The nucleolus is a dense, spherical structure within the nucleus of eukaryotic cells, often recognized as the most prominent sub-compartment of the cell’s control center. Discovered in the 1830s, this body immediately stood out due to its distinct, highly concentrated appearance under the microscope. It is composed of a complex mixture of ribosomal DNA (rDNA), ribosomal RNA (rRNA), and hundreds of different proteins. The nucleolus is a highly dynamic structure that is disassembled during cell division and rapidly reassembled afterward.
Defining the Nucleolus and Its Cellular Home
The nucleolus is unique among cellular components because it is a non-membrane-bound organelle; it is not enclosed by a lipid bilayer like the nucleus or mitochondria. Instead, it is a supramolecular assembly of proteins and nucleic acids that forms through liquid-liquid phase separation within the nucleoplasm. Its location is precisely within the nucleus of all eukaryotic cells, and its size can fluctuate significantly, occupying up to 25% of the nuclear volume in cells with high metabolic activity. A single nucleus can contain one to several nucleoli, depending on the organism and the cell type.
The formation of the nucleolus is organized around specific chromosomal sites known as Nucleolar Organizing Regions (NORs). These regions are sections of chromosomes that contain the tandemly repeated genes that code for ribosomal RNA (rRNA). In human cells, the NORs are found on the short arms of the five acrocentric chromosomes: 13, 14, 15, 21, and 22. The nucleolus only forms around active NORs; multiple NORs can cluster together to form a single, larger nucleolus. The visibility of the nucleolus changes throughout the cell cycle, disappearing as the cell prepares to divide and reforming around the NORs after mitosis is complete.
The Three Distinct Structural Regions
The internal organization of the nucleolus reveals three distinct structural regions that reflect the sequential steps of its primary function. These components are visible under an electron microscope and are named the Fibrillar Centers (FCs), the Dense Fibrillar Component (DFC), and the Granular Component (GC). The FCs are the innermost regions, where the ribosomal DNA (rDNA) genes are located. Transcription of the genes is thought to occur at the boundary between the Fibrillar Centers and the Dense Fibrillar Component.
The Dense Fibrillar Component completely surrounds the FCs and contains the newly synthesized ribosomal RNA (rRNA) transcripts along with proteins involved in their initial processing. Proteins such as fibrillarin, which modifies the rRNA, are concentrated in the DFC. The outermost layer is the Granular Component, which consists of larger, maturing particles. These particles represent the late-stage precursors of the ribosomal subunits, composed of processed rRNA bound to various ribosomal proteins.
Primary Function: The Ribosome Factory
The primary function of the nucleolus is the production and assembly of ribosomes, the structures responsible for translating messenger RNA into proteins. This process, known as ribosome biogenesis, is one of the most energetically expensive processes in a cell. The nucleolus synthesizes the majority of the cell’s ribosomal RNA (rRNA) molecules, specifically the 5.8S, 18S, and 28S rRNAs in eukaryotes.
Transcription of the rDNA genes is carried out by RNA polymerase I (Pol I), which produces a single, large precursor molecule known as the 47S pre-rRNA. This precursor transcript contains all three rRNA segments (18S, 5.8S, and 28S) interspersed with spacer regions that must be removed. The transcription occurs at the FC/DFC boundary, initiating a vectorial process where the RNA moves through the nucleolus for maturation.
As the pre-rRNA is transcribed, it is immediately coated with hundreds of proteins and small nucleolar RNAs (snoRNAs) that guide the necessary modifications and folding. These processing steps, including cleavage of the spacer regions and chemical modification of the rRNA, begin in the Dense Fibrillar Component and continue as the particle moves outward into the Granular Component. Simultaneously, ribosomal proteins synthesized in the cytoplasm are actively imported into the nucleus and then into the nucleolus.
Within the Granular Component, the processed rRNA and the imported ribosomal proteins are assembled to form the two distinct ribosomal subunits: the small 40S subunit and the large 60S subunit. The formation of these subunits ensures the correct structure is built before export. Once assembly is complete, the pre-ribosomal subunits are exported through the nuclear pores to the cytoplasm, where they combine to form a functional ribosome when protein synthesis is required.
Nucleolar Involvement in Cellular Stress and Disease
Beyond its role in ribosome production, the nucleolus functions as a central sensor for various forms of cellular stress. When a cell experiences conditions like DNA damage, nutrient deprivation, or heat shock, the nucleolar structure and function are rapidly altered, a phenomenon termed nucleolar stress. This stress response is often initiated by the disruption of ribosomal RNA synthesis or the assembly process itself.
In response to nucleolar stress, ribosomal proteins are released from the nucleolus into the nucleoplasm, where they interact with and stabilize the tumor suppressor protein p53. The stabilization and activation of p53 can then trigger cellular decisions, such as cell cycle arrest to allow for repair or, if the damage is too severe, the initiation of apoptosis, or programmed cell death. This p53-dependent pathway links the integrity of the ribosome factory directly to the cell’s fate.
The nucleolus’s role as a stress sensor and its direct involvement in regulating cell growth make it relevant to disease, particularly cancer. Cancer cells are characterized by rapid, uncontrolled proliferation, which requires high production of new ribosomes to support their protein synthesis demands. Consequently, the nucleoli in many aggressive cancer cells are often visibly enlarged, reflecting their hyperactive state of ribosome biogenesis. Dysfunctional nucleolar activity is linked to a variety of conditions, including tumorigenesis and neurodegenerative disorders.