14, 15,21 and 22. In humans, each NOR contains approximately one hundred tandemly repeated rRNA gene copies. The NORs of different chromosomes typically come together in interphase. Thus, a single nucleolus is often made up of rRNA genes from two or more different NORs. Some species have only a single NOR-bearing chromosome and thus a single nucleolus.
In addition to the well-established function of nucleoli in ribosome as­sembly, recent evidence suggests that nucleoli are also involved in several other cellular processes, including assembly and modification of various small ribonucleoprotein (RNPs), sequestration of important cell-cycle regulatory proteins, export of other no ribosomal RNAs, and control of cel­lular senescence or aging.
Referencesٌ
Olson, M.O., M. Dundr, and A. Szebeni. “The Nucleolus: An Old Factory with Un­expected Capabilities.” Trends in Cell Biology (2000) 10: 189-196.
Visintin, R., and A. Amon. “The Nucleolus: The Magician’s Hat for Cell Cycle Tricks.” Current Opinions in Cell Biology (2000) 12: 372-377.