Nucleoid in the cytoplasm. Structure of a bacterial cell

1. A prokaryotic cell is characterized by the presence
A) ribosome
B) mitochondria
B) a decorated core
D) plasma membrane
D) endoplasmic reticulum
E) one circular DNA

Answer

2. Prokaryotic cells are different from eukaryotic cells
A) the presence of ribosomes
B) lack of mitochondria
B) the absence of a formalized core
D) the presence of a plasma membrane
D) lack of organelles of movement
E) the presence of one ring chromosome

Answer

3. Establish a correspondence between the structure of cells and their type: 1-prokaryotic, 2-eukaryotic
A) do not have a formalized core
B) have a nuclear membrane
B) diploid or haploid
D) always haploid
D) do not have mitochondria, the Golgi complex
E) contain mitochondria, the Golgi complex

Answer

A1 B2 C2 D1 E1 E2

4. Why are bacteria classified as prokaryotes?
A) contain a nucleus in the cell, isolated from the cytoplasm
B) consist of many differentiated cells
B) have one ring chromosome
D) do not have a cell center, Golgi complex and mitochondria
D) do not have a nucleus isolated from the cytoplasm
E) have cytoplasm and plasma membrane

Answer

5. A bacterial cell is classified as a prokaryotic cell, since it
A) does not have a core covered with a shell
B) has a cytoplasm
B) has one DNA molecule embedded in the cytoplasm
D) has an outer plasma membrane
D) does not have mitochondria
E) has ribosomes where protein synthesis takes place

Answer

6. Cells of eukaryotic organisms, unlike prokaryotic ones, have
A) cytoplasm
B) core covered with a shell
B) DNA molecules
D) mitochondria
D) hard shell
E) endoplasmic reticulum

Answer

7. Establish a correspondence between the characteristics of the cell and its type: 1-prokaryotic, 2-eukaryotic
A) Membrane organelles are absent
B) There is a cell wall of murein
C) Hereditary material is represented by a nucleoid
D) Contains only small ribosomes
E) Hereditary material is represented by linear DNA
E) Cellular respiration occurs in mitochondria

Answer

A1 B1 C1 D1 E2 E2

8. Prokaryotic cells are different from eukaryotic cells
A) the presence of a nucleoid in the cytoplasm
B) the presence of ribosomes in the cytoplasm
C) ATP synthesis in mitochondria
D) the presence of the endoplasmic reticulum
D) the absence of a morphologically distinct nucleus
E) the presence of invaginations of the plasma membrane, performing the function of membrane organelles

Nuclear elements, or nucleoids of bacteria. Bacteria belong to prokaryotes, i.e., organisms that do not contain morphologically distinct nuclei. Bacteria have bodies called nucleoids, or chromatin bodies. They contain deoxyribonucleic acid (DNA) and act as a nucleus. Cell division is preceded by the division of discrete nucleoid bodies, which can be detected by specific reactions and staining methods, especially after preliminary special treatment of preparations. The functions of the nuclear apparatus of bacteria correspond to the functions of nuclei in eukaryotes, that is, they serve as carriers of the hereditary characteristics of the species and pass them on to offspring.[ ...]

Nucleoproteins are made up of proteins and nucleic acids. Since nucleic acids were originally isolated from plant and animal cells containing nuclei (nucleus - nucleus), it was assumed that they are found only in nuclei. Later, with the help of cytochemical methods, nucleic acids were detected, in addition to chromosomes, in mitochondria, ribosomes, in independent genetic elements - plasmids and hyaloplasm.[ ...]

Here C is the central nucleoid; - a parameter that characterizes the structure of various oncoviruses, which can change as a result of external or internal influences, respectively; N and B - respectively, the concentration of electrons and ionic formations that make up the oncovirus; m1 and m2 - respectively, the concentration of "working" electrons and ionic complexes' P, - the product of the photothermal reaction; E - EMP energy; kT - thermal energy.[ ...]

During the division of a bacterial cell, it is not possible to establish any reorganization in its nucleoid, comparable to the rearrangement of the nucleus during the division of cells of more highly organized organisms. Daughter nucleoids are formed as a result of either ligation of the original nucleoid, or divergence at an angle of its two halves.[ ...]

CS - cell wall, CPM - cytoplasmic membrane, H - nucleoid.[ ...]

CS - cell wall, CPM - cytoplasmic membrane, H - nucleoid, FMS - photosynthetic membrane structures, Uvel, x 40 OOO.[ ...]

[ ...]

In bacteria, DNA is less densely packed than in true nuclei; A nucleoid does not have a membrane, a nucleolus, or a set of chromosomes. Bacterial DNA is not associated with the main proteins - histones - and the nucleoside is located in the form of a bundle of fibrils.[ ...]

