When conditions are favorable, the bacterial cell multiplies. The main way bacteria reproduce is by simply dividing the cell in half (binary fission). At the beginning of division, the cell elongates, then the nucleoid divides. The nucleoid is represented by a supercoiled and tightly packed self-replicating DNA molecule - a replicon. Plasmids are also replicons. DNA replication is carried out with the participation of DNA polymerase enzymes. The process begins at a specific point in the DNA and occurs simultaneously in two opposite directions. Replication also ends at a specific place in the DNA. As a result of replication, the amount of DNA in the cell doubles. Newly synthesized DNA molecules, consisting of one mother and one newly synthesized strand, gradually disperse into the resulting daughter cells. It is believed that DNA replication takes up almost 80% of the total time spent by a bacterial cell dividing. Once DNA replication is complete, the process of cell division begins. First, a two-layer cytoplasmic membrane is synthesized, then peptidoglycan is synthesized between the layers of the membrane. The process ends with the formation of a partition.

During DNA replication and the formation of a dividing septum, the microorganism cell continuously grows. During this period, the following processes actively occur in the cell: synthesis of peptidoglycan of the cell wall and components of the cytoplasmic membrane, formation of new ribosomes and other organelles. At the last stage of division, the daughter cells are separated from each other, but in some types of bacteria the process does not go to completion, resulting in the formation of chains of cells (streptococci, tetracocci, etc.). When rod-shaped bacteria divide, the cells first grow in length. When the bacteria become twice as long, the rod narrows somewhat in the middle and then splits into two cells.

Some bacteria are characterized by another method of reproduction - budding, which is a type of binary fission. Bacteria of the genera reproduce by budding Hyphomicrobium,Pedomicrobium and others, united in the group of budding bacteria. These organisms have the appearance of elongated rods, sometimes pear-shaped, ending in hyphae. Reproduction in these bacteria begins with the formation of a bud at the end of the hypha or directly on the mother cell. The bud grows into a daughter cell, forms a flagellum and separates from the mother cell. Upon reaching a mature state, the flagellum is lost and the development process is repeated. Sometimes bacteria experience a sexual process called conjugation.

As a result of growth and reproduction, a colony of its descendants is formed from one cell of a microorganism. Microorganisms are characterized by a high reproduction rate, estimated by generation time, i.e. the time during which cell division occurs: in 24 hours, sometimes as many generations change as a person has in five thousand years. The rate of reproduction depends on a number of conditions and can be quite different for each type of bacteria. If the necessary nutrients are present in the medium, a favorable temperature, and an optimal reaction of the environment, the division of each cell, for example in E. coli, can be repeated every 20-30 minutes. At this rate of reproduction, one cell can produce 472 10 19 cells per day (72 generations). If we assume that 1 billion bacterial cells weigh 1 mg, then 472 10 19 cells will weigh 4720 tons. Such a mass of living matter could be obtained in the presence of ideal conditions that exclude cell death.

The high intensity of reproduction ensures the preservation of microorganisms on the earth's surface: when unfavorable conditions occur, they die en masse, but it is enough for a few cells to survive somewhere, and under optimal conditions they will again give rise to a huge number of organisms.

One of the vital functions of prokaryotes, like any other living beings, is reproduction. At its core, the process of bacterial reproduction can be characterized as an increase in the number of individuals, which occurs due to the division of bacteria.

Modern microbiology has described the patterns of mitosis, meiosis and amitosis - this is how eukaryotes divide, and prokaryotes reproduce by direct division.

Prokaryotes reproduce primarily by dividing the mother bacterial cell into 2 identical daughter cells. Under favorable conditions, binary fission occurs every 20 minutes, and if environmental conditions deteriorate, the time required for the cell to grow and divide increases. In case of unfavorable external conditions, prokaryotes stop reproducing for a while or completely.

The process of dividing the cell in half is immediately preceded by a period of growth of the cytoplasm and replication (doubling) of the bacterial chromosome, as in the photo.

Replication of the circular bacterial chromosome

The increase in cell size occurs as a result of a number of coordinated biosynthetic processes that are tightly controlled. The process of bacterial growth is not endless - once the prokaryotes reach a given critical size, division occurs.

Mechanism of bacterial DNA replication

When doubling the DNA of a nucleoid (analogue of the nucleus in a bacterial cell), the following scheme is implemented:

• initiation - the beginning of DNA division under the action of a replicon (an enzymatic apparatus, a section of DNA containing information about duplication);
• elongation - lengthening, growth of the chromosome chain;
• termination - completion of chain growth and DNA helixing during replication.

