The functions of the nucleolus in the cell are what? Nucleolus: structure and function

The cell is an elementary unit of living organisms on Earth and has a complex chemical organization of structures called organelles. These include the nucleolus, whose structure and functions we will study in this article.

Features of eukaryotic nuclei

The nucleated cells in their composition contain nemnbrannye organelles of rounded shape, denser than the karyoplasm, called nucleoli or nucleols. They were discovered in the 19th century. Nucleols have now been studied quite thoroughly thanks to electron microscopy. Almost until the 50s of the 20th century, the functions of the nucleoli were not determined, and the scientists considered this organelle, rather, as a reservoir of the reserve substances used during mitosis.

Modern studies have established that the organoid includes granules of the nucleoprotein nature. Moreover, biochemical experiments have confirmed that the organelle contains a large number of proteins. They also determine its high density. In addition to the proteins, the nucleolus contains RNA and a small amount of DNA.

Cell cycle

It is interesting that in the life of the cell, which consists of a period of rest (interphase) and division (meiosis - in sexual, mitosis - in somatic cells), the nucleoli remain unstable. Thus, in the interphase nucleus with nucleolus, whose functions - the preservation of the genome and the formation of protein-synthesizing organelles, are mandatory. 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.

Nucleolus organizer

In the 30-ies of the last century, scientists have found that the formation of nucleoli is controlled by certain areas 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, the spur frog contains two nucleolus-forming chromosomes in its karyotype and, correspondingly, two nucleols 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 the highly specialized tissues of the brain, blood, and also in the blastomeres of the crushing zygote.

Nucleol amplification

In the synthetic phase of the interphase, along with the self-duplication of DNA, there is an excessive replication of the number of rRNA genes. Since the main functions of the nucleolus are the production of ribosomes, in connection with the over-synthesis of DNA loci that carry information about RNA, the number of these organelles sharply increases. Nucleoproteins, not associated with chromosomes, begin to function autonomously. As a result, a nucleus is formed in the nucleus, distancing from the nucleolus-forming chromosomes. This phenomenon is called amplification of rRNA genes. Continuing to study the functions of the nucleolus in a cell, we note that their most active synthesis occurs in the prophase of the reductive division of meiosis, as a result of which first-order oocytes can contain several hundred nucleoli.

The biological significance of this phenomenon becomes understandable when one considers that in the early stages of embryogenesis: fragmentation and blastulation, a huge amount of ribosomes is needed, synthesizing the main building material - protein. Amplification is a fairly common process, it occurs in the ovogenesis of plants, insects, amphibians, yeasts, and also in some protista.

Histochemical composition of organelles

Let's continue the study of eukaryotic cells and their structures, and consider the nucleolus, the structure and functions of which are interrelated. It is established that it contains three types of elements:

1. Nucleonomes (filamentous formations). They are heterogeneous and contain fibrils and lumps. Entering into the composition of both plant and animal cells, nucleoneomes 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.
2. Vacuoles (light areas).
3. Granular granules (nucleolines).

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 an annular structure - pericameral chromatin.

So, we have established that the nucleolus consists of five formations: fibrillar and granular centers, chromatin, the 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, and also on the characteristics of their metabolism. There are 5 basic structural types of nucleols. The first is reticular, the most common and characterized by an abundance of dense fibrillar material, a nucleoprotein block and nucleon. The process of rewriting information from nucleolar organizers is very active, therefore 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 the protein-synthesizing organelles are formed, the reticular type of organization is inherent in both plant and animal cells. The ring-like 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.

Segregated species are inherent in cells that have experienced intoxication with carcinogens, antibiotics. And, finally, a compact type of nucleolus is characterized by a multitude of fibrillar centers and a small number of nucleons.

Protein nucleolar matrix

Let us continue our study of the internal structure of the nucleus structures and determine what are the functions of the nucleolus in the metabolism of the cell. It is known that about 60% of the dry weight of this organoid is accounted for by proteins that are part of chromatin, ribosomal particles, as well as actually nucleolar proteins. Let us dwell on them in more detail. Some of the proteids 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 protein fibrillarin is in a dense fibrillar component and, like nuclease, it carries out processing. Another protein is nucleolin. Together with fibrillarin it is located in the PFC and Fc nucleoli and in the nucleolar organizers of chromosomes prophase mitosis.

Such a polypeptide as nucleophosin is located in the granular zone and dense fibrillar component, it participates in the formation of ribosomes from 40 S and 60 S subunits.

What is the function of the nucleolus?

Synthesis of ribosomal RNA is the main task that a nucleolus should perform. At this time on its surface (namely, in the fibrillar centers) transcription occurs with the participation of the enzyme RNA polymerase. This nucleolus organizer synthesizes hundreds of pre-ribosomes, called ribonucleoprotein globules. Of these, ribosomal subunits are formed, which through the nuclear pores leave the karyoplasm and appear in the cytoplasm of the cell. A small subunit of 40S binds to the information RNA and only then a large subunit 40S is attached to them. A mature ribosome is formed, capable of translating - the synthesis of cellular proteins.

In this article we have studied the structure and functions of the nucleolus in plant and animal cells.