Eukaryotes, or Nuclear (lat. Eukaryota from Greekεύ- - good and κάρυον - core) - domain (superkingdom) alive organisms, cells which contain kernels. All organisms except bacteria And archaea, are nuclear ( viruses And viroids are also not eukaryotes, but not all biologists consider them living organisms).
Antibiotic resistance genes, or the gradual failure of antibiotics in populations, are often transferred to plasmids. If these plasmids are transferred from resistant cells to non-resistant cells, bacterial infection in populations may become much more difficult to control.
Prokaryotic cells are often viewed as "simpler" or "less complex" than eukaryotic cells. Prokaryotic cells typically have fewer visible structures, and the structures they do have are smaller than eukaryotic cells. Just because prokaryotic cells appear "simple" does not mean that they are in any way inferior or lesser than eukaryotic cells and organisms. Accepting this assumption can lead to serious trouble.
Animals, plants, mushrooms, as well as groups of organisms under the general name protista- all are eukaryotic organisms. They can be unicellular And multicellular but everyone has overall plan cell structure. It is believed that all these very dissimilar organisms have a common origin, therefore the nuclear group is considered as monophyletic taxon highest rank. According to the most common hypotheses, eukaryotes appeared 1.5–2 billion years ago. Played an important role in the evolution of eukaryotes symbiogenesis - symbiosis between a eukaryotic cell, apparently already having a nucleus and capable of phagocytosis, and absorbed by this cell bacteria- predecessors mitochondria And plastids.
Biologists are now learning that bacteria are capable of communicating and cooperating with each other at a level of complexity that rivals any communication system ever created by humans. Additionally, some archaeal cells are able to thrive in environments so hostile that no eukaryotic cell would survive for more than a few seconds. You're trying to live in a hot spring, salt Lake, a volcano or even deep underground. Prokaryotic cells are also capable of removing substances that eukaryotic cells could only dream of, in part due to their increased simplicity.
Structure of a eukaryotic cell
Eukaryotic cells are on average much larger prokaryotic, the difference in volume reaches thousands of times. Eukaryotic cells include about a dozen species various structures, known as organoids(or organelles, which, however, somewhat distorts the original meaning of this term), of which many are separated from the cytoplasm by one or more membranes (in prokaryotic cells internal organelles surrounded by a membrane are rare). Core- this is the part of the cell surrounded in eukaryotes by a double membrane (two elementary membranes) and containing genetic material: molecules DNA, “packed” into chromosomes. There is usually one nucleus, but there are also multinucleated cells.
Main differences between pro- and eukaryotes
Being bigger and more complex is not always better. These cells and organisms are just as locally adapted as any eukaryote, and in that sense have “evolved” just like any other living organism on Earth. One type of bacterial communication, also known as quorum sensing, is where small chemical signals are used to count the number of bacteria.
Many cells do not have mitochondria. The mitochondrion, an organelle that helps produce energy for the cell, is found only in eukaryotes, organisms with relatively large, complex cells. These organisms contrast with prokaryotes, which lack membrane-bound organelles like mitochondria. Eukaryotes include everything from the single-celled paramecium to plants, fungi, and animals. In short, many cells have mitochondria and many have mitochondria, and the difference is important.
Division into kingdoms
There are several options for dividing the eukaryotic superkingdom into kingdoms. The kingdoms were the first to be allocated plants And animals. Then the kingdom was allocated mushrooms, which, due to biochemical characteristics, according to most biologists, cannot be classified as one of these kingdoms. Also, some authors distinguish kingdoms protozoa,myxomycetes, chromists. Some systems have up to 20 kingdoms. By system Thomas Cavalier-Smith all eukaryotes are divided into two monophyletic taxa - Unikonta And Bikonta . The position of such eukaryotes as collodiction (Collodictyon) And Diphylleia , currently undefined.
By allowing the body to use oxygen, mitochondria supported the evolution of more complex organism. Scientists believe that the mitochondrion actually began as a free-living organism that was absorbed into another cell. Instead of digesting big cage contained the ancestor mitochondria within itself, providing food and shelter, while the premitochondria, in turn, gave the host cell the ability to use oxygen. Over time, mitochondria lost the ability to live outside the host cell and vice versa.
Eukaryotes without mitochondria
Scientists call this idea the “endosymbiosis theory.” Relatively simple organisms, such as bacteria and archaea, belong to a category of life called prokaryotes. Prokaryotes lack most of the structures found in eukaryotes, including any membrane-bound organelles. This includes the mitochondria and nucleus. Because bacteria do not have mitochondria, the vast majority of them cannot use oxygen as efficiently as eukaryotes. Unlike prokaryotes, eukaryotes have more complex structure, including membrane-bound organelles such as mitochondria.
