The cell nucleus is its most important organelle, the place of storage and reproduction of hereditary information. This is a membrane structure that occupies 10-40% of the cell, the functions of which are very important for the life of eukaryotes. However, even without the presence of a nucleus, the realization of hereditary information is possible. An example of this process is the vital activity of bacterial cells. Nevertheless, the structural features of the nucleus and its purpose are very important for a multicellular organism.
Location of the nucleus in the cell and its structure
The nucleus is located in the thickness of the cytoplasm and is in direct contact with the rough and smooth endoplasmic reticulum. It is surrounded by two membranes, between which is the perinuclear space. Inside the nucleus there is a matrix, chromatin and some nucleoli.
Some mature human cells do not have a nucleus, while others function under conditions of severe inhibition of its activity. In general, the structure of the nucleus (scheme) is presented as a nuclear cavity, limited by a karyolemma from the cell, containing chromatin and nucleoli fixed in the nucleoplasmnuclear matrix.
Structure of the karyolemma
For the convenience of studying the nucleus cell, the latter should be perceived as bubbles, limited by shells from other bubbles. The nucleus is a bubble with hereditary information located in the thickness of the cell. It is protected from its cytoplasm by a bilayer lipid membrane. The structure of the shell of the nucleus is similar to the cell membrane. In fact, they are distinguished only by the name and number of layers. Without all this, they are identical in structure and function.
The structure of the karyolemma (nuclear membrane) is two-layer: it consists of two lipid layers. The outer bilipid layer of the karyolemma is in direct contact with the rough reticulum of the cell endoplasm. Internal karyolemma - with the contents of the nucleus. There is a perinuclear space between the outer and inner karyomembrane. Apparently, it was formed due to electrostatic phenomena - repulsion of areas of glycerol residues.
The function of the nuclear membrane is to create a mechanical barrier that separates the nucleus from the cytoplasm. The inner membrane of the nucleus serves as a fixation site for the nuclear matrix - a chain of protein molecules that support the bulk structure. There are special pores in two nuclear membranes: messenger RNA enters the cytoplasm through them to the ribosomes. In the very thickness of the nucleus there are several nucleoli and chromatin.
Internal structure of the nucleoplasm
Features of the structure of the nucleus allow us to compare it with the cell itself. Inside the nucleus there is also a special environment (nucleoplasm),represented by a gel-sol, a colloidal solution of proteins. Inside it there is a nucleoskeleton (matrix), represented by fibrillar proteins. The main difference lies only in the fact that predominantly acidic proteins are present in the nucleus. Apparently, such a reaction of the environment is needed to preserve the chemical properties of nucleic acids and the occurrence of biochemical reactions.
Nucleolus
The structure of the cell nucleus cannot be completed without the nucleolus. It is a spiralized ribosomal RNA, which is in the maturation stage. Later, a ribosome will be obtained from it - an organelle necessary for protein synthesis. In the structure of the nucleolus, two components are distinguished: fibrillar and globular. They differ only by electron microscopy and do not have their own membranes.
The fibrillar component is in the center of the nucleolus. It is a strand of ribosomal-type RNA from which ribosomal subunits will be assembled. If we consider the core (structure and functions), then it is obvious that a granular component will subsequently be formed from them. These are the same maturing ribosomal subunits that are in the later stages of their development. They soon form ribosomes. They are removed from the nucleoplasm through the nuclear pores of the karyolemma and enter the membrane of the rough endoplasmic reticulum.
Chromatin and chromosomes
The structure and functions of the cell nucleus are organically linked: there are only those structures that are needed to store and reproduce hereditary information. There is also a karyoskeleton(nucleus matrix), the function of which is to maintain the shape of the organelle. However, the most important component of the nucleus is chromatin. These are chromosomes that play the role of file cabinets of various groups of genes.
Chromatin is a complex protein that consists of a polypeptide of a quaternary structure connected to a nucleic acid (RNA or DNA). Chromatin is also present in bacterial plasmids. Almost a quarter of the total weight of chromatin is made up of histones - proteins responsible for the "packaging" of hereditary information. This feature of the structure is studied by biochemistry and biology. The structure of the nucleus is complex precisely because of the chromatin and the presence of processes alternating its spiralization and despiralization.
