Nucleolus: Structure, Diagram & its Function

The nucleolus is a membrane-less cellular structure located within the nucleus of eukaryotic cells. It plays an important role in the synthesis and assembly of ribosomes, the cell’s protein factories. It participates in functions like stress response and cell cycle regulation. The nucleolus, described as the heart of the cell nucleus, is a cellular structure that plays a key role in various cellular processes. Despite its small size, its functions are very important, ranging from ribosome biogenesis to stress sensing and disease.

What is Nucleolus?

The nucleolus is a membrane-free cellular structure found within the nucleus of eukaryotic cells that is largely responsible for the production and assembly of ribosomes, the cellular machinery for protein synthesis. The nucleolus exhibits a dynamic nature, changing in size and shape in response to the cell’s metabolic activity, and is also involved in other cellular processes like cell cycle regulation and stress responses.

Diagram of Nucleolus

The labeled diagram of nucleolus is given below:

Diagram of Nucleolus

Structure of Nucleolus

The nucleolus has a typical membrane-less structure consisting of three main components:

• Fibrillar Center Component (FC): It is typically where ribosomal DNA transcription starts. This area contains the genes for the production of ribosomal RNA (rRNA). The FC is the site of active rDNA transcription.
• Dense Fibrillar Component (DFC): Surrounding the FC is the DFC. This region is where the initial processing of pre-ribosomal RNA occurs and the DFC is denser than the FC due to its high concentration of processed rRNA and proteins.
• Granular Component (GC): The outer layer of the nucleolus is the GC. It is described by its granular appearance and is the site where the final assembly of ribosomal takes place. The GC contains pre-ribosomal particles at various stages of assembly.

Composition of the Nucleolus

The nucleolus is composed of mixture of RNA, DNA and proteins, which are important for its various functions:

• Ribosomal RNA: The rRNA molecules are processed within the nucleolus and help in the formation of ribosomes. These RNA molecules form the structure and functions of ribosomes.
• Proteins: A large number of proteins are present in the nucleolus, many of which are involved in rRNA synthesis, processing. These include transcription factors like ribonucleoproteins, and various enzymes.
• Noncoding RNAs: The nucleolus also contains various small noncoding RNAs that helps in rRNA processing and assembly.
• DNA: The nucleolus contains chromosomal known as nucleolar organizing regions (NORs). These regions are rich in genes that encode ribosomal RNA.

Functions of the Nucleolus

The function of the nucleolus are as follows:

Ribosome Biogenesis

The primary function of the nucleolus is the production of ribosomes, the cellular machinery responsible for synthesis of protein. This process involve various stages:

• Transcription of Ribosomal RNA (rRNA): The nucleolus is the site where rRNA genes help in producing ribosomal RNA.
• Processing of rRNA: Now the rRNA is processed, modified, and assembled into ribosomal within the nucleolus.
• Assembly with Ribosomal Proteins: These rRNA components now combine with ribosomal proteins from the cytoplasm to form the large subunits of ribosomes.

Cell Regulation

The nucleolus helps in the regulation of the cell cycle. The size and activity of the nucleolus can change depending on the stage of the cell cycle, in response to cellular growth and signals.

Stress Response

The nucleolus acts as a sensor and responder to cellular stress. Under conditions such as nutrient deprivation, or oxidative stress, the nucleolus changes its structure, which can trigger a stress response, leading to cell cycle arrest and other cellular adaptations.

Nucleus Organization

The nucleolus helps in the organization of the nucleus by serving as a site of assembly for nuclear substructures. It can influence the arrangement of chromosomes and the organization of chromatin within the nucleus.

Disease Involvement

Changes in nucleolus structure and function are associated with various diseases, like cancer. Increased nucleolus size and activity can increase proliferation in tumor cells.

Difference between Nucleus and Nucleolus

The difference between Nucleus and Nucleolus are as follows:

Characteristic	Nucleus	Nucleolus
Location	Usually located in the center of the cell.	Found within the nucleus.
Structure	Surrounded by a double membrane called the nuclear envelope. Contains chromatin, which consists of DNA and proteins.	Composed of RNA and proteins, lacking a membrane.
Function	Controls cell activities and contains genetic information.	Involved in the production of ribosomal RNA (rRNA) and assembly of ribosomes.
Composition	Contains DNA, RNA, proteins, and nucleoplasm.	Primarily composed of RNA and associated proteins.
Role in Ribosome Synthesis	Not directly involved in ribosome synthesis.	Directly involved in the synthesis of ribosomal RNA, a crucial component of ribosomes.

Conclusion – Nucleolus

The nucleolus is a membrane-less cellular structure located within the nucleus of eukaryotic cells. From its primary role in ribosome biogenesis to its importance in cellular stress responses and disease and involvement of the nucleolus in cellular health and disease. The nucleolus, a critical structure within the cell nucleus, has been recognized for its role in various diseases. Earlier known as the site of ribosome biogenesis, the nucleolus also participates in cellular processes, making it an important factor in the development and growth of several diseases particularly cancer and aging, neurodegenerative disorders, and viral infections.

FAQs on Nucleolus

1. What is the Difference between the Nucleus and Nucleolus?

The nucleus is the large, membrane cellular structure that contains the cell’s genetic material, and acts as the control center for cellular activities. In contrast, the nucleolus is a smaller, membrane-less structure within the nucleus, responsible for synthesizing and assembling ribosomes.

2. Why is the Nucleolus called the Ribosome factory?

The nucleolus is called as the “ribosome factory” because it is the primary site where ribosomal RNA is synthesized and the initial assembly of ribosomes takes place.

3. Do Human cells have Nucleoli?

Human cells have nucleoli and these are found within the nucleus of almost all human cells, where they play an important role in producing and assembling ribosomes. The number and size of nucleoli depend on the cell type and its metabolic activity.

4. What is the Primary Function of Nucleolus?

The primary function of the nucleolus in a cell is the production of ribosomes, machinery responsible for protein synthesis.

5. How does the Structure of Nucleolus help in its Function?

The structure of the nucleolus has three different regions such as the fibrillar center, fibrillar component, and granular component, which helps its function like ribosome biogenesis. These compartments improve the production and assembly of ribosomal essential for protein synthesis.