In parallel with DNA replication, the cell itself grows, and the distance between the two new chromosomes attached via mesosomes to the cytoplasmic membrane gradually increases. A prokaryotic cell begins to divide some time after replication. Obviously, it is DNA duplication that initiates the separation process.

A similar process is absent for eukaryotic meiosis. The process of meiosis differs in many ways from the reproduction of prokaryotes. In addition, the division of the mother cell into two parts for gram-positive and gram-negative bacteria has its own characteristics.

Reproduction of gram-negative bacteria

Gram-negative bacteria have a relatively thin cell wall, on which a ring organelle, the septal ring, is located approximately in the center. The separation of bacteria occurs by contraction of the organelle and the formation of a constriction between the daughter cells, as can be seen in the photo.

The septal ring is a complex protein complex that includes more than 12 different proteins. It is formed by sequentially joining proteins to each other in a strict sequence.

Septal ring proteins perform the following functions necessary for reproduction:

• model the attachment of filaments (ring proteins) in a certain sequence to the Z-ring (immature form of the ring organelle);
• provide binding of the Z-ring to the membrane;
• coordinate the formation of a ring organelle with segregation (separation) of the chromosome;
• synthesize peptidoglycan, the most significant component of the bacterial cell wall, which provides osmotic protection;
• carry out hydrolysis of peptidoglycan to separate new cells.

The constriction in gram-negative bacteria covers all cell membranes - the cytoplasmic (inner) and outer membranes, as well as a thin layer of peptidoglycan associated with them by lipoprotein.

During meiosis of eukaryotes, a similar method of cell division by constriction does not occur.

Reproduction of gram-positive bacteria

The wall thickness of a gram-positive bacterium is more than twice that of a gram-negative bacterium.

The process of reproduction of a gram-positive bacterium is not similar to mitosis and differs from meiosis in eukaryotes. At the end of the DNA replication process, gram-positive bacteria do not create a constriction, but synthesize a transverse septum, as in the photo. During the synthesis process, as in gram-negative bacteria during the formation of a constriction, mesosomes take part, forming a partition from the edge to the center of the cellular structure.

The transverse binary division of a bacterial prokaryotic cell is always longitudinally and transversely symmetrical, which is another difference between the process and meiosis of the cellular structure of eukaryotes.

Under favorable conditions, direct binary division of bacterial cells can occur in one or several planes, which is impossible for meiosis. In the case when the cells do not diverge after separation, the formation of associations of different shapes occurs:

• when a cell is cut in one plane, chains of spherical or rod-shaped cells are formed (spherical diplococci, a chain of rod-shaped bacteria, as in the photo);
• when separated in different planes, cell accumulations of various shapes are observed (chains of streptococci, packets of sarcin, clusters of staphylococci).

The variety of forms of prokaryotes, which is visible in the photo, is completely impossible for meiosis of nuclear cells.

Such transverse division is characteristic not only of gram-positive bacteria, but also of filamentous cyanobacteria.

Multiple fission of cyanobacteria

One of the types of binary reproduction of prokaryotes is the multiple formation of daughter prokaryotes from the mother cell, typical of cyanobacteria, and completely uncharacteristic of meiosis.

A - reproduction of cyanobacteria of the genus Dermocarpa
B - reproduction of cyanobacteria of the genus Chroococcidiopsis

Initially, cytoplasmic growth and chromosome replication occur. Then, as can be seen in the video, within the additional fibrillar layer of the mother's body, successive binary divisions occur, which lead to the formation of baeocytes (small cells). Their number can range from 4 to 1000 units and is associated with the type of cyanobacterium. Baeocytes are released after the wall of the mother prokaryote ruptures, as seen in the video.

In addition to equal separation, some bacteria reproduce by budding.

Budding as a special case of binary fission

In photo- and chemotrophs, regardless of the food source (autotrophs or heterotrophs), it is possible to reproduce the organism by budding.

The mechanism of the process is as follows:

• a bud forms at the pole of the mother cell;
• the kidney grows to the size of the mother’s body (this can be seen in the photo), and a new cell wall is synthesized for the kidney;
• a full-fledged daughter cell is separated from the mother cell.