Young rapidly dividing cells of anasrobs contain nucleoids in the form of dumbbells, or Y-shaped figures (Fig. 46). Before sporulation, cell division stops, they sharply increase in size. At this time, there is an accumulation of a large amount of reserve nutrient - granulosa - deposited in the form of granules, due to which the cytoplasm becomes granular, and the cells themselves swell, taking the form of a lemon (clostridia) (Fig. 47-49) or a drumstick (plectridia) (Fig. 50, 51). Only in a part of proteolytic anaerobes, the cells do not change their original appearance, retaining the usual rod-shaped (bacillary) shape (Fig. 52).[ ...]

The genetic information of bacteria is not limited to the DNA located in the nucleoid of the bacterial cell. As already noted in the previous sections of the book, extrachromosomal elements, which received the general name of plasmids, also serve as carriers of hereditary properties. Unlike the DNA of nuclear equivalents, nucleoids, which are the organelles of a bacterial cell, plasmids are independent genetic elements. The loss of plasmids or their acquisition does not affect the biology of the cell (the acquisition of plasmids has a positive effect only on the population as a whole, increasing the viability of the species). Transmissible plasmids are those that initiate the properties of donors in host cells. At the same time, the latter receive a new quality - the ability to conjugate with recipient cells and give them their plasmids. Recipient cells, acquiring plasmids during conjugation, themselves turn into donors.[ ...]

Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have its analogue - the "nuclear equivalent" - the nucleoid (see Fig. 2, 5), which is an evolutionarily more primitive form of organization of the nuclear substance. Microorganisms that do not have a real nucleus, but have its analogue, belong to prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, most of the DNA is concentrated in one or more places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The nucleus is surrounded by a shell-membrane.[ ...]

When studying ultrathin sections of eubacteria spores, it was possible to establish that the central part is filled with sporoplasm containing several nucleoids. The sporoplasm is covered with a membrane that makes up the inner covering, called intina; the outer layer is the shell of the spore, which makes it highly resistant to the effects of reagents, making the spores difficult to stain. It does not occur in vegetative bacterial cells. However, some authors of manuals on microbiology without reason believe that the walls of spores do not contain compounds characteristic of the walls of vegetative cells, and consist of other substances. It is suggested that the calcium salt of diaminopimelic acid, which is part of the shell of heat-resistant spores, partially determines the thermal resistance of spores.[ ...]

The first sign of the onset of spore formation is a change in the morphology of the nucleoids, which take the form of spherical bodies. Further, several nucleoids approach at one of the poles of the cell, merge and form a longitudinally located convoluted chromatin (nuclear) strand (Fig. 53, 54 and scheme 1 in Table 32).[ ...]

Anaerobes, like other bacteria, lack a true nucleus surrounded by a membrane and possessing a set of chromosomes, a nucleolus and nuclear juice. Instead, there is an analogue of the nucleus - the nucleoid. Often, the nucleoid is simply called DNA-containing plasma (Fig. 43).[ ...]

The fundamental structure of cocci cells as a whole does not differ from that of other prokaryotic microorganisms. Coccal cells consist of a cell wall, a cytoplasmic membrane, cytoplasm with various inclusions, and a nucleoid.[ ...]

DNA in E. coli cells is a single double-stranded circular molecule, MW about 2 x 10®, which is approximately 3 x 10® nitrogenous base pairs. It performs the role of a functionally active chromosome, called the nucleoid. The latter is a haploid structure. Since the distance between the pairs of nitrogenous bases in E. coli DNA is about 3.4 Ä, the contour length of the DNA molecule of this species is about OD cm, which exceeds the length of the cell containing it by about 600 times, and the diameter is only 20 A (the diameter of a single E. coli cells is about 0.75). Therefore, it is believed that the chromosome (DNA) inside the bacteria of this species exists in the form of a “folded genome”, occupying 1/6 of the cell volume, i.e., in a folded (twisted) form in the form of about 50 loops, each of which is in a supertwisted form . Since the "folded genome" can be destabilized by treatment with RNase, it is believed that it also includes RNA. In addition, low molecular weight proteins have been found in its composition, the role of which has not yet been elucidated.[ ...]

Extremely small cell sizes are a characteristic, but not the main feature of bacteria. All bacteria are represented by a special type of cells, devoid of a true nucleus, surrounded by a nuclear membrane. An analogue of the nucleus in bacteria is the nucleoid - DNA-containing plasma, not separated from the cytoplasm by a membrane. In addition, bacterial cells are characterized by the absence of mitochondria, chloroplasts, as well as a special structure and composition of membrane structures and cell walls. Organisms whose cells lack a true nucleus are called prokaryotes (pre-nuclear) or protocytes (that is, organisms with a primitive cell organization).[ ...]