The presence of histones makes it possible to condense and complete the DNA strand in a small place - in the cell nucleus. This happens as follows: histones form nucleosomes, which are a structure like beads. H2B, H3, H2A and H4 are the major histone proteins. The nucleosome is formed by four pairs of each of the presented histones. At the same time, histone H1 is a linker: it is associated with DNA at the site of entry into the nucleosome. DNA packaging occurs as a result of the "winding" of a linear molecule around 8 histone structure proteins.
The structure of the nucleus, the scheme of which is presented above, suggests the presence of a solenoid-like structure of DNA completed on histones. The thickness of this conglomerate is about 30 nm. At the same time, the structure can be further compacted in order to take up less space and be less exposed tomechanical damage that inevitably occurs during the life of the cell.
Chromatin fractions
The structure, structure and functions of the cell nucleus are fixated on maintaining the dynamic processes of chromatin spiralization and despiralization. Therefore, there are two main fractions of it: strongly spiralized (heterochromatin) and slightly spiralized (euchromatin). They are separated both structurally and functionally. In heterochromatin, DNA is well protected from any influences and cannot be transcribed. Euchromatin is less protected, but genes can be duplicated for protein synthesis. Most often, sections of heterochromatin and euchromatin alternate throughout the length of the entire chromosome.
Chromosomes
The cell nucleus, the structure and functions of which are described in this publication, contains chromosomes. It is a complex and compactly packed chromatin that can be seen under light microscopy. However, this is only possible if a cell is located on the glass slide at the stage of mitotic or meiotic division. One of the stages is the spiralization of chromatin with the formation of chromosomes. Their structure is extremely simple: the chromosome has a telomere and two arms. Each multicellular organism of the same species has the same structure of the nucleus. His chromosome set table is also similar.
Implementation of kernel functions
The main features of the structure of the nucleus are related to the performance of certain functions and the need to control them. The nucleus plays the role of a repository of hereditary information, that is, it is a kind of file cabinet withwritten sequences of amino acids of all proteins that can be synthesized in the cell. This means that in order to perform any function, a cell must synthesize a protein, the structure of which is encoded in the gene.
In order for the nucleus to "understand" which particular protein needs to be synthesized at the right time, there is a system of external (membrane) and internal receptors. Information from them comes to the nucleus through molecular transmitters. Most often this is realized through the adenylate cyclase mechanism. This is how hormones (adrenaline, norepinephrine) and some drugs with a hydrophilic structure act on the cell.
The second mechanism of information transfer is internal. It is characteristic of lipophilic molecules - corticosteroids. This substance penetrates the bilipid membrane of the cell and goes to the nucleus, where it interacts with its receptor. As a result of the activation of receptor complexes located on the cell membrane (adenylate cyclase mechanism) or on the karyolemma, the activation reaction of a certain gene is triggered. It replicates, on its basis messenger RNA is built. Later, according to the structure of the latter, a protein is synthesized that performs a certain function.
The nucleus of multicellular organisms
In a multicellular organism, the structural features of the nucleus are the same as in a unicellular one. Although there are some nuances. First, multicellularity implies that a number of cells will have their own specific function (or several). This means that some genes will always bedespiralized while others are inactive.
For example, in adipose tissue cells, protein synthesis will be inactive, and therefore most of the chromatin is spiralized. And in cells, for example, the exocrine part of the pancreas, the processes of protein biosynthesis are ongoing. Therefore, their chromatin is despiralized. In those areas whose genes are replicated most often. At the same time, a key feature is important: the chromosome set of all cells of one organism is the same. Only because of the differentiation of functions in the tissues, some of them are turned off from work, while others are despiralized more often than others.
Nuclear cells of the body
There are cells, the structural features of the nucleus of which may not be considered, because as a result of their vital activity they either inhibit its function or completely get rid of it. The simplest example is red blood cells. These are blood cells, the nucleus of which is present only in the early stages of development, when hemoglobin is synthesized. As soon as there is enough of it to carry oxygen, the nucleus is removed from the cell in order to facilitate it without interfering with oxygen transport.
In general terms, an erythrocyte is a cytoplasmic sac filled with hemoglobin. A similar structure is characteristic of fat cells. The structure of the cell nucleus of adipocytes is extremely simplified, it decreases and shifts to the membrane, and the processes of protein synthesis are maximally inhibited. These cells also resemble "bags" filled with fat, although, of course, the varietythere are slightly more biochemical reactions in them than in erythrocytes. Platelets also do not have a nucleus, but they should not be considered full-fledged cells. These are fragments of cells necessary for the implementation of hemostasis processes.
