Nucleolus

The nucleolus (/njuːˈkliːələs, -kliˈoʊləs/, plural: nucleoli /-laɪ/) is the largest structure in the nucleus of eukaryotic cells. It is best known as the site of ribosome biogenesis. Nucleoli also participate in the formation of signal recognition particles and play a role in the cell's response to stress. Nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions called nucleolar organizing regions. Malfunction of nucleoli can be the cause of several human conditions called 'nucleolopathies' and the nucleolus is being investigated as a target for cancer chemotherapy. The nucleolus was identified by bright-field microscopy during the 1830s. Little was known about the function of the nucleolus until 1964, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increasing interest in the function and detailed structure of the nucleolus. They found that 25% of the frog eggs had no nucleolus and that such eggs were not capable of life. Half of the eggs had one nucleolus and 25% had two. They concluded that the nucleolus had a function necessary for life. In 1966 Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA. Three major components of the nucleolus are recognized: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). Transcription of the rDNA occurs in the FC. The DFC contains the protein fibrillarin, which is important in rRNA processing. The GC contains the protein nucleophosmin, (B23 in the external image) which is also involved in ribosome biogenesis. However, it has been proposed that this particular organization is only observed in higher eukaryotes and that it evolved from a bipartite organization with the transition from anamniotes to amniotes. Reflecting the substantial increase in the DNA intergenic region, an original fibrillar component would have separated into the FC and the DFC. Another structure identified within many nucleoli (particularly in plants) is a clear area in the center of the structure referred to as a nucleolar vacuole.Nucleoli of various plant species have been shown to have very high concentrations of iron in contrast to human and animal cell nucleoli. The nucleolus ultrastructure can be seen through an electron microscope, while the organization and dynamics can be studied through fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immunofluorescence experiments. Although usually only one or two nucleoli can be seen, a diploid human cell has ten nucleolus organizer regions (NORs) and could have more nucleoli. Most often multiple NORs participate in each nucleolus. In ribosome biogenesis, two of the three eukaryotic RNA polymerases (pol I and III) are required, and these function in a coordinated manner. In an initial stage, the rRNA genes are transcribed as a single unit within the nucleolus by RNA polymerase I. In order for this transcription to occur, several pol I-associated factors and DNA-specific trans-acting factors are required. In yeast, the most important are: UAF (upstream activating factor), TBP (TATA-box binding protein), and core binding factor (CBF)) which bind promoter elements and form the preinitiation complex (PIC), which is in turn recognized by RNA pol. In humans, a similar PIC is assembled with SL1, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), transcription initiation factors, and UBF (upstream binding factor). RNA polymerase I transcribes most rRNA transcripts 28S, 18S, and 5.8S) but the 5S rRNA subunit (component of the 60S ribosomal subunit) is transcribed by RNA polymerase III.