The cell is the 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
Nuclear cells contain non-membrane rounded organelles, denser than karyoplasm, and called nucleoli or nucleoli. They were discovered in the 19th century. Now nucleoli are quite 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 reserve 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 amount of proteins. It is they who determine its high density. In addition to proteins, the nucleolus contains RNA and a small amount of DNA.
Cell cycle
It is interesting that in the life of a cell, which consists ofperiod of rest (interphase) and division (meiosis - in sex, mitosis - in somatic cells), the nucleoli are not permanently preserved. So, in the interphase, the nucleus with the nucleolus, whose functions are the preservation of the genome and the formation of protein-synthesizing organelles, are necessarily present. At the beginning of cell division, namely in prophase, they disappear and are re-formed only at the end of telophase, remaining in the cell until the next division or until apoptosis - its death.
Nuclear organizer
In the 30s of the last century, scientists found that the formation of nucleoli is controlled by certain sections of some chromosomes. They contain genes that store information about the structure and functions of the nucleolus in the cell. There is a correlation between the number of nucleolar organizers and the organelles themselves. For example, the clawed frog contains in its karyotype two nucleolar-forming chromosomes and, accordingly, there are two nucleoli 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 brain tissues, blood, and also in the blastomeres of a crushing zygote.
Nucleol Amplification
In the synthetic stage of interphase, along with DNA self-duplication, there is an 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 that carry information about RNA. Nucleoproteins not associated withchromosomes begin to function autonomously. As a result, many nucleoli are formed in the nucleus, distancing themselves from the nucleolus-forming chromosomes. This phenomenon is called rRNA gene amplification. 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 clear, given that in the early stages of embryogenesis: crushing and blastulation, a huge number of ribosomes are needed to synthesize the main building material - protein. Amplification is a fairly common process; it occurs in the oogenesis of plants, insects, amphibians, yeasts, and also in some protists.
Histochemical composition of the organelle
Let's 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 kinds of elements:
- Nucleonema (filamentous formations). They are heterogeneous and contain fibrils and lumps. Being a part of both plant and animal cells, nucleonemes 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 (nucleolins).
From the point of view of chemical analysis, this organelle is almost entirely composed of RNA and protein, andDNA is located only on its periphery, forming a ring-shaped structure - perinucleolar chromatin.
So, we have established 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 organelles 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 nucleolus. The first - reticular, is the most common and is characterized by an abundance of dense fibrillar material, lumps of nucleoproteins and nucleone. 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 occur in cells that have completely lost the ability to transcribe, such as normoblasts and enterocytes.
Segregated species is inherent in cells that have experienced intoxication with carcinogens, antibiotics. And, finally, the compact type of the nucleolus is characterized by many fibrillar centers and a small amount ofnucleonem.
Protein nucleolar matrix
Let's continue the study of the internal structure of the structures of the nucleus and determine what are the functions of the nucleolus in cell metabolism. It is known that about 60% of the dry mass of this organelle is accounted for by the proteins that make up the chromatin, ribosomal particles, and also by the nucleolar proteins themselves. Let's dwell on them in more detail. Some of the proteins are involved in processing - the formation of mature ribosomal RNA. These include RNA polymerase 1 and nuclease, which remove extra triplets from the ends of the rRNA molecule. The fibrillarin protein is located in the dense fibrillar component and, like the nuclease, carries out processing. Another protein is nucleolin. Together with fibrillarin, it is found in the PFC and FC of the nucleoli and in the nucleolar organizers of the chromosomes of the prophase of mitosis.
A polypeptide such as nucleophosin is located in the granular zone and the dense fibrillar component, it is involved in the formation of ribosomes from 40 S and 60 S subunits.
What is the function of the nucleolus
The synthesis of ribosomal RNA is the main task that the nucleolus must perform. At this time, transcription occurs on its surface (namely, in fibrillar centers) with the participation of the RNA polymerase enzyme. On this nucleolar organizer, hundreds of pre-ribosomes, called ribonucleoprotein globules, are synthesized. They form ribosomal subunits, which leave the karyoplasm through the nuclear pores and end up in the cytoplasm of the cell. The 40S small subunit binds to messenger RNA and only then to themthe 40S large subunit is attached. A mature ribosome is formed, capable of carrying out translation - the synthesis of cellular proteins.
In this article, we studied the structure and functions of the nucleolus in plant and animal cells.