If the process of binary division has no restrictions, as in the case of meiosis

for eukaryotes, budding depends on the fact of prokaryotic aging. On average, the mother cell separates no more than 4 buds.

Budding has its own specific characteristics:

• only longitudinal symmetry is preserved (clearly visible in the photo);
• after budding, mother and daughter cells are obtained, whereas after binary division there is no mother cell - there are two equivalent daughter cells;
• the maternal and daughter organisms are not identical, the differences between them are clearly visible - the aging process is observed.

Under favorable physicochemical conditions, prokaryotes are able to divide exponentially and fill the entire world. However, in reality this does not happen, since there are factors that inhibit bacterial division.

Factors limiting division

Despite all the species diversity and adaptability, bacteria do not multiply indefinitely. Research has shown that the growth of the bacterial population occurs in accordance with the law of reproduction of microorganisms and can be described numerically and graphically.

Population growth associated with bacterial division consists of several phases:

• lag phase - a period of adaptation when it takes time to adapt to new living conditions; division is not of high importance;
• logarithmic phase – the period with the greatest number of divisions and exponential population growth;
• stationary phase - the time when the growth of a bacterial colony tends to zero, the division of bacteria is equalized with the number of deaths due to limited food resources;
• growth slowdown - occurs due to a significant reduction in food resources and the accumulation of toxic waste products.

Unfavorable conditions provoke the cessation of bacterial division and, as a consequence, the inevitable death of the population.

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Bacteria are the oldest group of organisms currently existing on Earth. The first bacteria probably appeared more than 3.5 billion years ago and for almost a billion years they were the only living creatures on our planet. Since these were the first representatives of living nature, their body had a primitive structure.

Over time, their structure became more complex, but to this day bacteria are considered the most primitive single-celled organisms. It is interesting that some bacteria still retain the primitive features of their ancient ancestors. This is observed in bacteria living in hot sulfur springs and anoxic mud at the bottom of reservoirs.

Most bacteria are colorless. Only a few are purple or green. But the colonies of many bacteria have a bright color, which is caused by the release of a colored substance into the environment or pigmentation of cells.

The discoverer of the world of bacteria was Antony Leeuwenhoek, a Dutch naturalist of the 17th century, who first created a perfect magnifying microscope that magnifies objects 160-270 times.

Bacteria are classified as prokaryotes and are classified into a separate kingdom - Bacteria.

Body Shape

Bacteria are numerous and diverse organisms. They vary in shape.

Name of the bacterium	Bacteria shape	Bacteria image
Cocci		Ball-shaped
Bacillus		Rod-shaped
Vibrio		Comma-shaped
Spirillum		Spiral
Streptococci		Chain of cocci
Staphylococcus		Clusters of cocci
Diplococcus		Two round bacteria enclosed in one mucous capsule

Methods of transportation

Among bacteria there are mobile and immobile forms. Motiles move due to wave-like contractions or with the help of flagella (twisted helical threads), which consist of a special protein called flagellin. There may be one or more flagella. In some bacteria they are located at one end of the cell, in others - at two or over the entire surface.

But movement is also inherent in many other bacteria that lack flagella. Thus, bacteria covered on the outside with mucus are capable of gliding movement.

Some aquatic and soil bacteria lacking flagella have gas vacuoles in the cytoplasm. There may be 40-60 vacuoles in a cell. Each of them is filled with gas (presumably nitrogen). By regulating the amount of gas in the vacuoles, aquatic bacteria can sink into the water column or rise to its surface, and soil bacteria can move in the soil capillaries.

Habitat

Due to their simplicity of organization and unpretentiousness, bacteria are widespread in nature. Bacteria are found everywhere: in a drop of even the purest spring water, in grains of soil, in the air, on rocks, in polar snow, desert sands, on the ocean floor, in oil extracted from great depths, and even in the water of hot springs with a temperature of about 80ºC. They live on plants, fruits, various animals and in humans in the intestines, oral cavity, limbs, and on the surface of the body.

Bacteria are the smallest and most numerous living creatures. Due to their small size, they easily penetrate into any cracks, crevices, or pores. Very hardy and adapted to various living conditions. They tolerate drying, extreme cold, and heating up to 90ºC without losing their viability.

There is practically no place on Earth where bacteria are not found, but in varying quantities. The living conditions of bacteria are varied. Some of them require atmospheric oxygen, others do not need it and are able to live in an oxygen-free environment.

In the air: bacteria rise to the upper atmosphere up to 30 km. and more.