The protoplast of cyanophycea does not have a nucleus, like the protoplast of bacteria. In the protoplast, a more transparent uncolored part is distinguished - the centroplasm - and the peripheral, containing pigments - the chromatoplasm. In the centroplasm there are more compacted areas, or nucleoids, which are nuclear equivalents, in which DNA and RNA are found. Cyanophyceous cells have cavities filled with gas - gas vacuoles. Under the microscope, they look like black bodies; are natural for planktonic genera, as they contribute to their stay in suspension. There are no structures containing pigments in the centroplasm. The nucleoids of blue-green algae are not separated from the cytoplasm by the nuclear envelope in the same way as in bacteria.[ ...]

In bacteria, sexual reproduction is sometimes observed. An electron microscopic study of Escherichia coli made it possible to detect prstoplasmic bridges between pairs of this bacterium, one of the cells being male and the other female. In this case, a partial transfer of the nucleoid deoxyribonucleic acid from the male cell to the female occurs.[ ...]

From all that has been said, it follows that the place of synthesis of proteins and all enzymes in the cell are ribosomes. Figuratively speaking, these are, as it were, "factories" of the protein, as it were, an assembly shop, where all the materials necessary for the assembly of the polypeptide chain of the protein and amino acids are supplied. The nature of the synthesized protein depends on the structure of i-RNA, on the order of arrangement of nucleoids in it, and the structure of i-RNA reflects the structure of DNA, so that in the end the specific structure of the protein, i.e. the order in which various amino acids are arranged in it, depends on the order arrangement of nucleoids in DNA, from the structure of DNA.[ ...]

There is still no consensus on the question of the nuclear structures of bacteria. The presence of a nuclear substance, consisting mainly of deoxyribonucleic acid (DNA), is undoubted. According to some researchers, the nuclear substance in bacterial cells is in a diffuse (sprayed) state. Other scientists found a differentiated (isolated) nucleus. Electron microscopy made it possible to identify nucleus-like formations in some types of bacteria - nucleoids (from lat. piyeue - nucleus). However, compared with the nuclei of cells of higher organisms, these formations have a simpler structure. Nucleoids are not separated from the cytoplasm by a membrane and therefore do not have a permanent shape.[ ...]

A wide variety of geographical and ecological conditions, within which certain types of microorganisms can spread and exist in nature, also leaves its mark on the chemical composition of cells and is reflected in the biochemical functions of the microbial population. Modern methods of laboratory experiment make it possible to divide a microbial cell into its organelles and study separately the chemical composition of flagella, membranes, protoplast, membranes, ribosomes, nucleoids, as well as the contents of the protoplast: various reserve nutrients - glycogen, volatility, fats, pigments, vitamins and others metabolites.[ ...]

The hereditary properties of bacteria or individual traits are encoded in units of heredity - genes, linearly located in the chromosome along the DNA strand. Therefore, a gene is a fragment of a DNA strand. Each trait corresponds to a specific gene, and often an even smaller segment of DNA - a codon. In other words, information about all the properties of bacteria is located in a linear order in the DNA strand. However, bacteria have another feature. The nuclei of eukaryotes usually contain several chromosomes, their number in the nucleus is constant in each species. The bacterial nucleoid contains only one ring of DNA strand, that is, one chromosome. However, the amount of hereditary characteristics of a bacterial cell is not exhausted by the stock of information contained in one chromosome or in an annularly closed double-stranded DNA helix. Plasmids contain DNA, which also carries genetic information transmitted from the mother cell to the daughter cell.[ ...]

The centroplasm of blue-green algae cells consists of hyaloplasm and various rods, fibrils and granules. The latter are chromatin elements that are stained with nuclear dyes. Hyaloplasm and chromatin elements in general can be considered an analogue of the nucleus, since these elements contain DNA; during cell division, they divide longitudinally, and the halves are equally distributed among the daughter cells. But, unlike a typical nucleus, in the cells of blue-green algae around the chromatin elements it is never possible to detect the nuclear membrane and nucleoli. It also contains ribosomes containing RNA, vacuoles and polyphosphate granules.[ ...]

The decisive direct proof of the genetic rule of DNA was provided by the development of genetic engineering methods, which made it possible to design recombinant DNA molecules with desired properties. To date, the possibilities of genetic engineering have been shown by the example of cloning many genes of various organisms. As for circumstantial evidence, they have been known for a very long time and there are several of them. DNA is characterized by specific localization in cells, since it is found only in cell nuclei (chromosomes), mitochondria (in animals) and chloroplasts (in plants). In many microorganisms, DNA is localized only in the nuclear region (nucleoid) or in the cytoplasm in the form of plasmids. For organisms of each species, a certain amount of DNA per cell is characteristic (Table 10).[ ...]

Hyphae cells contain one or more nuclei. In some phycomycetes, a sporangiophore cell may contain many tens or even hundreds of nuclei. Polynucleus is also observed in the conidiophores of ascomycetes; in basidiomycetes it is less pronounced. The structure of the nucleus in fungi is typical of eukaryotes in plant organisms. The nucleus is covered with a shell - nucleolemma - and has a nucleolus and chromatin substance or chromatin thread, which breaks up at a certain age stage of the cell into a constant number of chromosomes inherent in each genus. The presence of a nuclear membrane in the nuclei of eukaryotic microorganisms (fungi and algae) is a hallmark of the structure of their cells. Nucleoids of prokaryotic microorganisms are devoid of membranes, and chromatin threads do not break up into chromosomes.[ ...]