Differences between eukaryotes and prokaryotes
The most important, fundamental feature of eukaryotic cells is associated with the location of the genetic apparatus in the cell. The genetic apparatus of all eukaryotes is located in the nucleus and is protected by the nuclear envelope (in Greek, “eukaryote” means having a nucleus). DNA eukaryotes are linear (in prokaryotes the DNA is circular and is located in a special region of the cell - nucleoid, which is not separated by a membrane from the rest of the cytoplasm). It is associated with proteins histones and other chromosomal proteins that bacteria do not have.
Most eukaryotes have mitochondria, and all multicellular eukaryotes do. However, several monolocular eukaryotes do not have mitochondria. This entire type of eukaryote lives as parasites. These special eukaryotes are thought to be descended from primitive eukaryotes that never had mitochondria, or that had "secondary loss," meaning that their ancestors had mitochondria at some point but later lost them. In addition, some multicellular eukaryotes do not have mitochondria in certain cells.
For example, human red blood cells do not have mitochondria, a device that either reduces the size of the cells or prevents them from using the oxygen they carry. Several other eukaryotic organelles have important common features with mitochondria. The chloroplast is very similar. In fact, chloroplasts are thought to have evolved from blue-green algae, which eventually lost the ability to live outside cells, like mitochondria. Chloroplasts allow some eukaryotes, such as plants and algae, to use sunlight to obtain energy and oxygen for their cells, which is then used by their mitochondria.
In the life cycle of eukaryotes, there are usually two nuclear phases (haplophase and diplophase). The first phase is characterized by a haploid (single) set of chromosomes, then, merging, two haploid cells(or two nuclei) form a diploid cell (nucleus) containing a double (diploid) set of chromosomes. Sometimes during the next division, and more often after several divisions, the cell again becomes haploid. Such life cycle and in general, diploidity is not typical for prokaryotes.
In addition, the hydrogenosome plays a similar role to mitochondria, but functions under conditions unfavorable for oxygen. They were originally known as fungi and single-celled eukaryotes, but have recently been discovered in very small simple animals that live in oxygen-poor seafloors.
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The third, perhaps the most interesting difference, is the presence in eukaryotic cells of special organelles that have their own genetic apparatus, reproduce by division and are surrounded by a membrane. These organelles are mitochondria And plastids. In their structure and life activity they are strikingly similar to bacteria. This circumstance has prompted modern scientists to believe that such organisms are descendants of bacteria that entered into symbiotic relationship with eukaryotes. Prokaryotes are characterized by a small number of organelles, and none of them are surrounded by a double membrane. Prokaryotic cells do not have an endoplasmic reticulum, Golgi apparatus, or lysosomes.
Who are eukaryotes
Much of this will not fit with your current idea of evolution and will surprise you. Here are a few key words about what we are talking about. The lines in the "family tree" of evolution not only branch out, but also flow together: the two become one. And therefore the family tree is not a tree. In a eukaryotic cell, several other cells are stacked inside each other, like in a Russian doll. And perhaps you are composed much more in a simple way- living mosaic. “Competition” and survival of the fittest are the evolution of only one principle among others. Together is as important as against each other. . He also smuggles viruses and harpoons cnidarians, leukocytes and neurons, Russian symbiogenetics and the nasty Bengal fig tree.
Another important difference between prokaryotes and eukaryotes - the presence of eukaryotes endocytosis, including many groups - phagocytosis. Phagocytosis (literally “eating by a cell”) is the ability of eukaryotic cells to capture, enclose in a membrane vesicle, and digest a wide variety of solid particles. This process provides an important protective function in the body. It was first opened I. I. Mechnikov at starfish. The appearance of phagocytosis in eukaryotes is most likely associated with average size (more about size differences is written below). The sizes of prokaryotic cells are disproportionately smaller, and therefore, in the process of evolutionary development of eukaryotes, they had the problem of supplying the body with a large amount of food. As a result, the first real, mobile predators.
And it is important to be able to appreciate all this: after all, we are all alive ourselves and therefore are part of evolution. Absolutely appears before our eyes new image thinking that has what it takes to change our worldview far beyond biology, and hardly anyone will look like it. Often things are completely different than you think. This also applies to life, that is, to life, to the biological - that is, to all the trees and grasses, birds, worms and bacteria that are around us and, of course, for ourselves.
