22 Nov 2019
What is a Nucleolus Function? – Definition
The nucleolus Function 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.
The nucleolus 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.
What Is The Function Of The Nucleolus?
In eukaryotic cells, the nucleolus has a well-ordered structure with four main ultrastructural components. The components can be further identified as:
- Fibrillar Centers: It is the place where the ribosomal proteins are formed.
- Granular Components: Before ribosomes are formed, these components have rRNA that binds to ribosomal proteins.
- Dense Fibrillar Components: It has a new transcribed RNA that connects to the ribosomal proteins.
- Nucleolar vacuoles: It is present only in plant cells.
The ultrastructure of the nucleolus can be easily visualized through an electron microscope. The arrangement of the nucleolus within the cell can be clearly studied by the techniques – fluorescent recovery after photobleaching and fluorescent protein tagging.
The nucleolus of several plant species has very high concentrations of iron in contrast to the human and animal cell nucleolus.
Nucleolus Function In Animal Cell
Estable and Sotelo (1951) described the structure of a nucleolus under the light microscope. According to them, the nucleolus consists of a continuous coiled filament called the nucleolonema embedded in a homogenous matrix, the pars amorpha. The first description of the nucleolar ultrastructure was given by Borysko and Bang (1951) and Bernhard (1952).
They described two main nucleolar components, a filamentous one corresponding to the nucleolonema and a homogenous one corresponding to the pars amorpha (matrix).
Nucleolus Function In Plant Cell
The definition of nucleolus can be described as the place where the biogenesis of ribosomal subunits and ribosomal rRNA occurs. Or that of a non-membrane bound structure that is also composed of nucleic acids, found within the nucleus. The rRNA is assembled and transcribed within this structure and several human diseases can occur when malfunctioning of the nucleoli happens.
Barbara McClintock first described the formation of nucleoli, which are formed around the nucleolar organizing regions (NORs), specific genetic loci. The nucleolus is defined as a “genetically determined element” due to this non-random organization.
What Is The Main Function Of The Nucleolus?
The Nucleus Of Many Eukaryotic Cells Contains A Structure Called A Nucleolus. 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). It’s 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, Whichoccurs Through A System Of Nucleolarpassages. These Passages Allow Macromolecules With A Molecular Weight Up To 2,000 Kdato 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.
(i) Ribosome formation or biogenesis of ribosomes.
(ii) Synthesis and storage of RNA:
It produces 70-90% of cellular RNA in many cells. It is a source of RNA. The chromatin in the nucleolus contains genes or ribosomal DNA (rDNA) for coding ribosomal RNA. Chromatin containing DNA gives rise to fibrils containing RNA. Granules containing RNA already produces ribosomes.
(iii) Protein synthesis:
Maggis (1960) and others have suggested that protein synthesis takes place in the nucleolus. Other studies confirm the above views. In eukaryotes, the gene coding for RNA contains a chain of at least 100-1000 repeating copies of DNA. This DNA is given off from the chromosomal fiber in the forms of loops. The DNA loops are associated with proteins to form nucleoli.