A cell is an elementary unit of living organisms on Earth and has a complex chemical organization of structures called organelles. These include the nucleolus, the structure and functions of which we will study in this article.
Features of eukaryotic nuclei
Nucleated cells in their composition contain round-shaped non-membrane organelles, denser than karyoplasm, and called nucleoli or nucleoli. They were discovered back in the 19th century. Nowadays, nucleols have been fully studied thanks to electron microscopy. Almost until the 50s of the 20th century, the functions of the nucleoli were not determined, and scientists considered this organelle, rather, as a reservoir of spare substances used during mitosis.
Modern research has established that the organoid includes granules of a nucleoprotein nature. Moreover, biochemical experiments have confirmed that the organelle contains a large number of proteins. It is they who determine its high density. In addition to proteids, RNA and a small amount of DNA are present in the nucleoli.
Cell cycle
It is interesting that in the life of a cell, which consists of a period of rest (interphase) and division (meiosis - in reproductive cells , mitosis - in somatic cells), the nucleoli persist. So, in the interphase, the nucleus with the nucleolus, whose functions are to preserve the genome and the formation of protein synthesizing organelles, are necessarily present. At the beginning of cell division, namely in prophase, they disappear and re-form only at the end of the telophase, remaining in the cell until the next division or until apoptosis - its death.
The nucleolar organizer
In the 30s of the last century, scientists found that the formation of nucleoli is controlled by certain sections of certain chromosomes. They contain genes that store information about what structure and what are the functions of the nucleolus in the cell. There is a correlation between the number of nucleolar organizers and the organelles themselves. For example, a Spur frog contains two nucleolar-forming chromosomes in its karyotype and, accordingly, two nucleoli are located in the nuclei of its somatic cells.
Since the functions of the nucleolus, as well as its presence, are closely related to cell division and the formation of ribosomes, the organelles themselves are absent in highly specialized tissues of the brain, blood, and also in the blastomeres of the fragmented zygote.
Amplification Nucleol
In the synthetic stage of interphase, along with DNA self-doubling, there is excessive replication of the number of rRNA genes. Since the main functions of the nucleolus are the production of ribosomes, the number of these organelles sharply increases due to the oversynthesis of DNA loci carrying RNA information. Nucleoproteins that are not associated with chromosomes begin to function autonomously. As a result, in the nucleus, many nucleols are formed, spacing from the nucleolus-forming chromosomes. This phenomenon is called amplification of rRNA genes. Continuing to study the functions of the nucleolus in the cell, we note that their most active synthesis occurs in the prophase of the reduction division of meiosis, as a result of which first-order oocytes can contain several hundred nucleoli.
The biological significance of this phenomenon becomes understandable, given that in the early stages of embryogenesis: fragmentation and blastulation, a huge number of ribosomes are synthesized, which synthesize the main building material - protein. Amplification is a fairly common process; it occurs in the ovogenesis of plants, insects, amphibians, yeast, as well as in some protists.
The histochemical composition of organelles
We continue the study of eukaryotic cells and their structures, and consider the nucleolus, the structure and functions of which are interconnected. It is established that it contains three types of elements:
- Nucleonemia (filamentous formations). They are heterogeneous and contain fibrils and clumps. Being a part of both plant and animal cells, nucleonems form fibrillar centers. The cytochemical structure and functions of the nucleolus also depend on the presence of a matrix in it - a network of supporting protein molecules of the tertiary structure.
- Vacuoles (light areas).
- Granular granules (nucleolin).
From the point of view of chemical analysis, this organoid is almost entirely composed of RNA and protein, and DNA is located only on its periphery, forming a ring-shaped structure - perinucleolar chromatin.
So, we found that the nucleolus consists of five formations: fibrillar and granular centers, chromatin, protein reticulum and a dense fibrillar component.
Types of Nucleoli
The biochemical structure of these organoids depends on the type of cells in which they are present, as well as on the characteristics of their metabolism. There are 5 main structural types of nucleos. The first is reticular, the most common and is characterized by an abundance of dense fibrillar material, clumps of nucleoproteins and nucleons. The process of rewriting information from the nucleolar organizers is very active, so the fibrillar centers are poorly visible in the field of view of the microscope.
Since the main functions of the nucleolus in the cell are the synthesis of ribosomal subunits from which protein synthesizing organelles are formed, the reticular type of organization is inherent in both plant and animal cells. The ring-shaped type of nucleoli is found in connective tissue cells: lymphocytes and endotheliocytes, in which rRNA genes are practically not transcribed. Residual nucleoli are found in cells that have completely lost the ability to transcribe, for example, in normoblasts and enterocytes.
A segregated species is inherent in cells that have experienced toxicity with carcinogens, antibiotics. And, finally, the compact type of nucleolus is characterized by many fibrillar centers and a small number of nucleons.
Protein nucleolar matrix
We continue the study of the internal structure of the nucleus structures and determine what the functions of the nucleolus are in the metabolism of the cell. It is known that about 60% of the dry mass of this organoid is accounted for by the proteins that make up chromatin, ribosomal particles, and also the nucleolar proteins themselves. Let us dwell on them in more detail. Part of the proteids is involved in processing - the formation of mature ribosomal RNA. These include RNA polymerase 1 and nuclease, which remove excess triplets from the ends of the rRNA molecule. The fibrillarin protein is located in a dense fibrillar component and, like nuclease, carries out processing. Another protein is nucleolin. Together with fibrillarin, it is located in PFK and FC of nucleoli and in nucleolar organizers of mitosis prophase chromosomes.
A polypeptide such as nucleophosin is located in the granular zone and dense fibrillar component, it is involved in the formation of ribosomes from 40 S and 60 S subunits.
What function does the nucleolus perform
The synthesis of ribosomal RNA is the main task that a nucleol must perform. At this time, transcription occurs on its surface (namely, in the fibrillar centers) with the participation of the RNA polymerase enzyme. At this nucleolar organizer, hundreds of pre-ribosomes, called ribonucleoprotein globules, are synthesized. Of these, ribosomal subunits are formed, which leave the karyoplasm through the nuclear pores and end up in the cell cytoplasm. The small 40S subunit binds to messenger RNA and only then does the large 40S subunit attach to them. A mature ribosome is formed, capable of translating - the synthesis of cellular proteins.
In this article, we studied the structure and functions of the nucleolus in plant and animal cells.