The structure and functions of the cell have undergone a number of changes in the course of evolution. The appearance of new organelles was preceded by transformations in the atmosphere and lithosphere of the young planet. One of the significant acquisitions was the cell nucleus. Eukaryotic organisms received, due to the presence of separate organelles, significant advantages over prokaryotes and quickly began to dominate.
The cell nucleus, the structure and functions of which are somewhat different in different tissues and organs, has improved the quality of RNA biosynthesis and the transmission of hereditary information.
Origin
To date, there are two main hypotheses about the formation of a eukaryotic cell. According to the symbiotic theory, organelles (such as flagella or mitochondria) were once separate prokaryotic organisms. The ancestors of modern eukaryotes devoured them. The result was a symbiotic organism.
The core was formed as a result of protrusion inwardsection of the cytoplasmic membrane. This was a necessary acquisition on the way to mastering a new way of nutrition, phagocytosis, by the cell. The capture of food was accompanied by an increase in the degree of cytoplasmic mobility. Genophores, which were the genetic material of a prokaryotic cell and attached to the walls, fell into a zone of strong "flow" and needed protection. As a result, a deep invagination of a section of the membrane containing attached genophores was formed. This hypothesis is supported by the fact that the shell of the nucleus is inextricably linked with the cytoplasmic membrane of the cell.
There is another version of the development of events. According to the viral hypothesis of the origin of the nucleus, it was formed as a result of infection of an ancient archaean cell. A DNA virus infiltrated it and gradually gained complete control over life processes. Scientists who consider this theory more correct, give a lot of arguments in its favor. However, to date, there is no conclusive evidence for any of the existing hypotheses.
One or more
Most of the cells of modern eukaryotes have a nucleus. The vast majority of them contain only one such organelle. There are, however, cells that have lost the nucleus due to some functional features. These include, for example, erythrocytes. There are also cells with two (ciliates) and even several nuclei.
Structure of the cell nucleus
Regardless of the characteristics of the organism, the structure of the nucleus is characterized by a set of typicalorganelles. It is separated from the inner space of the cell by a double membrane. In some places, its inner and outer layers merge, forming pores. Their function is to exchange substances between the cytoplasm and the nucleus.
The organelle space is filled with karyoplasm, also called nuclear sap or nucleoplasm. It contains chromatin and the nucleolus. Sometimes the last of the named organelles of the cell nucleus is not present in a single copy. In some organisms, nucleoli, on the contrary, are absent.
Membrane
The nuclear membrane is formed by lipids and consists of two layers: outer and inner. In fact, this is the same cell membrane. The nucleus communicates with the channels of the endoplasmic reticulum through the perinuclear space, a cavity formed by two layers of the membrane.
The outer and inner membranes have their own structural features, but are generally quite similar.
Closest to cytoplasm
The outer layer passes into the membrane of the endoplasmic reticulum. Its main difference from the latter is a significantly higher concentration of proteins in the structure. The membrane in direct contact with the cytoplasm of the cell is covered with a layer of ribosomes from the outside. It is connected to the inner membrane by numerous pores, which are rather large protein complexes.
Inner layer
The membrane facing the cell nucleus, unlike the outer one, is smooth, not covered with ribosomes. It limits the karyoplasm. A characteristic feature of the inner membrane is a layer of nuclear lamina lining it from the side,in contact with the nucleoplasm. This specific protein structure maintains the shape of the envelope, is involved in the regulation of gene expression, and also promotes attachment of chromatin to the nuclear membrane.
Metabolism
The interaction of the nucleus and cytoplasm is carried out through nuclear pores. They are rather complex structures formed by 30 proteins. The number of pores on one core can be different. It depends on the type of cell, organ and organism. So, in humans, the cell nucleus can have from 3 to 5 thousand pores, in some frogs it reaches 50,000.
The main function of the pores is the exchange of substances between the nucleus and the rest of the cell space. Some molecules pass through the pores passively, without additional expenditure of energy. They are small in size. Transportation of large molecules and supramolecular complexes requires the consumption of a certain amount of energy.
RNA molecules synthesized in the nucleus get into the cell from the karyoplasm. Proteins necessary for intranuclear processes are transported in the opposite direction.
Nucleoplasma
Nuclear juice is a colloidal solution of proteins. It is bounded by the nuclear envelope and surrounds the chromatin and nucleolus. Nucleoplasm is a viscous liquid in which various substances are dissolved. These include nucleotides and enzymes. The former are essential for DNA synthesis. Enzymes are involved in transcription as well as DNA repair and replication.