The cytoplasm is a colloidal solution, the dispersed phase of which is complex protein compounds and substances close to fats, and the dispersion medium is water. In some forms of bacteria, the cytoplasm contains inclusions - droplets of fat, sulfur, glycogen, etc. Permanent components of bacterial cells are special outgrowths of the cytoplasmic membrane - mesosomes, which contain enzymatic redox systems. In these formations, there are mainly processes associated with the respiration of bacteria. In small inclusions - ribosomes containing ribonucleic acid, protein biosynthesis is carried out. Most bacterial species do not have a distinct nucleus. The nuclear substance, represented by DNA, is not separated from the cytoplasm and forms a nucleoid. Transportation of substances necessary for the life of the cell, and removal of metabolic products is carried out through special channels and cavities, separated from the cytoplasm by a membrane that has the same structure as the cytoplasmic one. This structural formation is called the endoplasmic reticulum (reticulum).

Initially, bacteria were considered to be the simplest forms of life, referring to animals. Only with the advent of improved microscopes, new methods in research work, scientists were able to fully explore the cells of bacteria and find out the biological role of each cell structure. It was possible to find out that bacterial cells have an ordered structure and each of the bacteria is a separate individual that does not have a basic resemblance to animals.

It can distinguish between permanent and temporary cellular structures. The main ones, many of which are also found in animal cells, include:

• cell wall;
• cytoplasmic membrane;
• cytoplasm;
• nucleoid (nuclear apparatus);
• inclusions;
• ribosomes.

Temporary ones include:

• mesosome;
• fat droplets;
• drank (villi);
• thylakoids;
• flagella;
• chromatophores;
• polysaccharides.

Between the membrane, which is also found in animals, and the capsule is a cell wall. Due to its density, it acts as a skeleton. It is invisible without a special microscope. It is necessary for cells to maintain their permanent shape. So, for example, it is clearly visible in such a large bacterium as Beggiatoa mirabilis. L-shaped bacteria and mycoplasma do not have such a wall, other species do. It occupies 20% of the total weight of the microorganism, its thickness is about 50 nm.

The cell wall is essential for bacteria to:

• protection from negative environmental factors;
• participation in exchange processes;
• fission process;
• transport of metabolites;
• selective intake of nutrients.

Having studied the sections, the scientists found that it is permeated with pores through which nutrients enter it and are removed from the cell. The wall itself is layered, and the compaction and thickness are the same throughout the cell.

The plasmalemma separates the cytoplasm from the cell wall. It is a semi-permeable structure consisting of three layers. Of the total dry matter, it is 8-15%. Basically, its composition consists of proteins (50-70%) and lipids (20-50%).

In the life of a cell, its functions are as follows:

• plays the role of an obstacle;
• participates in metabolic processes;
• has a selective throughput;
• takes part in cell growth.

When the cell grows, the cytoplasmic membrane creates peculiar protrusions, which are called mesosomes. Their function has not yet been fully established, but many scientists have suggested that they are required for respiratory and metabolic processes.

Cytoplasm

The main volume of the entire bacterium is occupied by the cytoplasm, which is also present in animals, which is a semi-liquid mass, in which about 90% of water. The cytoplasm consists of the cytosol and includes the following components:

• ribosomes;
• various inclusions;
• enzymes;
• internal membranes;
• nuclear component;
• soluble RNA elements;
• metabolic products.

It creates the internal cavity of the cell and ensures the interconnection between all its components.

Nucleoid

Animals, and all eukaryotes, have a nucleus, but bacteria do not. The nucleoid is the prototype of the nucleus. It is a double-stranded DNA, folded into a coil and freely located in the central zone. Unlike animal cells, bacteria do not have a well-defined nucleus. The nucleoid does not have nucleoli, nuclear envelope and basic proteins, while animal cells do.

Despite the fact that the nucleoid does not have many features of the nucleus, it is usually referred to as a separate structure of the cell. Often the nucleoid consists of separate parts. This peculiarity of it is explained by the fact that it is the carrier of genetic information, and by the way the actual cell division occurs. In many types of prokaryotes, in addition to the nucleoid that performs the function of the nucleus, there are plasmids, which are DNA molecules and are characterized by the ability to renew.

Ribosomes

Ribosomes are called ribonucleic particles having the shape of a sphere and a diameter of 15 to 20 nm. The cells of many bacteria have 5-20 thousand ribosomes. In one RNA molecule there are several ribosomes interconnected according to the principle of beads on a piece of jewelry. Ribosomes combined in this way are called polysomes. The main function of ribosomes is protein synthesis, which is the most important process in the life of bacteria and animals.

