Cell nucleus and its functions

The structure and function of the cell underwent a number of changes in the course of evolution. The appearance of new organelles was preceded by transformations in the atmosphere and lithosphere of a young planet. One of the significant acquisitions was the cell nucleus. Eukaryotic organisms, thanks to the presence of isolated organelles, had significant advantages over prokaryotes and quickly began to dominate.

The cell nucleus, whose structure and functions are somewhat different in different tissues and organs, made it possible to improve the quality of RNA biosynthesis and the transfer of hereditary information.

Origin

To date, there are two main hypotheses about the formation of a eukaryotic cell. According to the symbiotic theory, organelles (for example, flagella or mitochondria) were once separate prokaryotic organisms. The ancestors of modern eukaryotes absorbed them. As a result, a symbiotic organism was formed.

The nucleus was formed as a result of protrusion into the cytoplasmic membrane. This was a necessary acquisition on the path of developing a new way of eating, phagocytosis, by the cell. The seizure of food was accompanied by an increase in the degree of mobility of the cytoplasm. Genophors, which were the genetic material of the prokaryotic cell and attached to the walls, fell into the zone of a strong "current" and needed protection. As a result, a deep invagination of the region of the membrane containing the 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 the cell of the ancient archaea. The DNA virus was introduced into it and gradually gained full control over the life processes. Scientists who consider this theory more correct, lead a lot of arguments in its favor. However, to date, there is no comprehensive proof for any of the existing hypotheses.

One or more

Most of the cells of modern eukaryotes have a nucleus. The overwhelming number of them contains only one similar organelle. There are, however, cells that have lost the core due to some functional features. These include, for example, erythrocytes. There are also cells with two (infusoria) and even several nuclei.

Structure of the cell nucleus

Regardless of the characteristics of the body, the structure of the nucleus is characterized by a set of typical organelles. From the inner space of the cell it is fenced off by a double membrane. Its internal and external layers in some places merge, forming pores. Their function is to exchange substances between the cytoplasm and the nucleus.

The space of the organelle is filled with a karyoplasm, also called nuclear juice or nucleoplasm. It contains chromatin and nucleolus. Sometimes the last of the named organelles of the cell nucleus is not present in a single specimen. In some organisms, however, the nucleoli are absent.

Membrane

The nuclear envelope is formed by lipids and consists of two layers: the outer and the inner. In fact, it 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 envelope.

The outer and inner membranes have their own peculiarities in the structure, but on the whole they are quite similar.

Nearest to the cytoplasm

The outer layer passes into the membrane of the endoplasmic reticulum. Its main difference from the latter is a much higher concentration of proteins in the structure. The membrane directly in contact with the cytoplasm of the cell is covered with a layer of ribosomes from the outside. With the inner membrane, it is joined by numerous pores, which are quite large protein complexes.

Inner layer

The membrane turned into 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 the layer of the nuclear lamina lining it from the side in contact with the nucleoplasm. This specific protein structure supports the shape of the envelope, participates in the regulation of gene expression, and also promotes the attachment of chromatin to the core membrane.

Metabolism

The interaction of the nucleus and the 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 3 to 5 thousand pores, in some frogs it reaches 50,000.

The main function of pores is the metabolism between the nucleus and the rest of the cell space. Some molecules penetrate through the pores passively, without additional expenditure of energy. They are small in size. Transportation of large molecules and supramolecular complexes requires consumption of a certain amount of energy.

Of the karyoplasm, RNA molecules synthesized into the nucleus fall into the cell. In the opposite direction, the proteins necessary for intranuclear processes are transported.

Nucleoplasm

Nuclear juice is a colloidal solution of proteins. It is bounded by the shell of the nucleus and surrounds the chromatin and the nucleolus. Nucleoplasma is a viscous liquid in which various substances are dissolved. These include nucleotides and enzymes. The former are necessary for the synthesis of DNA. Enzymes are involved in transcription, as well as DNA repair and replication.

The structure of nuclear juice varies depending on the state of the cell. The two are stationary and appear during the fission period. The first is characteristic for the interphase (the time between divisions). In this case, nuclear juice is characterized by a uniform distribution of nucleic acids and unstructured DNA molecules. During this period, the hereditary material exists in the form of a 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 definite structure, the nuclear envelope collapses, and the karyoplasm mixes with the cytoplasm.

Chromosomes

The main functions of the nucleoprotein structures of the chromatin transformed for the time of division are storage, realization and transmission of hereditary information, which contains the cellular nucleus. Chromosomes are characterized by a definite form: they are divided into parts or shoulders by a primary constriction, also called intact. By its arrangement, three types of chromosomes are distinguished:

• Rod-like or acrocentric: for them, the placement of integers is almost at the end, one arm turns out to be very small;
• Variegated or submetacentric have shoulders of unequal length;
• Equilateral or metacentric.

A set of chromosomes in a cell is called a karyotype. In each species it is fixed. In this case, different cells of the same organism can contain a diploid (double) or haploid (single) set. The first variant is characteristic for somatic cells, which mainly make up the body. Haploid set is the privilege of sex cells. Human somatic cells contain 46 chromosomes, sexual - 23.

Chromosomes of the diploid set are pairs. Identical nucleoprotein structures in the 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 a DNA molecule that encodes a specific protein.

Nucleolus

The cell nucleus has one more organoid - the nucleolus. It does not separate from the karyoplasm by the membrane, but it is easy to see during the study of the cell using a microscope. Some nuclei may have several nucleoli. There are also those in which such organelles are absent altogether.

In shape, the nucleolus resembles a sphere, it is rather small in size. It contains various proteins. The main function of the nucleolus is the synthesis of ribosomal RNAs and the ribosomes themselves. They are necessary for the creation of polypeptide chains. Nucleoli are formed around special areas of the genome. They were called nucleolus organizers. Here, the genes of ribosomal RNA are contained. The nucleolus, among other things, is the site with the highest concentration of protein in the cell. Some of the proteins are needed to perform the functions of the organoid.

The nucleolus consists of two components: granular and fibrillar. The first is the maturing subunits of the ribosomes. In the fibrillar center, ribosomal RNA is synthesized. The granular component surrounds the fibrillar component located in the center of the nucleolus.

Cell nucleus and its functions

The role played by the core is inextricably linked to its structure. Internal structures of the organoid jointly realize the most important processes in the cell. It contains genetic information that determines the structure and functions of the cell. The nucleus is responsible for storing and transmitting hereditary information during mitosis and meiosis. In the first case, the daughter cell receives an identical genetic set of genes. As a result of meiosis, sex cells with a haploid set of chromosomes are formed.

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.

The effect on protein synthesis has one more expression. The nucleus, controlling processes inside the cell, unites all its organoids 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 ribosomal subunits, which are responsible for the formation of all 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 cell. The appearance of organoids with their own membrane allowed to increase the efficiency of intracellular processes. The formation of a nucleus surrounded by a double lipid shell played a very important role in this evolution. Protection of hereditary information by the membrane made it possible to learn new ways of life by ancient unicellular organisms. Among them was phagocytosis, which, according to one version, led to the appearance of a symbiotic organism, which later became the progenitor of the modern eukaryotic cell with all its characteristic organoids. The cell nucleus, the structure and functions of some new structures allowed the use of oxygen in metabolism. A consequence of this was a dramatic change in the Earth's biosphere, the foundation was laid for the formation and development of multicellular organisms. Today, eukaryotic organisms, to which a person belongs, dominate the planet, and nothing foreshadows any changes in this plan.