What is the function of the nucleolus in the cell?

The nucleolus is a prominent, non-membrane-bound structure located within the nucleus of eukaryotic cells. Its primary function is the production and assembly of ribosomes. The nucleolus synthesizes ribosomal RNA (rRNA) and combines it with proteins to form ribosomal subunits, which are then transported to the cytoplasm for protein synthesis. In addition to its role in ribosome biogenesis, the nucleolus is involved in regulating the cell cycle and responding to cellular stress. Its size and activity often reflect the cell's metabolic activity, becoming larger in cells with high rates of protein production.

Nucleolus not bound by a membrane but has a distinct composition and organization. Its structure can be divided into three main components:

01. Fibrillar Center (FC)
  • Contains the DNA regions (nucleolar organizer regions, or NORs) where ribosomal RNA (rRNA) genes are located.
  • Site of rRNA gene transcription initiation.
02. Dense Fibrillar Component (DFC)
  • Surrounds the fibrillar center.
  • Site where the newly transcribed rRNA is processed and modified.
  • Contains small nucleolar RNAs (snoRNAs) and associated proteins involved in rRNA processing.
03. Granular Component (GC)
  • Surrounds the dense fibrillar component.
  • Contains ribosomal proteins and pre-ribosomal particles.
  • Site where late stages of ribosome assembly occur, leading to the formation of ribosomal subunits.

Functions of Nucleolus 

The nucleolus is a dense, membrane-less structure found within the nucleus of eukaryotic cells. It plays a central role in many critical cellular functions, most notably ribosome biogenesis. In addition to its involvement in the production and assembly of ribosomes, the nucleolus also plays roles in cellular stress response, regulation of the cell cycle, and protein sequestration. Here is a detailed explanation of its primary functions:

01. Ribosome Production

One of the main functions of the nucleolus is the production of ribosomes, the molecular machines responsible for protein synthesis. Ribosomes consist of two subunits: a large and a small subunit. These subunits are assembled in the nucleolus from ribosomal RNA (rRNA) and ribosomal proteins.

The nucleolus serves as the site for the synthesis of rRNA, which forms the structural core of ribosomes. Ribosomal proteins, which are synthesized in the cytoplasm, are imported into the nucleolus where they combine with rRNA to form the small and large ribosomal subunits. These subunits are subsequently transported out of the nucleus through nuclear pores into the cytoplasm, where they come together to form functional ribosomes capable of translating mRNA into proteins.

02. rRNA Transcription

One of the primary functions of the nucleolus is the transcription of rRNA, a vital component of ribosomes. Ribosomal DNA (rDNA) genes, located in specific chromosomal regions known as nucleolar organizer regions (NORs), serve as templates for rRNA synthesis.

RNA polymerase I is responsible for transcribing these rDNA genes, resulting in the production of a large precursor rRNA molecule, known as 45S pre-rRNA. This precursor is the first step in generating the various types of rRNA needed for ribosome assembly.

The activity of rRNA transcription is tightly regulated, ensuring that ribosome production matches the cellular demand for protein synthesis. During periods of high growth and protein synthesis, nucleoli expand in size and increase the transcriptional output of rRNA to meet the cellular needs for ribosomes. Conversely, in times of cellular stress or when protein synthesis slows, nucleolar activity decreases.

03. Ribosomal Assembly

In the nucleolus, ribosomal assembly begins when the newly transcribed rRNAs are processed and combined with ribosomal proteins that are imported from the cytoplasm. The assembly of the small and large ribosomal subunits occurs in a step-wise fashion, with each subunit being formed by the integration of different rRNA species (18S, 5.8S, 28S, and 5S rRNAs) and specific ribosomal proteins.

Once ribosomal subunits are assembled, they are exported from the nucleolus to the cytoplasm, where they mature further and participate in protein synthesis by translating mRNA into polypeptides.

04. Regulation of Cell Cycle

The nucleolus also has a role in regulating the cell cycle, particularly by interacting with proteins that control cell division. One such protein is p53, a tumor suppressor that helps regulate the cell cycle and prevent uncontrolled cell growth. Nucleolus size and activity vary depending on the cell's growth and proliferation status. During periods of active cell division, the nucleolus is enlarged and highly active, producing more ribosomes to support increased protein synthesis. During mitosis, the nucleolus disassembles and its components are redistributed to daughter cells, ensuring proper chromosomal segregation and the reformation of the nucleus.

5. Response to Cellular Stress

The nucleolus acts as a sensor for cellular stress, such as DNA damage, oxidative stress, or nutrient deprivation. When cells encounter such stressors, the nucleolus can adjust its activities, primarily by altering ribosome biogenesis. In response to stress, the production of ribosomes often decreases, which helps to conserve cellular energy and resources during challenging conditions.

Moreover, stress can lead to nucleolar reorganization, affecting its structure and function. Under certain conditions, stress-induced nucleolar changes can lead to the release or activation of proteins like p53, triggering stress responses such as cell cycle arrest, apoptosis (programmed cell death), or DNA repair mechanisms.

6. Sequestration of Proteins

The nucleolus can also act as a temporary storage site for various proteins, including those involved in RNA processing, stress response, and signaling pathways. Proteins that are not immediately needed for their specific cellular functions can be sequestered in the nucleolus and held there until they are required. This protein sequestration helps maintain cellular homeostasis by regulating the availability and activity of these proteins, thus allowing the cell to fine-tune its responses to various internal and external signals.


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SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2


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