There are especially many of them in the soil. 1 g of soil can contain hundreds of millions of bacteria.

In water: in the surface layers of water in open reservoirs. Beneficial aquatic bacteria mineralize organic residues.

In living organisms: pathogenic bacteria enter the body from the external environment, but only under favorable conditions cause diseases. Symbiotic live in the digestive organs, helping to break down and absorb food, and synthesize vitamins.

External structure

The bacterial cell is covered with a special dense shell - a cell wall, which performs protective and supporting functions, and also gives the bacterium a permanent, characteristic shape. The cell wall of a bacterium resembles the wall of a plant cell. It is permeable: through it, nutrients freely pass into the cell, and metabolic products exit into the environment. Often, bacteria produce an additional protective layer of mucus on top of the cell wall - a capsule. The thickness of the capsule can be many times greater than the diameter of the cell itself, but it can also be very small. The capsule is not an essential part of the cell; it is formed depending on the conditions in which the bacteria find themselves. It protects the bacteria from drying out.

On the surface of some bacteria there are long flagella (one, two or many) or short thin villi. The length of the flagella can be many times greater than the size of the body of the bacterium. Bacteria move with the help of flagella and villi.

Internal structure

Inside the bacterial cell there is dense, immobile cytoplasm. It has a layered structure, there are no vacuoles, therefore various proteins (enzymes) and reserve nutrients are located in the substance of the cytoplasm itself. Bacterial cells do not have a nucleus. A substance carrying hereditary information is concentrated in the central part of their cell. Bacteria, - nucleic acid - DNA. But this substance is not formed into a nucleus.

The internal organization of a bacterial cell is complex and has its own specific characteristics. The cytoplasm is separated from the cell wall by the cytoplasmic membrane. In the cytoplasm there is a main substance, or matrix, ribosomes and a small number of membrane structures that perform a variety of functions (analogues of mitochondria, endoplasmic reticulum, Golgi apparatus). The cytoplasm of bacterial cells often contains granules of various shapes and sizes. The granules may be composed of compounds that serve as a source of energy and carbon. Droplets of fat are also found in the bacterial cell.

In the central part of the cell, the nuclear substance is localized - DNA, which is not delimited from the cytoplasm by a membrane. This is an analogue of the nucleus - a nucleoid. The nucleoid does not have a membrane, a nucleolus, or a set of chromosomes.

Eating methods

Bacteria have different feeding methods. Among them there are autotrophs and heterotrophs. Autotrophs are organisms that are capable of independently producing organic substances for their nutrition.

Plants need nitrogen, but cannot absorb nitrogen from the air themselves. Some bacteria combine nitrogen molecules in the air with other molecules, resulting in substances that are available to plants.

These bacteria settle in the cells of young roots, which leads to the formation of thickenings on the roots, called nodules. Such nodules form on the roots of plants of the legume family and some other plants.

The roots provide carbohydrates to the bacteria, and the bacteria to the roots provide nitrogen-containing substances that can be absorbed by the plant. Their cohabitation is mutually beneficial.

Plant roots secrete a lot of organic substances (sugars, amino acids and others) that bacteria feed on. Therefore, especially many bacteria settle in the soil layer surrounding the roots. These bacteria convert dead plant debris into plant-available substances. This layer of soil is called the rhizosphere.

There are several hypotheses about the penetration of nodule bacteria into root tissue:

• through damage to epidermal and cortex tissue;
• through root hairs;
• only through the young cell membrane;
• thanks to companion bacteria producing pectinolytic enzymes;
• due to stimulation of the synthesis of B-indoleacetic acid from tryptophan, always present in plant root secretions.

The process of introduction of nodule bacteria into root tissue consists of two phases:

• infection of root hairs;
• process of nodule formation.

In most cases, the invading cell actively multiplies, forms so-called infection threads and, in the form of such threads, moves into the plant tissue. Nodule bacteria emerging from the infection thread continue to multiply in the host tissue.

Plant cells filled with rapidly multiplying cells of nodule bacteria begin to rapidly divide. The connection of a young nodule with the root of a legume plant is carried out thanks to vascular-fibrous bundles. During the period of functioning, the nodules are usually dense. By the time optimal activity occurs, the nodules acquire a pink color (thanks to the leghemoglobin pigment). Only those bacteria that contain leghemoglobin are capable of fixing nitrogen.

Nodule bacteria create tens and hundreds of kilograms of nitrogen fertilizer per hectare of soil.

