The structure and functions of the cell during evolution underwent a number of changes. 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. Thanks to the presence of isolated organelles, eukaryotic organisms gained 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
Today, there are two main hypotheses about the formation of a eukaryotic cell. According to the symbiotic theory of organelles (for example, flagella or mitochondria) were once separate prokaryotic organisms. The ancestors of modern eukaryotes swallowed them. As a result, a symbiotic organism was formed.
In this case, the nucleus was formed as a result of an inward protrusion of the cytoplasmic membrane. This was a necessary acquisition on the path to the development of a new method of nutrition by the cell, phagocytosis. The capture of food was accompanied by an increase in the degree of mobility of the cytoplasm. Genophores, which were the genetic material of a prokaryotic cell and attached to the walls, fell into the zone of strong “current” and needed protection. As a result, a deep indentation of a portion of the membrane containing attached geneophores formed. This hypothesis is supported by the fact that the membrane of the nucleus is inextricably linked to 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 cells of the ancient archaea. A DNA virus was introduced into it and gradually gained complete control over life processes. Scientists who believe this theory is more correct give a lot of arguments in its favor. However, to date there is no comprehensive 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 similar organelle. However, there are cells that have lost the nucleus due to some functional features. These include, for example, red blood cells. There are also cells with two (ciliates) and even several nuclei.
Cell nucleus structure
Regardless of the characteristics of the organism, 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 karyoplasm, also called nuclear juice or nucleoplasm. It contains chromatin and nucleolus. Sometimes the last of these 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: external and internal. 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 the 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 coated with a layer of ribosomes on the outside. It is connected to the inner membrane by numerous pores, which are rather large protein complexes.
Inner layer
The membrane turned into the cell nucleus, unlike the outer one, is smooth, not covered by ribosomes. It limits karyoplasm. A characteristic feature of the inner membrane is the layer of the nuclear laminas lining it from the side in contact with the nucleoplasm. This specific protein structure maintains the shape of the membrane, 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 cytoplasm is carried out through the nuclear pores. They are quite 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 metabolism between the nucleus and the rest of the cell space. Some molecules pass through the pores passively, without additional energy costs. They are small in size. Transportation of large molecules and supramolecular complexes requires the consumption of a certain amount of energy.
From karyoplasm, RNA molecules synthesized in the nucleus enter the cell. In the opposite direction, 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 chromatin and nucleolus. Nucleoplasm is a viscous liquid in which various substances are dissolved. These include nucleotides and enzymes. The first are necessary for DNA synthesis. 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. There are two of them - stationary and arising during the period of division. The first is characteristic of interphase (time between divisions). At the same time, nuclear juice is characterized by a uniform distribution of nucleic acids and unstructured DNA molecules. During this period, hereditary material exists in the form of chromatin. Cell division is accompanied by the conversion of chromatin to chromosomes. At this time, the structure of the karyoplasm changes: the genetic material acquires a certain structure, the nuclear membrane is destroyed, and the karyoplasm mixes with the cytoplasm.
Chromosomes
The main functions of the nucleoprotein structures of chromatin converted to fission are storage, sale and transmission of hereditary information that the cell nucleus contains. Chromosomes are characterized by a certain form: they are divided into parts or shoulders by a primary constriction, also called a coelom. According to its location, three types of chromosomes are distinguished:
- rod-shaped or acrocentric: they are characterized by the placement of coelomera almost at the end, one shoulder is very small;
- different shoulders or submetacentric shoulders of unequal length;
- equal shoulders or metacentric.
A set of chromosomes in a cell is called a karyotype. In each species, it is fixed. Moreover, different cells of one organism may contain a diploid (double) or haploid (single) set. The first option is characteristic of somatic cells, which mainly make up the body. The haploid kit is the privilege of germ cells. Somatic human cells contain 46 chromosomes, sex - 23.
The diploid chromosomes are pairs. Identical nucleoprotein structures in a pair are called allelic. They have the same structure and perform the same functions.
The structural unit of chromosomes is a gene. It is a section of a DNA molecule that encodes a specific protein.
Nucleolus
The cell nucleus has another organoid - this is the nucleolus. It is not separated from the karyoplasm by a membrane, but it can be easily seen during the study of the cell with a microscope. Some kernels may have multiple nucleoli. There are those in which such organoids are absent altogether.
The shape of the nucleolus resembles a sphere, has a fairly small size. It consists of various proteins. The main function of the nucleolus is the synthesis of ribosomal RNAs and the ribosomes themselves. They are necessary for creating polypeptide chains. Nuclei form around special sections of the genome. They are called nucleolar organizers. It contains the genes of ribosomal RNA. The nucleolus, among other things, is the site with the highest concentration of protein in the cell. Part of the proteins is necessary to perform the functions of an organoid.
Two components are distinguished in the nucleolus: granular and fibrillar. The first is a maturing subunit of ribosomes. 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.
The cell nucleus and its functions
The role played by the core is inextricably linked with its structure. The internal structures of the organoid together realize the most important processes in the cell. It contains 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 an identical maternal set of genes. As a result of meiosis, germ cells are formed with a haploid set of chromosomes.
Another equally important function of the nucleus is the regulation of intracellular processes. It is carried out as a result of the control of the synthesis of proteins responsible for the structure and functioning of cellular elements.
The effect on protein synthesis has another expression. The nucleus, controlling the processes inside the cell, combines 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 ribosome subunits, which are responsible for the formation of the same protein from amino acids. Ribosomes are indispensable in the process of transcription.
A eukaryotic cell is a more perfect structure than a prokaryotic cell . The appearance of organelles with their own membrane has increased 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 allowed the ancient unicellular organisms to master new ways of life. Among them was phagocytosis, which, according to one version, led to the appearance of a symbiotic organism, which later became the progenitor of a modern eukaryotic cell with all its organoids. The cell nucleus, structure and functions of some new structures have enabled the use of oxygen in metabolism. The consequence of this was a fundamental change in the biosphere of the Earth, the foundation was laid for the formation and development of multicellular organisms. Today, eukaryotic organisms, which include humans, dominate the planet, and nothing portends changes in this regard.