Usually people think that all living things somehow diverged in a straight line, on lines with branches to the left and to the right: birds from dinosaurs, people from monkeys, plants from algae, and all at the same time from primitive cell or from several. If we try to imagine this as a whole, before inner eye a tree appears. A thick, gnarled oak tree that rises in its branches to different types, and somewhere out there, we crouch, man.
Most bacteria have cell wall, different from eukaryotic (not all eukaryotes have it). In prokaryotes, it is a durable structure consisting mainly of mureina(in archaea from pseudomureine). The structure of murein is such that each cell is surrounded by a special mesh sac, which is one huge molecule. Among eukaryotes, many have a cell wall protista, mushrooms and plants. In mushrooms it consists of chitin And glucans, y lower plants- from cellulose And glycoproteins, diatoms synthesize a cell wall from silicic acids, higher plants it consists of cellulose, hemicellulose and pectin. Apparently, for larger eukaryotic cells it has become impossible to create a cell wall of high strength from a single molecule. This circumstance could force eukaryotes to use different material for the cell wall. Another explanation is that the common ancestor of eukaryotes lost its cell wall due to the transition to predation, and then the genes responsible for the synthesis of murein were also lost. When some eukaryotes return to osmotrophic In nutrition, the cell wall appeared again, but on a different biochemical basis.
This kind of structure is probably inevitable when you hear the word “evolution.” And if the question then arises of what makes this tree sprout and grow, then even before the question is even thought about, it is in the struggle for existence, in the survival of the fittest, in the whole known idea of competition and, of course, in mutation and choice. So, you are into Darwin's theories. More precisely: in their modern sophistications, Darwin knew nothing about mutation - and their somewhat crude form, like an engraving. But when you try to think of something as complex as evolution in general, it always gets a little rough around the edges.
The metabolism of bacteria is also diverse. In general, there are four types of nutrition, and all are found among bacteria. These are photoautotrophic, photoheterotrophic, chemoautotrophic, chemoheterotrophic (phototrophic use the energy of sunlight, chemotrophic use chemical energy). Eukaryotes either synthesize energy from sunlight themselves or use ready-made energy of this origin. This may be due to the emergence of predators among eukaryotes, for which the need to synthesize energy has disappeared.
And in many ways you completely agree with these ideas. Because all this - mutation and selection, lines of branched species, competition - certainly plays a role important role in development biological species. But otherwise you are wrong. Because very little, as recent research has shown, is fundamentally different in its evolution than Darwin's big picture suggests.
His major work, On the Origin of Species, appeared more than 20 years later. Isn't it always a suspicion? Even if a lot happens over almost four billion years, as long as someone suspects, there is life on earth. Tree of branching species: in the evolution of cells, for example, there are none. Or, more precisely, in family tree Cells not only have bifurcations, but also branches grow into each other. And this is much more important for their evolution.
Another difference is the structure of the flagella. In bacteria they are thin - only 15–20 nm in diameter. These are hollow strands of protein flagellina. The structure of eukaryotic flagella is much more complex. They are a cell outgrowth surrounded by a membrane and contain a cytoskeleton (axoneme) of nine pairs of peripheral microtubules and two microtubules in the center. Unlike rotating prokaryotic flagella, eukaryotic flagella bend or wriggle.
Cells exist in two different options: There are cells without a nucleus and cells with a nucleus, prokaryotic and eukaryotic cells. All from eukaryotic cells higher organisms, of course, ourselves. Prokaryotic cells are bacterial cells.
Comparison of prokaryotic structure bacterial cell and eukaryotic cell. In addition to the question “with or without a core?” There are even more differences between the two cell types. Eukaryotic cells are more complex and larger, but first of all there is something inside that does not exist in prokaryotic cells: eukaryotic cells have mitochondria and, if they are plant cells, chloroplasts.
The two groups of organisms we are considering, as already mentioned, are very different in their average sizes. The diameter of a prokaryotic cell is usually 0.5–10 μm, while the same figure for eukaryotes is 10–100 μm. The volume of such a cell is 1000–10000 times greater than that of a prokaryotic cell.
Ribosomes prokaryotes are small (70S-type). Eukaryotic cells contain both larger 80S-type ribosomes located in the cytoplasm and prokaryotic-type 70s ribosomes located in mitochondria and plastids.
Both chloroplasts and mitochondria are small oval bodies in the cytoplasm that are used to produce energy. Chloroplasts, in which green chlorophyll is found, can be observed in almost astronomical numbers every day as they transform sunlight into chemical energy during photosynthesis. Mitochondria and chloroplasts are no less important for a eukaryotic cell than for the stomach or intestines themselves.
