Nucleolus

Nucleolus

Diagram of a human cell nucleus

The nucleolus is a prominent sub-structure within the cell nucleus of eukaryotic cells. It is primarily involved in the synthesis and assembly of ribosomes, the cellular machinery responsible for protein synthesis. The nucleolus is not surrounded by a membrane, distinguishing it from other organelles within the cell.

Structure

The nucleolus is composed of three main components: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). These regions are involved in different stages of ribosomal RNA (rRNA) processing and ribosome assembly.

• Fibrillar Center (FC): This region contains the DNA that encodes rRNA. It is the site where rRNA transcription begins.
• Dense Fibrillar Component (DFC): Surrounding the FC, the DFC is where the initial stages of rRNA processing occur.
• Granular Component (GC): This area contains maturing ribosomal subunits and is involved in the final stages of ribosome assembly.

Function

The primary function of the nucleolus is the biogenesis of ribosomes. This involves the transcription of rRNA genes, processing of the rRNA, and assembly of ribosomal subunits. The nucleolus also plays a role in the regulation of the cell cycle, stress responses, and the assembly of signal recognition particles.

Ribosome Biogenesis

Ribosome biogenesis begins with the transcription of rRNA genes by RNA polymerase I. The resulting rRNA is then processed and assembled with ribosomal proteins imported from the cytoplasm to form the small and large ribosomal subunits. These subunits are then exported to the cytoplasm where they combine to form functional ribosomes.

Cell Cycle Regulation

The nucleolus is involved in the regulation of the cell cycle. It is known to sequester proteins that are involved in cell cycle progression, thereby influencing cell division.

Stress Response

Under conditions of cellular stress, such as nutrient deprivation or DNA damage, the nucleolus can alter its structure and function. This can lead to changes in ribosome production and the activation of stress response pathways.

Image of a nucleolus

Clinical Significance

The nucleolus is implicated in several diseases, particularly cancer. Alterations in nucleolar structure and function are often observed in cancer cells, where increased ribosome biogenesis supports rapid cell proliferation. The nucleolus is also involved in the pathogenesis of certain genetic disorders, such as nucleolar stress syndromes.

Related Pages

• Ribosome
• Cell nucleus
• RNA
• Protein synthesis

File:Widespread-Expression-of-BORISCTCFL-in-Normal-and-Cancer-Cells-pone.0022399.s011.ogv

See Also

• Ribosomal RNA
• RNA polymerase I
• Cell cycle
• Cancer biology