Capsule

In addition to the main structures, solid, gaseous and liquid inclusions are isolated in the cytoplasm - these are products of metabolic processes and a supply of nutrients.

The capsule is a mucosal that has clear demarcations from the environment and is closely associated with the cell wall. There is no such organelle in animal cells. You can see it only under a special light microscope by staining. It is not a life-forming organelle of the cell; when it is lost, the microorganism does not lose its viability. In a bacterium such as leuconostoc, more than one microbial cell is included in one capsule. Antigens are concentrated in the capsule, which determine the peculiarity, virulence and ability to induce an immune response of bacteria.

It also protects the microorganism from such negative effects:

• drying;
• mechanical impact;
• infections.

In many species, attachment of the microorganism to the nutrient medium is indispensable without it.

Organelles of movement

To move around, bacteria have special organelles ─ flagella, which are also found in protozoa. They are composed of protein molecules and look like thin, filamentous, elongated structures. The length of the flagella itself is several times longer than the microorganism. The flagella are 3 to 12 µm long and 12 to 20 nm thick. They are fixed to the cytoplasmic membrane with the help of special disks. You can see them only under an electron microscope or after pre-treatment with coloring preparations under a light microscope. They do not provide the life of the microorganism, and their number in different individuals can vary from 1 to 50. Depending on where the flagella are attached, they distinguish:

• monotrichous, having one flagellum;
• amphitrichous, having two or several pieces collected in a bundle, located polarly relative to each other;
• lophotrichous, have a bundle on one side;
• peritrichous, have flagella along the entire length of the cell;
• atrichia, do not have flagella at all.

They are mainly found in microorganisms with a tortuous form and living in a liquid medium, as an exception, they are found in cocci. The movement of bacteria, as a rule, is erratic, but under the influence of external factors, they make a movement that has a certain direction, called taxis.

There are several types of purposeful movement:

• chemotaxis - when in the external environment there is a different saturation with chemicals;
• aerotaxis ─ various accumulation of oxygen;
• phototaxis ─ varying degrees of illumination;
• magnetotaxis is the ability to respond to a magnetic field.

Villi

The villi are somewhat shorter than the flagella and are present in both motile and non-motile microorganisms. They can only be seen under an electron microscope. On the surface of them can be from a thousand to hundreds of thousands. They are of two types: ordinary and sexual. Ordinary villi are responsible for attachment, adhesion and water-salt metabolism. The sex organs are responsible for the transfer of hereditary information.

controversy

These formations, which are not characteristic of animal cells, have an oval shape and a size of 0.8 to 1.5 microns. There are two types of spores: bacilli that do not exceed the size of the bacterium itself, and clostridia that are larger than the bacterium itself. They can be in the central part of the cell, towards the end and at the very end. All bacteria have the same spore structure. In the center is sporoplasm, which includes proteins and nucleic acids. It has a carrier of hereditary information, ribosomes and a dimly expressed membrane. The multi-layered shell protects the spore from negative impacts and makes it stable. The main purpose of spores is to keep the bacteria alive, despite such adverse conditions as high temperature and chemical treatment. They can lie dormant for hundreds of years.

A deep study of the structure of bacterial cells has made it possible for mankind to fight many diseases and apply them in various fields of their activity.

The bacterial nucleoid looks like a diffuse mass of DNA, but it is characterized by high orderliness and a non-random arrangement of genes.

Bacteria do not have nucleosomes, but DNA organization is aided by various proteins associated with the nucleoid

Just as it takes place for the nucleus and cytoplasm of a eukaryotic cell, in bacteria, transcription occurs throughout the entire mass of the nucleoid, translation - on its peripheral zone.

RNA polymerase plays an important role in nucleoid organization.

Fundamental difference cells prokaryotes from eukaryotic cells is that they do not have a nuclear membrane. The presence of a nuclear membrane in eukaryotes provides for the existence of compartments that separate the processes of transcription and translation. In prokaryotes, these processes are not separated by a membrane, and mRNA can be translated during transcription. The simultaneous occurrence of these processes has important consequences for the regulation of the activity of some genes.

As shown in the picture below, chromosomal DNA bacteria has the appearance of an amorphous mass, a nucleoid that occupies most of the volume in the center of the cytoplasm. A nucleoid is made up of chromosomal DNA and its associated proteins. Bacteria do not contain nucleosomes, which are involved in DNA packaging of eukaryotic and archaeal chromosomes. However, bacterial DNA is compact and packed with numerous nucleoid-associated proteins, which are listed in the figure below.

Among the most important of these proteins include topoisomerases. They control the supercoiling of DNA, which plays an important role in its compaction, and ensure the flow of processes such as replication and transcription, which require the unwinding of the DNA molecule. Proteins of the SMC family, which maintain the structural organization of chromosomes, are also involved in the organization of the nucleoid. This is evidenced by the phenotype of the corresponding mutants, but the specific mechanism of their participation remains unclear.