The structure of nuclear juice changes depending on the state of the cell. There are two of them - stationary andoccurring during division. The first is characteristic of interphase (the time between divisions). At the same time, nuclear juice is distinguished by a uniform distribution of nucleic acids and unstructured DNA molecules. During this period, the hereditary material exists in the form of chromatin. The division of the cell nucleus is accompanied by the transformation of chromatin into chromosomes. At this time, the structure of the karyoplasm changes: the genetic material acquires a certain structure, the nuclear envelope is destroyed, and the karyoplasm is mixed with the cytoplasm.
Chromosomes
The main functions of the nucleoprotein structures of the chromatin transformed at the time of division are the storage, implementation and transmission of hereditary information contained in the cell nucleus. Chromosomes are characterized by a certain shape: they are divided into parts or arms by a primary constriction, also called the coelomere. According to its location, three types of chromosomes are distinguished:
- rod-shaped or acrocentric: they are characterized by the placement of the coelomere almost at the end, one arm is very small;
- diversified or submetacentric have arms of unequal length;
- equilateral or metacentric.
The set of chromosomes in a cell is called a karyotype. Each type is fixed. In this case, different cells of the same organism may contain a diploid (double) or haploid (single) set. The first option is typical for somatic cells, which mainly make up the body. The haploid set is a privilege of germ cells. human somatic cellscontain 46 chromosomes, sex - 23.
Chromosomes of the diploid set make pairs. Identical nucleoprotein structures included in a pair are called allelic. They have the same structure and perform the same functions.
The structural unit of chromosomes is the gene. It is a section of the DNA molecule that codes for a specific protein.
Nucleolus
The cell nucleus has one more organelle - the nucleolus. It is not separated from the karyoplasm by a membrane, but it is easy to notice when examining the cell with a microscope. Some nuclei may have multiple nucleoli. There are also those in which such organelles are completely absent.
The shape of the nucleolus resembles a sphere, has a fairly small size. It contains various proteins. The main function of the nucleolus is the synthesis of ribosomal RNA and the ribosomes themselves. They are necessary for the creation of polypeptide chains. Nucleoli form around special regions of the genome. They are called nucleolar organizers. It contains the ribosomal RNA genes. The nucleolus, among other things, is the place with the highest concentration of protein in the cell. Part of the proteins is necessary to perform the functions of the organoid.
The nucleolus consists of two components: granular and fibrillar. The first is the maturing ribosome subunits. In the fibrillar center, the synthesis of ribosomal RNA is carried out. The granular component surrounds the fibrillar component located in the center of the nucleolus.
Cell nucleus and its functions
The role thatplays the core, is inextricably linked with its structure. The internal structures of the organoid jointly implement the most important processes in the cell. It houses the genetic information that determines the structure and function of the cell. The nucleus is responsible for the storage and transmission of hereditary information during mitosis and meiosis. In the first case, the daughter cell receives a set of genes identical to the parent. As a result of meiosis, germ cells are formed with a haploid set of chromosomes.
Another no less important function of the nucleus is the regulation of intracellular processes. It is carried out as a result of controlling the synthesis of proteins responsible for the structure and functioning of cellular elements.
Influence on protein synthesis has another expression. The nucleus, controlling the processes inside the cell, unites all its organelles into a single system with a well-functioning mechanism of work. Failures in it lead, as a rule, to cell death.
Finally, the nucleus is the site of synthesis of ribosome subunits, which are responsible for the formation of the same protein from amino acids. Ribosomes are indispensable in the process of transcription.
The eukaryotic cell is a more perfect structure than the prokaryotic one. The appearance of organelles with their own membrane made it possible to increase the efficiency of intracellular processes. The formation of a nucleus surrounded by a double lipid membrane played a very important role in this evolution. The protection of hereditary information by the membrane made it possible for ancient unicellular organisms to masterorganisms in new ways of life. Among them was phagocytosis, which, according to one version, led to the emergence of a symbiotic organism, which later became the progenitor of the modern eukaryotic cell with all its characteristic organelles. The cell nucleus, the structure and functions of some new structures made it possible to use oxygen in metabolism. The consequence of this was a cardinal change in the Earth's biosphere, the foundation was laid for the formation and development of multicellular organisms. Today, eukaryotic organisms, which include humans, dominate the planet, and nothing foreshadows changes in this regard.
