The eukaryotic cell nucleus functions as the cell’s command center, housing the vast majority of the genetic material. It is enclosed by the nuclear envelope, a double membrane that regulates the passage of molecules between the nucleus and the cytoplasm. Within this boundary, the genetic material (chromatin) is organized alongside specialized, non-membrane-bound sub-compartments. These structures are dedicated to the synthesis and processing of various components required by the cell.
Identifying the Dark-Stained Structure
The large, dark-staining spherical body visible inside the nucleus is called the nucleolus. Its distinct appearance under a light microscope, where it appears much denser and darker than the surrounding nuclear material, results from its unique composition. The nucleolus is highly concentrated with nucleic acids, specifically ribosomal RNA (rRNA), and numerous associated proteins.
This high concentration of macromolecules readily absorbs basic histological stains, causing the structure to stand out prominently against the lighter nuclear background. The nucleolus is a dynamic structure and is not enclosed by a membrane, allowing for the continuous exchange of molecules with the rest of the nucleus. Its size correlates with the cell’s activity level, becoming larger in cells that are rapidly growing or synthesizing large amounts of protein.
The Role in Ribosome Production
The primary function of the nucleolus is to serve as the cellular factory for producing ribosomes, the machines responsible for synthesizing all cellular proteins. This process, known as ribosome biogenesis, begins with the transcription of ribosomal DNA (rDNA) genes. These genes, which encode for the 5.8S, 18S, and 28S ribosomal RNA molecules, are transcribed by RNA Polymerase I.
The initial product is a long precursor molecule of ribosomal RNA (pre-rRNA), which then undergoes extensive chemical modification and cleavage. This multi-step processing pathway involves hundreds of specialized proteins and small nucleolar RNAs (snoRNAs). Simultaneously, ribosomal proteins synthesized in the cytoplasm are imported into the nucleolus to begin assembly with the processed rRNA.
Assembly of the large (60S) and small (40S) ribosomal subunits occurs sequentially within the nucleolus, bringing the rRNA and ribosomal proteins together. Once partially assembled, these subunits are exported through the nuclear pores and into the cytoplasm. Only in the cytoplasm do the large and small subunits join to form a functional ribosome capable of protein synthesis.
Structural Components of the Nucleolus
The internal organization of the nucleolus is highly compartmentalized, featuring three distinct morphological regions visible using electron microscopy. These regions reflect the different stages of the ribosome biogenesis pathway. The Fibrillar Centers (FCs) are the lightest-staining areas and contain the ribosomal DNA genes and inactive RNA Polymerase I machinery.
Surrounding the Fibrillar Centers is the Dense Fibrillar Component (DFC), the site of active rRNA transcription and initial pre-rRNA processing. The outermost region is the Granular Component (GC), where the final assembly of the ribosomal subunits takes place, incorporating processed rRNA and imported ribosomal proteins. The GC is the largest region and contains the nearly complete ribosomal subunits awaiting export.
The formation of the nucleolus is initiated by specific chromosomal segments known as Nucleolar Organizing Regions (NORs). NORs are the chromosomal locations containing the tandemly repeated clusters of ribosomal DNA genes. In human cells, NORs are found on the short arms of the five acrocentric chromosomes: 13, 14, 15, 21, and 22.