Proteins in eukaryotic cells close to SMC, participate in the bonding of chromosomes to each other and their condensation in mitosis and meiosis. These proteins of various nature, associated with the nucleoid, are involved in maintaining the required level of its supercoiling and compaction. However, it remains to be seen how such a state of nucleoid homeostasis is achieved and maintained.

Although the nucleoid has amorphous structure, individual genes are arranged in it in an orderly manner. The position of the genes in the nucleoid reflects their relative location on the chromosome map. Fortunately, the first confirmation of this was obtained by studying the properties of B. subtilis mutants defective in the spoIIIE gene. The mutant of this organism is unable to properly segregate the chromosome during the asymmetric division that accompanies the early stages of spore formation. Instead, the division septum closes around one copy of the chromosome. In this mutant, certain genes almost always fall into a small compartment near the pole, while others are always excluded from it.

This observation allows assume that before division, the chromosome is always in a certain place and in a certain orientation.

Direct data were received in research using in situ hybridization and fluorescent labeling (FISH). This method allows you to directly track the position in the cell of certain genes. However, when using it, before the hybridization of the probe with DNA, it is necessary to fix the preparations and carry out other hard actions. Another approach is to use a green fluorescent protein conjugate with the DNA-binding LacI protein. This conjugate can attach to binding sites located at different locations in the cell. Based on all these experiments, it was shown that the genes do not freely diffuse through the bacterial cell, but are localized in certain, strictly limited places.

Generally speaking, the region of a chromosome containing oriC, is located at one end of the nucleoid, and the region containing terC is at the opposite end. The genes that are located between these two points on the genetic map are more or less proportionally distributed over the nucleoid.

Bacteria in the device transcriptions one catalytic basic RNA polymerase is used, consisting of two a-, one b- and one b-subunits. Promoter specificity is initially determined by various sigma(a) factors, which are also required for transcription initiation but are subsequently cleaved from the core. Transcriptional regulation is carried out by a large set of additional regulators, which usually bind to DNA close to the promoter in order to activate or repress the initiation of transcription. Other regulatory factors act at the level of termination (cessation) of transcription or changes in mRNA stability.

Most of the molecules basic RNA polymerase located in the nucleoid at the center of the cell. Therefore, this is probably where most of the transcription takes place. On the contrary, ribosomes and various proteins involved in translation are concentrated along the periphery of the cell. Thus, even in the absence of a nuclear envelope, transcription and translation in a bacterial cell are spatially separated, similar to the case in a eukaryotic cell. However, there are various data in favor of the fact that transcription and translation in bacteria are sometimes closely coupled with each other.

These data are not inconsistent with the available results, which indicate that RNA polymerase and ribosomes located in different places in the cell. It is possible that both processes occur at the border of the central, or core, and peripheral regions of the cell. So far, we know little about the organization of the central, or core, and peripheral regions of the nucleoid, as well as about the details of the general organization of this structure.

The structure of any organism (and the mechanism, by the way, too) directly depends on the functions performed. For example, for a person, the easiest way to get around is walking, so we have legs, a car is designed to drive, so it has wheels instead of legs. Similarly, the functions of a bacterial cell determine its structure. And each of its internal structures exactly corresponds to its functions.

Bacteria stood at the origins of life on our planet. Their contribution to the formation of minerals and fertile soils is difficult to overestimate. They maintain the balance between carbon dioxide and oxygen in the atmosphere. Their ability to destroy dead organisms allows you to return the necessary nutrients to nature. In the human body, many processes, for example, digestion, cannot proceed without their participation. But the same bacterial cells that help the body survive can, under certain conditions, bring disease or death.

Depending on the purpose, bacteria differ in structure. So, microorganisms that produce oxygen must have chloroplasts; cells capable of moving are always equipped with flagella; bacteria that survive in an aggressive environment cannot do without a protective capsule, etc. Some of the structural elements of the cell exist permanently, its other components arise as needed or are inherent only to certain types of bacteria. But each element of its structure is an example of the ideal correspondence of the structure to the functions performed.

How is a bacterium

A bacterial organism is just one cell. Instead of the usual organs responsible for certain functions, it has only peculiar inclusions called organelles. Their set may be different depending on the type of cell or the conditions of its existence, but a certain mandatory set of internal structures is always present in bacteria. They characterize the cell as bacterial.

A bacterial cell belongs to prokaryotes - non-nuclear unicellular organisms. This means that in its structure there is no membrane that separates the nucleus from the cytoplasm. The role of the nucleus in bacteria is performed by the nucleoid (a closed DNA molecule). In a prokaryotic cell, there are basic and additional organelles (structures). Its main structures include:

• nucleoid;
• cell wall (gram-positive or gram-negative protective layer);
• cytoplasmic membrane (thin layer between the cell wall and the cytoplasm);
• the cytoplasm, which contains the nucleoid and ribosomes (RNA molecules).

