Nucleolus

The nucleolus is the largest structure found inside the nucleus of eukaryotic cells. Its primary role is the production of ribosomes, but it also helps assemble signal recognition particles and is involved in how a cell reacts to stress. It is made up of proteins, DNA, and RNA, and forms around specific chromosomal regions known as nucleolar organizing regions. When the nucleolus does not work properly, it can lead to several human disorders called nucleolopathies, and researchers are exploring it as a target for cancer chemotherapy.

The nucleolus was first seen using a bright-field microscope in the 1830s. In 1839, Theodor Schwann wrote that Matthias Schleiden had noticed small bodies inside cell nuclei, which he named "Kernkörperchen." An English translation in 1947 called this structure the "nucleolus." For many years, its function was unknown. That changed in 1964, when John Gurdon and Donald Brown studied nucleoli in African clawed frog eggs. They found that eggs without a nucleolus could not survive, while those with one or two could. This showed the nucleolus is essential for life. In 1966, Max L. Birnstiel and his team used nucleic acid hybridization to prove that the DNA inside the nucleolus codes for ribosomal RNA.

Under an electron microscope, the nucleolus shows three main parts: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). Ribosomal DNA is transcribed in the FC. The DFC contains the protein fibrillarin, which helps process rRNA. The GC contains nucleophosmin, another protein involved in ribosome assembly. This three-part structure is typical of higher eukaryotes and may have evolved from a simpler two-part arrangement when anamniotes gave way to amniotes. Some nucleoli, especially in plants, also have a clear central area called a nucleolar vacuole. Plant nucleoli often contain high levels of iron, unlike those in human and animal cells. Researchers can study nucleolar structure and dynamics using fluorescent protein tagging and FRAP, or mark the nucleolus with antibodies against the PAF49 protein. Although a typical human cell has ten nucleolus organizer regions, usually only one or two nucleoli are visible at a time, and multiple NORs often combine to form each nucleolus.

Ribosome assembly in the nucleolus requires two of the three eukaryotic RNA polymerases: Pol I and Pol III. First, RNA polymerase I transcribes rRNA genes as a single unit. This transcription depends on several factors, such as UAF, TBP, and CBF in yeast, or SL1, UBF, and other initiation factors in humans. Pol I produces most rRNA transcripts (28S, 18S, and 5.8S), while Pol III transcribes the 5S rRNA separately. The initial transcript is a long 45S pre-rRNA that includes internal and external transcribed spacers. These spacers are removed through processing guided by small nucleolar RNAs (snoRNAs), which bind to specific sequences and work as part of small nucleolar ribonucleoproteins (snoRNPs). After processing, the rRNA subunits are ready to be assembled into larger ribosomal subunits. In yeast, the 5S rDNA is located in the intergenic spacer and is transcribed by Pol III inside the nucleolus. In higher eukaryotes and plants, the 5S DNA lies outside the nucleolus organizer region and is transcribed by Pol III in the nucleoplasm before joining the other ribosomal components.