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.

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 ribonucleoproteins (RNPs), sequestration of important cell-cycle regulatory proteins, export of other nonribosomal RNAs, and control of cellular senescence or aging.

see also Chromosome, Eukaryotic; Nuclear Transport; Nucleus; Ribosome; RNA; Transcription

A. Gregory Matera

Bibliography

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.

Nucleolus

The nucleolus is the part of the nucleus (the cell's control center) of a cell that helps produce ribosomes. Ribosomes are those parts of a cell that help make proteins. The nucleolus is easily recognized as a dark, dense area near the center of the nucleus.

Most cells have only one nucleolus, although some have two or more. When a cell nucleus is stained in order to observe it better under a microscope, the nucleolus is always seen as a dark-stained body. Its shape is usually irregular, probably because it is not walled off from the rest of the nucleus by any type of membrane. It looks like a mass rather than something that is sharply defined.

The nucleolus has an important job, however, in that it assembles ribosomes. After the nucleolus assembles them, the ribosomes leave the nucleus through its membrane's pores and enter the cell's cytoplasm (the jelly-like contents of a cell). Here they go to work making proteins. Since nucleoli are indirectly involved in making proteins, they perform a key function in the cell. Rapidly growing cells require a great deal of protein, which means that they must also have a lot of ribosomes. The name nucleolus means "little nucleus," and the nucleolus does resemble a small nucleus within a large nucleus.

[See alsoCell; Nucleus ]

nucleolus A clearly defined, often spherical area of the eukaryotic nucleus, composed of densely packed fibrils and granules. Its composition is similar to that of chromatin, except that it is very rich in RNA and protein. It is the site of the synthesis of ribosomal RNA, which forms a major part of ribosomes. The assembly of ribosomes starts in the nucleolus, but is completed in the cytoplasm.

nu·cle·o·lus / n(y)oōˈklēələs/ • n. (pl. -li / -ˌlī; -ˌlē/ ) Biol. a small dense spherical structure in the nucleus of a cell during interphase.DERIVATIVES: nu·cle·o·lar / -lər/ adj.

nucleolus A clearly defined, often spherical area of the eukaryotic nucleus, composed of densely packed fibrils and granules. Its composition is similar to that of chromatin, except that it is very rich in RNA and protein. It is the site of origin of ribosomes.