The cell acquires additional organelles (organelles) under adverse conditions. They can appear and disappear depending on the environment. Optional cell structures include capsules, pili, spores, various inclusions such as plasmids or volutin grains.

Nucleus in a nuclear-free cell

Nucleoid (“nucleus-like”) is one of the most important organelles in a prokaryotic cell that performs the functions of the nucleus. It is responsible for the storage and transfer of genetic material. A nucleoid is a DNA molecule closed in a ring corresponding to one chromosome. This circular molecule looks like a random weave of threads. However, based on its functions (the exact distribution of genes among daughter organisms), it becomes clear that the bacterial chromosome has a highly ordered structure.

As a rule, this organelle does not have a permanent external form, but it can be easily distinguished against the background of a gel-like cytoplasm in an electron microscope. When examining with a conventional light microscope, the bacterium must first be stained, because in its natural state, the bacteria are transparent and invisible against the background of the glass slide. After special staining, the region of the nuclear vacuole of the bacterium becomes clearly visible.

The DNA molecule (nucleoid) consists of 1.6 x 107 base pairs. A nucleotide is a separate “brick”, a link that makes up all nuclear nucleic acids (DNA, RNA). Thus, a nucleotide is only a single small part of a nucleoid. The length of the DNA molecule in the unfolded state can be a thousand times longer than the length of the bacterial cell itself.

Some bacterial cells contain additional stores of hereditary information - plasmids. These are extrachromosomal genetic elements consisting of double-stranded DNA. They are much smaller than a nucleoid and contain "only" 1500–40,000 base pairs. These plasmids can contain up to hundreds of genes. Their existence can be completely autonomous, although under certain conditions, additional genes are easily integrated into the main DNA chain.

Framework for unicellular

The cell wall performs a shaping function, that is, it simultaneously works as a "skeleton" for the cell and replaces its skin. This hard outer shell:

• protects bacterial "insides";
• responsible for the shape of bacteria;
• transports nutrients in and removes waste out.

Bacterial cells are round (cocci), sinuous (vibrio, spirilla), rod-shaped. There are microorganisms that look like cones, stars, cubes, or have a C-shaped appearance.

The mechanical and physiological functions (protection and transport) of the bacterial cell wall depend on its structure. It is convenient to study the structure of the cell wall using the Gram method. This Dane proposed a method for staining bacteria with aniline dyes. Depending on the reaction of the cell membrane to the paint, there are:

1. Gram-positive (stainable) bacteria. Their shell consists of one layer, the outer membrane is absent.
2. Gram-negative bacteria have a shell that does not retain the dye (after washing, the wall becomes discolored). Their outer shell is much thinner than that of gram-positive ones, while it has two layers - the outer membrane and the bacterial wall located under it.

This separation of bacteria is of great importance in medical research - most often, pathogenic microbes have a gram-positive wall. If the analysis revealed gram-positive bacteria, then there is reason for concern. Gram-negative cells are much safer. Some of them are constantly present in the body and can pose a threat only in case of uncontrolled reproduction. These are the so-called opportunistic bacteria.

The outer membrane of Gram-negative bacteria extends the functions of the bacterial wall. Its permeability and transport properties change. The outer membrane has various channels (pores) that selectively let substances into the cell - useful ones pass freely, and toxins are rejected. That is, the outer layer of a gram-negative cell acts as a "sieve" for molecules. This can explain the great resistance of gram-negative organisms to adverse conditions: all kinds of poisons, chemicals, enzymes, antibiotics.

In biology, the "layer cake" of the cell wall and cytoplasmic membrane is called the cell wall.

What is CPM and mesosomes

Between the cell wall and the cytoplasm is another organelle - the cytoplasmic membrane (CPM). Its functions include limiting the internal contents of the cell, maintaining its shape, protecting it from the penetration of aggressive factors and the unimpeded admission of nutrients. In fact, this is another molecular "sieve".

Electrons (energy) and the transport of materials necessary for the existence of the cell freely pass through the cytoplasmic membrane. There are two active processes that flow through the membrane:

• endocytosis - the penetration of substances into the bacteria;
• exocytosis - excretion of waste.

In the process of endocytosis, the membrane forms internal folds, which then transform into vesicles (vacuoles). Depending on the functions performed, two types of endocytosis are distinguished:

1. Phagocytosis ("eating"). This function is available to some types of bacteria, they are called phagocytes. Such cells create a kind of bag from the cytoplasmic membrane, enveloping the absorbed particle (phagocytic vacuole). An example is blood leukocytes, "eating" foreign particles or bacteria.
2. Pinocytosis ("drinking") is the absorption of liquids. In this case, bubbles of various sizes are formed, sometimes very small.

Exocytosis (excretion) works in the opposite direction. With its help, undigested residues and cellular secretion are removed from the cell.