Metabolism

Bacteria differ from each other in their metabolism. For some it occurs with the participation of oxygen, for others - without it.

Most bacteria feed on ready-made organic substances. Only a few of them (blue-green, or cyanobacteria) are capable of creating organic substances from inorganic ones. They played an important role in the accumulation of oxygen in the Earth's atmosphere.

Bacteria absorb substances from the outside, tear their molecules into pieces, assemble their shell from these parts and replenish their contents (this is how they grow), and throw unnecessary molecules out. The shell and membrane of the bacterium allows it to absorb only the necessary substances.

If the shell and membrane of a bacterium were completely impermeable, no substances would enter the cell. If they were permeable to all substances, the contents of the cell would mix with the medium - the solution in which the bacterium lives. To survive, bacteria need a shell that allows necessary substances to pass through, but not unnecessary substances.

The bacterium absorbs nutrients located near it. What happens next? If it can move independently (by moving a flagellum or pushing mucus back), then it moves until it finds the necessary substances.

If it cannot move, then it waits until diffusion (the ability of molecules of one substance to penetrate into the thicket of molecules of another substance) brings the necessary molecules to it.

Bacteria, together with other groups of microorganisms, perform enormous chemical work. By converting various compounds, they receive the energy and nutrients necessary for their life. Metabolic processes, methods of obtaining energy and the need for materials for building the substances of their bodies are diverse in bacteria.

Other bacteria satisfy all their needs for carbon necessary for the synthesis of organic substances in the body at the expense of inorganic compounds. They are called autotrophs. Autotrophic bacteria are capable of synthesizing organic substances from inorganic ones. Among them are:

Chemosynthesis

The use of radiant energy is the most important, but not the only way to create organic matter from carbon dioxide and water. Bacteria are known that use not sunlight as an energy source for such synthesis, but the energy of chemical bonds occurring in the cells of organisms during the oxidation of certain inorganic compounds - hydrogen sulfide, sulfur, ammonia, hydrogen, nitric acid, ferrous compounds of iron and manganese. They use the organic matter formed using this chemical energy to build the cells of their body. Therefore, this process is called chemosynthesis.

The most important group of chemosynthetic microorganisms are nitrifying bacteria. These bacteria live in the soil and oxidize ammonia formed during the decay of organic residues to nitric acid. The latter reacts with mineral compounds of the soil, turning into salts of nitric acid. This process takes place in two phases.

Iron bacteria convert ferrous iron into oxide iron. The resulting iron hydroxide settles and forms the so-called bog iron ore.

Some microorganisms exist due to the oxidation of molecular hydrogen, thereby providing an autotrophic method of nutrition.

A characteristic feature of hydrogen bacteria is the ability to switch to a heterotrophic lifestyle when provided with organic compounds and the absence of hydrogen.

Thus, chemoautotrophs are typical autotrophs, since they independently synthesize the necessary organic compounds from inorganic substances, and do not take them ready-made from other organisms, like heterotrophs. Chemoautotrophic bacteria differ from phototrophic plants in their complete independence from light as an energy source.

Bacterial photosynthesis

Some pigment-containing sulfur bacteria (purple, green), containing specific pigments - bacteriochlorophylls, are able to absorb solar energy, with the help of which hydrogen sulfide in their bodies is broken down and releases hydrogen atoms to restore the corresponding compounds. This process has much in common with photosynthesis and differs only in that in purple and green bacteria the hydrogen donor is hydrogen sulfide (occasionally carboxylic acids), and in green plants it is water. In both of them, the separation and transfer of hydrogen is carried out due to the energy of absorbed solar rays.

This bacterial photosynthesis, which occurs without the release of oxygen, is called photoreduction. Photoreduction of carbon dioxide is associated with the transfer of hydrogen not from water, but from hydrogen sulfide:

6СО 2 +12Н 2 S+hv → С6Н 12 О 6 +12S=6Н 2 О

The biological significance of chemosynthesis and bacterial photosynthesis on a planetary scale is relatively small. Only chemosynthetic bacteria play a significant role in the process of sulfur cycling in nature. Absorbed by green plants in the form of sulfuric acid salts, sulfur is reduced and becomes part of protein molecules. Further, when dead plant and animal remains are destroyed by putrefactive bacteria, sulfur is released in the form of hydrogen sulfide, which is oxidized by sulfur bacteria to free sulfur (or sulfuric acid), forming sulfites in the soil that are accessible to plants. Chemo- and photoautotrophic bacteria are essential in the nitrogen and sulfur cycle.

