Felice Fontana discovered the nucleolus in 1774. The first documented accounts about nucleolus were made by Wagner (1835) and Valentin (1836, 1839). The nucleolus is a small, dense structure present in the nucleus. It has a spherical structure. The nucleus is present in the cell. The nucleolus is visible during the interphase of the cell division under a highly magnified microscope. The nucleolus region is dense. This is due to the presence of proteins, DNA and RNA. The plural form of the nucleolus is “nucleoli”. Nucleoli are non–membrane-bound bodies. These are found within the nucleus.

Discoveries

The basic building or unit of life is termed as a “cell”. Robert Hooke in 1665 discovered a cell from a cork cell. In 1674, Lee Wenhoek discovered cells from living cells in pond water. In 1833, Robert Brown coined the term nucleus. Purkinje coined the term protoplasm for fluid substance in 1849. Plants and animals are composed of cells, and the cell is the basic unit of life. A cell arises from a preexisting cell. Felice Fontana discovered the nucleolus in 1774. Nucleoli were first discovered in 1950. Then, in the early 1960s, a booklet of studies led to discoveries in the modern era of cell biology.

The Nucleolus

The nucleolus is a significant sub-nuclear structure. It resides within the nuclear matrix. The granular material consists of ribosomal subunits. These have already been formed but have not yet matured. These materials are waiting to be exported to the cytoplasm. The threadlike fibrillar part is present in the nucleolus. These thread-like parts of a nucleolus are predominantly composed of rRNA molecules. These are also associated proteins that have joined together to form fibrils.

In the modern era, researchers concluded that nucleoli manufacture the subunits. These are combined to form ribosomes. The ribosomes are the cell’s protein-producing factories. The ribosomal requirements depend on the size of nucleoli. In cells, the nucleolus produces large amounts of protein. 

Components of nucleolus

 Nucleolus has four main ultrastructural components as follows:

• Granular Components – These components have rRNA. These are bound to ribosomal proteins.
• Fibrillar Centers- The ribosomal protein formation occurs here.
• Dense Fibrillar Components- Dense Fibrillar has new transcribed RNA. This connects to the ribosomal proteins.
• Nucleolar vacuoles- These types of vacuoles are present only in plant cells.

Difference Between Nucleus and Nucleolus

Nucleolus Detection Methods 

• The electron microscope helps to visualise the nucleolus present in between a cell. 
• Nucleoli are non–membrane-bound bodies. These are found within the nucleus. The nucleolus is the site of transcribed ribosomal RNA. The “SYTO RNA” select Green Fluorescent Cell Stain is a cell-permeant nucleic acid stain that is selective for RNA.
• DAPI staining of nuclei identifies the nucleolus. This appears as a black cavity in the nucleus. Because a threefold lower concentration of DNA in the nucleolus is present in comparison to the surrounding nucleoplasm. The nucleolus is visible in a dark region of low DNA density by DAPI DNA stain. This happens because of the presence of the abundant amount of ribosomal RNA and proteins in the nucleolus.

The function of the nucleolus

• The nucleolus structure consists of a small, dense, spherical structure in the nucleus during interphase. 
• In cells, the ribosomal units combine into complete ribosomes.
• The nucleus controls and regulates the activities of the cell such as growth, reproduction, metabolism, etc. 
• The nucleus carries the genes that carry the hereditary information.
•  Nucleoli are small bodies present in the nucleus.
•  The nucleolus is responsible for the production of ribosomes in eukaryotic cells.

Conclusion

In summary, we have learnt the nucleolus definition and gathered nucleolus notes for preparation. The nucleolus is an organelle present in the nucleus. This is the largest nuclear organelle. It plays an important role in the transcription and processing of rRNA. It plays a major role in facilitating ribosome biogenesis. The nucleolus is responsible for producing and assembling ribosomes. The assembled ribosomes are transported to the cytoplasm. Here ribosomes serve as the sites for protein synthesis. The nucleolus is assembled around arrays of ribosomal DNA genes by forming specific chromosomal features. These specific chromosomal features are known as “nucleolar organising regions”. NORs are the sites of ribosomal DNA transcription.