In addition, the cytoplasmic membrane:

• regulates fluid pressure inside the cell;
• receives and processes chemical information from the outside;
• participates in the process of cell division;
• responsible for the growth of flagella and their movement;
• regulates cell wall synthesis.

The inner bacterial membrane, depending on the functions performed by the cell, forms mesosomes (internal folds). An example is the lamellae and thylakoids in unicellular organisms that live by photosynthesis. Thylakoids are stacks of flat sacs formed by internal folds of the membrane (mesosomes) in which photosynthesis takes place, and lamellae are the same elongated mesosomes that connect stacks of thylakoids.

In gram-positive bacteria, the mesosomes are well developed and rather complexly organized, in contrast to gram-positive ones. There are three types of mesosomes:

• lamellar (lamellae);
• vesicles (vesicles with a supply of nutrients);
• tubules (tubular mesosomes).

Microbiologists have not yet come to a final conclusion - whether mesosomes are the main structure of a bacterial cell or only enhance its functions.

Ribosomes are the basis of protein life

The cytoplasm of bacteria is the internal semi-liquid (colloidal) component of the cell, in which all organelles (nucleoid, plasmids, mesosomes and other inclusions) are located. One of the main functions of the cytoplasm is to create comfortable conditions for ribosomes.

The ribosome is the most important non-membrane organelle of the cell, consisting of two parts: large and small subunits (polypeptides that make up the protein complex). The function of ribosomes is protein synthesis in the cell. Ribosomes are ribonucleoprotein particles up to about 20 nm in size. In a cell, there can be from 5,000 to 90,000 of them at the same time. These are the smallest and most numerous organelles of prokaryotes. Most of the bacterial RNA is located in the ribosomes, in addition, they include proteins.

Ribosomes are responsible for the synthesis of proteins from amino acids. The process proceeds according to the scheme laid down in the genetic information of RNA. It is believed that the evolution of ribosomes began in the pre-protein era. Over time, the biosynthesis apparatus improved, but RNA continues to play the main function in it. Thus, ribosomes - suppliers of the main component of the vital activity of protein forms - themselves rely on RNA, and not on the protein component.

The problem of the origin of life on Earth is a kind of paradox - DNA (deoxyribonucleic acid), which carries genetic information, cannot reproduce itself, it needs some kind of catalyst, and proteins, an excellent catalyst, cannot be formed without DNA. A paradox arises: chickens and eggs or “what happened before?”.

It turned out that in the beginning there was RNA (ribonucleic acid)! All key stages of protein biosynthesis (information transfer, catalyst operation, amino acid transport) were taken over by RNA, which forms the basis of ribosomes. This served as one of the proofs of the existence of life "before DNA". The hypothesis of the "RNA world" has not yet found experimental confirmation, but nucleic acid research remains one of the "hottest" areas of science.

Additional structures of prokaryotes

Like any living being, a bacterial cell seeks to protect itself by creating various additional elements. Surface structures include:

1. Capsule. It is a superficial mucous layer that forms around the cell in response to the environment. The capsule not only gives the bacteria extra protection, but can also contain a reserve of nutrients for a rainy day.
2. Flagella. Long (longer than the cell itself) very thin threads attached to the CPM and the wall act as a motor for the free movement of bacteria. They can be located over the entire surface of the bacterium or grow in bunches along its edges.
3. Drank (villi). They differ from flagella in size (thinner and much shorter). The function of pili is not to move, but they are responsible for attaching (binding) bacteria to other microorganisms or surfaces. They also take part in the water-salt metabolism and the nutritional process.
4. Disputes. This is a guarantee for microorganisms to survive any adverse factors (lack of water or food, aggressive environment). They form inside bacteria, mostly Gram-positive. However, this method provides only survival, but not reproduction (as in the case of fungal spores).

Internal additional inclusions can be both active (chlorosomes of photosynthetic cells) and passive (food reserves). Bacteria that live in water have gas vacuoles, tiny air bubbles that keep them buoyant.

Nutrients of bacteria are deposited in various granules (lipids, volutin). Lipids provide the bacterium with a carbon reserve that provides energy in the absence of other sources. Volutin (grains containing polyphosphates) becomes a source of phosphorus when there is not enough of it in the environment. Volutin reserves can also serve as a source of energy, although their role is not so significant. Additional structures of cyanobacteria are nitrogen reserves, for sulfur bacteria - deposits of molecular sulfur. The main characteristic of all inclusions with reserves "for a rainy day" is that they are necessarily isolated from the cytoplasm and cannot affect the cell under normal conditions. Otherwise, there may be an overdose of chemical elements and the bacterium will suffer.

The structures of a bacterial cell, both basic and additional, clearly perform their functions, preserving and prolonging its viability. The information contained in the RNA and DNA of prokaryotes allows the cell to quickly respond to changing conditions of existence and take the necessary measures to preserve the microorganism and successfully perform all the functions inherent in it by nature.