Sporulation

Spores form inside the bacterial cell. During the process of sporulation, the bacterial cell undergoes a number of biochemical processes. The amount of free water in it decreases and enzymatic activity decreases. This ensures the resistance of the spores to unfavorable environmental conditions (high temperature, high salt concentration, drying, etc.). Sporulation is characteristic of only a small group of bacteria.

Spores are an optional stage in the life cycle of bacteria. Sporulation begins only with a lack of nutrients or accumulation of metabolic products. Bacteria in the form of spores can remain dormant for a long time. Bacterial spores can withstand prolonged boiling and very long freezing. When favorable conditions occur, the spore germinates and becomes viable. Bacterial spores are an adaptation to survive in unfavorable conditions.

Reproduction

Bacteria reproduce by dividing one cell into two. Having reached a certain size, the bacterium divides into two identical bacteria. Then each of them begins to feed, grows, divides, and so on.

After cell elongation, a transverse septum gradually forms, and then the daughter cells separate; In many bacteria, under certain conditions, after dividing, cells remain connected in characteristic groups. In this case, depending on the direction of the division plane and the number of divisions, different shapes arise. Reproduction by budding occurs as an exception in bacteria.

Under favorable conditions, cell division in many bacteria occurs every 20-30 minutes. With such rapid reproduction, the offspring of one bacterium in 5 days can form a mass that can fill all seas and oceans. A simple calculation shows that 72 generations (720,000,000,000,000,000,000 cells) can be formed per day. If converted into weight - 4720 tons. However, this does not happen in nature, since most bacteria quickly die under the influence of sunlight, drying, lack of food, heating to 65-100ºC, as a result of struggle between species, etc.

The bacterium (1), having absorbed enough food, increases in size (2) and begins to prepare for reproduction (cell division). Its DNA (in a bacterium the DNA molecule is closed in a ring) doubles (the bacterium produces a copy of this molecule). Both DNA molecules (3,4) find themselves attached to the wall of the bacterium and, as the bacterium elongates, move apart (5,6). First the nucleotide divides, then the cytoplasm.

After the divergence of two DNA molecules, a constriction appears on the bacterium, which gradually divides the body of the bacterium into two parts, each of which contains a DNA molecule (7).

It happens (in Bacillus subtilis) that two bacteria stick together and a bridge is formed between them (1,2).

The jumper transports DNA from one bacterium to another (3). Once in one bacterium, DNA molecules intertwine, stick together in some places (4), and then exchange sections (5).

The role of bacteria in nature

Gyre

Bacteria are the most important link in the general cycle of substances in nature. Plants create complex organic substances from carbon dioxide, water and mineral salts in the soil. These substances return to the soil with dead fungi, plants and animal corpses. Bacteria break down complex substances into simple ones, which are then used by plants.

Bacteria destroy complex organic substances of dead plants and animal corpses, excretions of living organisms and various wastes. Feeding on these organic substances, saprophytic bacteria of decay turn them into humus. These are a kind of orderlies of our planet. Thus, bacteria actively participate in the cycle of substances in nature.

Soil formation

Since bacteria are distributed almost everywhere and occur in huge numbers, they largely determine various processes occurring in nature. In autumn, the leaves of trees and shrubs fall, above-ground shoots of grasses die, old branches fall off, and from time to time the trunks of old trees fall. All this gradually turns into humus. In 1 cm3. The surface layer of forest soil contains hundreds of millions of saprophytic soil bacteria of several species. These bacteria convert humus into various minerals that can be absorbed from the soil by plant roots.

Some soil bacteria are able to absorb nitrogen from the air, using it in vital processes. These nitrogen-fixing bacteria live independently or settle in the roots of legume plants. Having penetrated the roots of legumes, these bacteria cause the growth of root cells and the formation of nodules on them.

These bacteria produce nitrogen compounds that plants use. Bacteria obtain carbohydrates and mineral salts from plants. Thus, there is a close relationship between the legume plant and the nodule bacteria, which is beneficial to both one and the other organism. This phenomenon is called symbiosis.

Thanks to symbiosis with nodule bacteria, leguminous plants enrich the soil with nitrogen, helping to increase yield.

