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Created by: Team Biology at, Last Updated: April 27, 2024


Embark on a journey into the nucleus to uncover the nucleolus, a pivotal entity in cellular function and ribosome assembly. This comprehensive guide presents the nucleolus in all its glory, from its structure to its indispensable role in synthesizing ribosomes, the cell’s protein factories. Through illustrative examples, we delve into how the nucleolus regulates protein production, impacts gene expression, and supports cell health and growth. Ideal for students, educators, and biology enthusiasts, this guide enriches your understanding of the nucleolus, offering insights into its vital contributions to life at the microscopic level.

What is a Nucleolus?

The nucleolus is a prominent structure within the nucleus of eukaryotic cells, identifiable under a microscope as a dense area devoid of a surrounding membrane. It is not a static structure; rather, its appearance and size can change based on the cell’s metabolic activity, particularly its rate of protein synthesis. The primary function of the nucleolus is the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomes, the cellular machines responsible for protein synthesis. The nucleolus forms around specific chromosomal regions called nucleolar organizing regions (NORs), which contain genes for rRNA.

Structure of Nucleolus

The nucleolus is a prominent sub-organelle within the nucleus of eukaryotic cells, notable for its lack of a surrounding membrane. It is formed around specific chromosomal regions called nucleolar organizing regions (NORs), which contain the genes for ribosomal RNA (rRNA) synthesis. The structure of the nucleolus can be broadly divided into three main components, reflecting its functions in the production and assembly of ribosome subunits:

  1. Fibrillar Centers (FCs): These are less dense areas within the nucleolus where the transcription of rRNA genes takes place. The newly synthesized rRNA is then processed and modified as it moves to the surrounding areas.
  2. Dense Fibrillar Components (DFCs): Surrounding the fibrillar centers, DFCs are regions rich in newly transcribed rRNA and the proteins involved in its processing. These areas are more electron-dense compared to FCs when viewed under an electron microscope.
  3. Granular Component (GC): This is the most peripheral region of the nucleolus, consisting of maturing ribosomal subunits. It contains partially assembled ribosomes and the proteins necessary for their final assembly.

The dynamic structure of the nucleolus reflects its active role in ribosome synthesis and assembly. Its size and shape can vary depending on the cell’s metabolic activity, particularly the demand for protein synthesis.

Functions of Nucleolus

Functions of Nucleolus

The nucleolus plays several critical roles in the cell, primarily related to the production of ribosomes, which are essential for protein synthesis. Its main functions include:

  1. Ribosomal RNA Synthesis: The nucleolus is the site of transcription of rRNA genes by RNA polymerase I. This rRNA forms the core structural and functional component of ribosomes.
  2. Ribosome Assembly: Within the nucleolus, rRNA combines with ribosomal proteins imported from the cytoplasm to form the large and small subunits of ribosomes. These subunits are then transported out of the nucleus into the cytoplasm, where they fully assemble into functional ribosomes.
  3. Ribosomal Protein Modification: The nucleolus also plays a role in the chemical modification of ribosomal proteins and rRNA, including methylation and pseudouridylation, which are crucial for the proper functioning of ribosomes.
  4. Cell Cycle Regulation: Beyond its primary role in ribosome biogenesis, the nucleolus is involved in several cellular processes, including the regulation of the cell cycle. It interacts with various proteins that play key roles in cell cycle regulation and stress responses.
  5. Stress Response: The nucleolus responds to cellular stress by adjusting its activities. Under conditions of stress, nucleolar function can be altered, and the organelle can participate in the activation of stress response pathways, including the p53 signaling pathway, which can lead to cell cycle arrest or apoptosis.

Nucleolus vs Nucleus

The nucleus and nucleolus are integral components of eukaryotic cells, each serving distinct functions vital to cellular operations. Their differences lie not only in function but also in structure and location within the cell.


The nucleus is the command center of a eukaryotic cell, encased in a double-layered membrane known as the nuclear envelope. This envelope is punctuated with pores that regulate the flow of materials between the nucleus and the cytoplasm. Inside, the nucleus contains the cell’s genetic material in the form of DNA, which is organized with proteins into chromatin. During specific stages of the cell cycle, chromatin condenses to form visible chromosomes.


The nucleolus is a dense region within the nucleus, not enclosed by a membrane. It forms around nucleolar organizing regions on chromosomes, where genes for ribosomal RNA (rRNA) are located. The primary function of the nucleolus is to synthesize rRNA and combine it with proteins imported from the cytoplasm to assemble ribosomal subunits, which are then transported out to the cytoplasm for protein synthesis.

Difference Between Nucleus and Nucleolus

Difference Between Nucleus and Nucleolus


Nucleus Nucleolus
Large, typically the most prominent organelle in a cell. Very small in comparison, located within the nucleus.
Bound by a nuclear envelope, consisting of two membranes. It has no limiting membrane; it’s a region within the nucleus.
Contains chromosomes made of DNA and proteins. Does not contain chromosomes but is rich in RNA.
Rich in DNA, the cell’s genetic material. Primarily composed of RNA and proteins involved in ribosome production.
Regulates gene expression, stores genetic information, and controls cell division. Primarily responsible for the synthesis of ribosomal RNA (rRNA) and assembly of ribosomes.
Present throughout the cell cycle, undergoes changes during cell division. May vary in size and visibility depending on the cell’s activity, particularly protein synthesis.


Where is the nucleolus located in the cell?

The nucleolus is located within the nucleus of eukaryotic cells, forming around nucleolar organizing regions on certain chromosomes.

Is nucleolus an organelle?

Yes, the nucleolus is considered an organelle, specifically a non-membrane-bound organelle found within the nucleus.

What would happen if there is no nucleolus in the cell?

Without a nucleolus, cells would fail to produce ribosomes efficiently, severely impairing protein synthesis and affecting cell growth and division.

The nucleolus plays a pivotal role in cell functionality, primarily orchestrating ribosomal RNA synthesis and ribosome assembly. Beyond its central task, it influences various cellular processes, including stress response and cell cycle regulation. This multifunctional organelle exemplifies the intricate and dynamic nature of cellular components, underpinning essential mechanisms vital for life.

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