For a long time, it was believed that eukaryotic cells are a further development of the prokaryotic, that is, a little further or at the top of the same evolutionary branch. Mitochondria and chloroplasts were thought to have evolved within the cell itself during evolution, e.g. cell membrane, invaginating inside, into some kind of bag in which the corresponding biochemical processes.
Apparently, the time of emergence of these groups also differs. The first prokaryotes arose in the process of evolution about 3.5 billion years ago, from them about 1.2 billion years ago eukaryotic organisms evolved.
All types of eukaryotes are characterized by both types of reproduction. 1) Asexual reproduction: -unicellular: -mitotic division; -schizogony (multiple fission); -budding; -spore formation; -multicellular: -vegetative reproduction; -spore formation. 2) Sexual reproduction: unicellular: - conjugation (sexual process that occurs when two vegetative cells merge); -copulation (the process of fusion of two sex cells or two individuals); multicellular: -without fertilization; - with fertilization.
However, as we know today, this is not the case. Neither mitochondria nor chloroplasts belong to the cell at all. They were added from outside. They were once solitary, independent living beings, namely bacteria, that is, prokaryotic cells. The story went something like this. On early stage evolution, approximately 1.5-2 billion years ago, the precursors of modern mitochondria and chloroplasts - at that time independent bacteria - came into contact with the precursors of today's ubiquitous eukaryotic cells.
These pre-chloroplasts and pre-mitochondria probably even served as food for the precursor cell of eukaryotes. However, in some cases the meal may not be completed. This collaboration seems to be working well. The partners stayed together until today, the first victim became a fixed functional part of the cell - a cellular organelle. Although we cannot reconstruct the details of the events of that time, we certainly know from genetic testing that such a merger occurred.
Eukaryotes are the most progressive organized organisms. In our article we will look at which of the representatives of living nature belongs to this group and what organizational features allowed them to occupy a dominant position in the organic world.
Who are eukaryotes
According to the definition of the concept, eukaryotes are organisms whose cells contain a formed nucleus. These include the following kingdoms: Plants, Animals, Fungi. And it doesn’t matter how complex their body is. Microscopic amoeba, Volvox colonies - they are all eukaryotes.
Although cells of real tissues can sometimes lack a nucleus. For example, it is not found in red blood cells. Instead, this blood cell contains hemoglobin, which carries oxygen and carbon dioxide. Such cells contain a nucleus only in the first stages of their development. Then this organelle is destroyed, and at the same time the ability of the entire structure to divide is lost. Therefore, having fulfilled their functions, such cells die.
Structure of eukaryotes
All eukaryotic cells have a nucleus. And sometimes not even one. This double-membrane organelle contains in its matrix genetic information encrypted in the form of DNA molecules. The core consists of a surface apparatus, which ensures the transport of substances, and a matrix - its internal environment. The main function of this structure is the storage of hereditary information and its transmission to daughter cells formed as a result of division.
The internal environment of the kernel is represented by several components. First of all, this is karyoplasm. It contains nucleoli and chromatin threads. The latter consist of proteins and nucleic acids. It is during their spiralization that chromosomes are formed. They are directly carriers of genetic information. Eukaryotes are organisms that, in some cases, can form two types of nuclei: vegetative and generative. A striking example of this is ciliates. Its generative nuclei carry out the preservation and transmission of the genotype, and its vegetative nuclei - regulation
Main differences between pro- and eukaryotes
Prokaryotes do not have a formed nucleus. The only thing that belongs to this group of organisms is Bacteria. But this structural feature does not mean at all that there are no carriers of genetic information in the cells of these organisms. Bacteria contain circular DNA molecules called plasmids. However, they are located in the form of clusters in certain place cytoplasm and do not have a common membrane. This structure is called a nucleoid. There is one more difference. The DNA in prokaryotic cells is not associated with nuclear proteins. Scientists have established the existence of plasmids in eukaryotic cells. They are found in some semi-autonomous organelles, such as plastids and mitochondria.
Progressive structural features
Eukaryotes include organisms that are distinguished by more complex structural features at all levels of organization. First of all, this concerns the method of reproduction. The nucleoid of bacteria provides the simplest of them - in two. Eukaryotes are organisms that are capable of all types of reproduction of their own kind: sexual and asexual, parthenogenesis, conjugation. This ensures the exchange of genetic information, the appearance and consolidation in the genotype of a number of useful signs, and therefore better adaptation of organisms to constantly changing conditions environment. This feature allowed eukaryotes to occupy a dominant position in the system of the organic world.
So, eukaryotes are organisms whose cells have a formed nucleus. These include plants, animals and fungi. The presence of a core is a progressive structural feature that provides high level development and adaptation.