What is Nucleolus? Function, Definition & More

The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cells. The nucleolar organizer regions of chromosomes, which contain the genes for pre‐ribosomal ribonucleic acid (rRNA), serve as the foundation for the nucleolar structure. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell, and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre‐rRNA. During transcription, ribosomal and non‐ribosomal proteins attach to the rRNA.

Subsequently, there is modification and cleavage of pre‐rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre‐ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified. These include assembly of signal recognition particles, sensing cellular stress, and transport of human immunodeficiency virus 1 (HIV‐1) messenger RNA.

What Is Nucleolus?

The nucleolus is a round body located inside the nucleus of a eukaryotic cell. It is not surrounded by a membrane but sits in the nucleus. The nucleolus makes ribosomal subunits from proteins and ribosomal RNA, also known as rRNA. It then sends the subunits out to the rest of the cell where they combine into complete ribosomes. Ribosomes make proteins; therefore, the nucleolus plays a vital role in making proteins in the cell.

Nucleolus function

Nucleolus function

The nucleolus is the most prominent structure in a cell nucleus. It is the site of ribosomal RNA (rRNA) transcription, pre-rRNA processing, and ribosome subunit assembly. The nucleolus is a dynamic structure that assembles around the clusters of rRNA gene repeats during late telophase, persists throughout interphase, and then disassembles as cells enter mitosis. Owing to the difference in density between the nucleolus and the surrounding nucleoplasm, it is readily visible in either life or fixed cells viewed by phase contrast or differential interference contrast (DIC) optics (upper-left panel).

Thanks to the advent of fluorescent protein (FP) technology, nucleoli can also be detected by fluorescence microscopy in cell lines expressing FP-tagged nucleolar proteins. An example is shown in the inset of the upper-left panel, in which PP1γ, a protein phosphatase that accumulates in the nucleolus, is tagged with YFP and stably expressed in HeLa cells.

Nucleolus Function

The nucleolus, whose primary function is to assemble ribosomes, is the largest structure in the cell nucleus.

The Nucleus organizer regions of chromosomes, which harbor the genes for pre‐rRNA, are the foundation for the Nucleus.

All active nucleoli contain at least two ultrastructural components, the nucleolar dense fibrillar component representing early pre‐ribosomal complexes and the granular component containing more mature pre‐ribosomal particles.

Most nucleoli in higher eukaryotes also contain fibrillar centers, which are the interphase equivalents of the Nucleus organizer regions.

The Nucleus disassembles at the beginning of mitosis and begins to reassemble in telophase.

Ribosome assembly begins with the transcription of pre‐rRNA by RNA polymerase I.



Ribosomal and non‐ribosomal proteins and 5S RNA associate with the pre‐rRNA during and after transcription.

The pre‐rRNA is modified and processed into rRNA with the aid of non‐ribosomal proteins and small nucleolar RNAs.

The Nucleus has numerous other functions including assembly of signal recognition particles, modification of transfer RNAs, and sensing cellular stress.

Nucleolus Function In Animal Cell/Plant Cell

The Nucleus is one of the most important components of the cell nucleus and is far the most easily recognized substructure in the nucleus of eukaryotic cells. It can easily be determined by phase-contrast microscopy and with the use of different dyes. The nucleolus, also known as the ribosome factory, is composed of protein, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA).

The Nucleus function can be described thus: a long ribosomal RNA (rRNA) precursor molecule is transcribed from DNA, at the nucleus, is being processed into three mature RNAs that are packed together with certain types of proteins in order to create small and large ribosomal subunits. As soon as the subunits have been assembled, these will then be transported out of the nucleolus for the cytoplasm to use in functions such as protein synthesis, or translation. The nucleolus is not a static structure by nature. It will eventually disassemble during mitosis and will transform back in the early G1 phase. The formation of this nucleus component does not cause ribosomal RNA or rRNA gene expression but rather, produced nucleoli are actually the result of rRNA processing and transcription.

Nucleolus structure & location

Nucleolus structure & location


The nucleolus is located in the center of the nucleus of a cell. It can vary in size depending on the type of organism. The main components of the nucleolus are RNA, DNA, and proteins.


The nucleolus has one main function. That main function is the production of subunits which then together form ribosomes. This production of ribosomes indirectly involves the nucleolus in protein synthesis. Furthermore, the nucleolus is involved in about 50% of RNA synthesis.

Structure Of The Nucleus

The nucleus of many eukaryotic cells contains a structure called a Nucleus. As the nucleus is the “brain” of the cell, the nucleolus could loosely be thought of as the brain of the nucleus. The nucleolus takes up around 25% of the volume of the nucleus.
This structure is made up of proteins and ribonucleic acids (RNA). Its main function is to rewrite ribosomal RNA (rRNA) and combine it with proteins. This results in the formation of incomplete ribosomes. There is an uninterrupted chain between the nucleoplasm and the interior parts of the nucleolus, which occurs through a system of nucleolarpassages. These passages allow macromolecules with a molecular weight up to 2,000 kDa to be easily circulated throughout the nucleolus.
Because of its close relationship to the chromosomal matter of the cell and its important role in producing ribosomes, the nucleolus is thought to be the cause of a variety of different human diseases.

What Is The Function Of The Nucleus?

This Organelle Has Two Major Functions: It Stores The Cell’s Hereditary Material, Or Dna, And It Coordinates The Cell’s Activities, Which Include Growth, Intermediary Metabolism, Protein Synthesis, And Reproduction (Cell Division). Only The Cells Of Advanced Organisms, Known As Eukaryotes, Have A Nucleus.

What Is Nucleus And What Is Its Function?

Function Of The Nucleus. The Nucleus Is An Organelle Found In Eukaryotic Cells. Inside Its Fully-enclosed Nuclear Membrane, It Contains The Majority Of The Cell’s Genetic Material. This Material Is Organized As DNA Molecules, Along With A Variety Of Proteins, To Form Chromosomes.

Where Is The Nucleolus Found In A Plant Or Animal Cell?

It Is Found In Both Plant And Animal Cells. But In RBCs Or Red Blood Cells, The Nucleus Is Enucleated.

What Is The Main Function Of Nucleolus?

The Nucleus Makes Ribosomal Subunits From Proteins And Ribosomal Rna, Also Known As rRNA. It Then Sends The Subunits Out To The Rest Of The Cell Where They Combine Into Complete Ribosomes. Ribosomes Make Proteins; Therefore, The Nucleus Plays A Vital Role In Making Proteins In The Cell.

Does Plant Cell Have Nucleus?

The Nucleus Houses The Cell’s Dna And Directs The Synthesis Of Proteins And Ribosomes. Plant Cells Have A Cell Wall, A Large Central Vacuole, Chloroplasts, And Other Specialized Plastids, Whereas Animal Cells Do Not.