The cell nucleus is its most important organelle, a place of storage and reproduction of hereditary information. This is a membrane structure, occupying 10-40% of the cell, the functions of which are very important for the life of eukaryotes. However, even without a kernel, the implementation 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.
The 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 a certain number of 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 bounded by a karyolemma from the cell, containing chromatin and nucleoli fixed in the nucleoplasm by a nuclear matrix.
The structure of the karyolemma
For the convenience of studying nucleus cells, the latter should be understood as vesicles bounded by membranes from other vesicles. A nucleus is a vesicle with hereditary information located in the thickness of a cell. From its cytoplasm, he is protected by a bilayer lipid membrane. The structure of the membrane of the nucleus is similar to a 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 endoplasm of the cell. Internal karyolemma - with the contents of the nucleus. Between the outer and inner karyomembrane there is a perinuclear space. Apparently, it was formed due to electrostatic phenomena - repulsion of areas of glycerin residues.
The function of the nuclear membrane is to create a mechanical barrier separating the nucleus and 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: through them, informational RNA enters the cytoplasm to the ribosomes. In the very core are several nucleoli and chromatin.
The internal structure of the nucleoplasm
The structural features of the nucleus make it possible to compare it with the cell itself. Inside the nucleus there is also a special medium (nucleoplasm) represented by gel sol, a colloidal solution of proteins. Inside it there is a nucleoskeleton (matrix), represented by fibrillar proteins. The main difference is only that the core contains predominantly acidic proteins. Apparently, such a reaction of the medium 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 at the stage of maturation. Later, a ribosome is obtained from it - the organelle necessary for protein synthesis. Two components are distinguished in the structure of the nucleolus: fibrillar and globular. They differ only by electron microscopy and do not have their own membranes.
The fibrillar component is located in the center of the nucleolus. It is a ribosomal type RNA strand from which ribosomal subunits will be assembled. If we consider the core (structure and functions), 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. Of these, ribosomes soon form. 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 connected: only those structures are present that are needed to store and reproduce hereditary information. There is also a karyoskeleton (core matrix) whose function 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 card files of various groups of genes.
Chromatin is a complex protein that consists of a quaternary structure polypetide linked 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 - the proteins responsible for the "packaging" of hereditary information. This structural feature is studied by biochemistry and biology. The structure of the nucleus is complex precisely because of chromatin and the presence of processes alternating between its spiralization and despiralization.
The presence of histones makes it possible to condense and complement 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 main 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 e to the nucleosome. DNA packaging occurs as a result of the "winding" of a linear molecule on 8 proteins of the histone structure.
The structure of the nucleus, the scheme of which is presented above, suggests the presence of a solenoid-like structure of DNA equipped with histones. The thickness of this conglomerate is about 30 nm. In this case, the structure can continue to condense in order to occupy less space and be less exposed to mechanical damage that inevitably occurs during the life of the cell.
Chromatin fractions
The structure, structure and functions of the cell nucleus are fixated on supporting the dynamic processes of chromatin spiralization and despiralization. Therefore, there are two main fractions of it: highly spiralized (heterochromatin) and low-spiralized (euchromatin). They are divided both structurally and functionally. In heterochromatin, DNA is well protected from any influences and cannot be transcribed. Euchromatin is less protected, but genes can double for protein synthesis. Most often, the 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. This is a complex and compactly packed chromatin, which can be seen with light microscopy. However, this is only possible if a cell in the stage of mitotic or meiotic division is located on a glass slide. One of the steps is chromatin spiralization with the formation of chromosomes. Their structure is extremely simple: the chromosome has a telomere and two shoulders. Each multicellular organism of the same species has the same nuclear structure. His chromosome set table is also similar.
Kernel function implementation
The main features of the structure of the nucleus are associated with the performance of certain functions and the need for their control. The nucleus plays the role of a repository of hereditary information, that is, it is a kind of card index with recorded amino acid sequences of all proteins that can be synthesized in the cell. So, to perform any function, the cell must synthesize a protein whose structure is encoded in the gene.
In order for the nucleus to "understand" what specific protein needs to be synthesized at the right time, there is a system of external (membrane) and internal receptors. Information from them enters the nucleus through molecular transmitters. Most often this is realized through the adenylate cyclase mechanism. So, hormones (adrenaline, norepinephrine) and some drugs with a hydrophilic structure act on the cell.
The second mechanism for transmitting information is internal. It is characteristic of lipophilic molecules - corticosteroids. This substance penetrates through the bilipid membrane of the cell and is directed to the nucleus, where it interacts with its receptor. As a result of activation of receptor complexes located on the cell membrane (adenylate cyclase mechanism) or on the karyolemma, the activation reaction of a specific gene is triggered. It is replicated, and information RNA is built on its basis. Later, according to the structure of the latter, a protein is synthesized that performs a certain function.
The core of multicellular organisms
In a multicellular organism, the structural features of the nucleus are the same as in a unicellular organism. Although there are some nuances. Firstly, multicellularity implies that a number of cells will have their own specific function (or several). This means that some genes will constantly be despiralized, while others will be inactive.
For example, in the cells of adipose tissue, protein synthesis will be inactive, and therefore most of the chromatin is spiralized. And in the cells, for example, of the exocrine part of the pancreas, the processes of protein biosynthesis are ongoing. Because 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 despiralize more often than others.
Nuclear-free cells of the body
There are cells whose structural features of the nucleus may not be considered, because as a result of their vital functions they either inhibit its function or completely get rid of it. The simplest example is red blood cells. These are blood cells, the core of which is present only in the early stages of development, when hemoglobin is synthesized. As soon as its quantity is sufficient for oxygen transfer, the nucleus is removed from the cell in order to facilitate its not to interfere with oxygen transport.
In general, a red blood cell is a cytoplasmic sac filled with hemoglobin. A similar structure is also 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 suppressed as much as possible. These cells also resemble “bags” filled with fat, although, of course, the variety of biochemical reactions in them is slightly greater than in red blood cells. 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 hemostatic processes.