Distribution in nature

Microorganisms are ubiquitous. The only exceptions are the craters of active volcanoes and small areas at the epicenters of exploded atomic bombs. Neither the low temperatures of Antarctica, nor the boiling streams of geysers, nor saturated salt solutions in salt pools, nor the strong insolation of mountain peaks, nor the harsh irradiation of nuclear reactors interfere with the existence and development of microflora. All living beings constantly interact with microorganisms, often being not only their repositories, but also their distributors. Microorganisms are natives of our planet, actively exploring the most incredible natural substrates.

Soil microflora

The number of bacteria in the soil is extremely large - hundreds of millions and billions of individuals per gram. There are much more of them in soil than in water and air. The total number of bacteria in soils changes. The number of bacteria depends on the type of soil, their condition, and the depth of the layers.

On the surface of soil particles, microorganisms are located in small microcolonies (20-100 cells each). They often develop in the thickness of clots of organic matter, on living and dying plant roots, in thin capillaries and inside lumps.

The soil microflora is very diverse. Here there are different physiological groups of bacteria: putrefaction bacteria, nitrifying bacteria, nitrogen-fixing bacteria, sulfur bacteria, etc. among them there are aerobes and anaerobes, spore and non-spore forms. Microflora is one of the factors in soil formation.

The area of ​​development of microorganisms in the soil is the zone adjacent to the roots of living plants. It is called the rhizosphere, and the totality of microorganisms contained in it is called the rhizosphere microflora.

Microflora of reservoirs

Water is a natural environment where microorganisms develop in large numbers. The bulk of them enters the water from the soil. A factor that determines the number of bacteria in water and the presence of nutrients in it. The cleanest waters are from artesian wells and springs. Open reservoirs and rivers are very rich in bacteria. The largest number of bacteria is found in the surface layers of water, closer to the shore. As you move away from the shore and increase in depth, the number of bacteria decreases.

Clean water contains 100-200 bacteria per ml, and polluted water contains 100-300 thousand or more. There are many bacteria in the bottom sludge, especially in the surface layer, where the bacteria form a film. This film contains a lot of sulfur and iron bacteria, which oxidize hydrogen sulfide to sulfuric acid and thereby prevent fish from dying. There are more spore-bearing forms in silt, while non-spore-bearing forms predominate in water.

In terms of species composition, the microflora of water is similar to the microflora of soil, but there are also specific forms. By destroying various waste that gets into the water, microorganisms gradually carry out the so-called biological purification of water.

Air microflora

The microflora of the air is less numerous than the microflora of soil and water. Bacteria rise into the air with dust, can remain there for some time, and then settle on the surface of the earth and die from lack of nutrition or under the influence of ultraviolet rays. The number of microorganisms in the air depends on the geographical zone, terrain, time of year, dust pollution, etc. each speck of dust is a carrier of microorganisms. Most bacteria are in the air above industrial enterprises. The air in rural areas is cleaner. The cleanest air is over forests, mountains, and snowy areas. The upper layers of air contain fewer microbes. The air microflora contains many pigmented and spore-bearing bacteria, which are more resistant than others to ultraviolet rays.

Microflora of the human body

The human body, even a completely healthy one, is always a carrier of microflora. When the human body comes into contact with air and soil, various microorganisms, including pathogenic ones (tetanus bacilli, gas gangrene, etc.), settle on clothing and skin. The most frequently exposed parts of the human body are contaminated. E. coli and staphylococci are found on the hands. There are over 100 types of microbes in the oral cavity. The mouth, with its temperature, humidity, and nutrient residues, is an excellent environment for the development of microorganisms.

The stomach has an acidic reaction, so the majority of microorganisms in it die. Starting from the small intestine, the reaction becomes alkaline, i.e. favorable for microbes. The microflora in the large intestines is very diverse. Each adult excretes about 18 billion bacteria daily in excrement, i.e. more individuals than people on the globe.

Internal organs that are not connected to the external environment (brain, heart, liver, bladder, etc.) are usually free of microbes. Microbes enter these organs only during illness.

Bacteria in the cycle of substances

Microorganisms in general and bacteria in particular play a large role in the biologically important cycles of substances on Earth, carrying out chemical transformations that are completely inaccessible to either plants or animals. Different stages of the cycle of elements are carried out by organisms of different types. The existence of each individual group of organisms depends on the chemical transformation of elements carried out by other groups.

Nitrogen cycle

The cyclic transformation of nitrogenous compounds plays a primary role in supplying the necessary forms of nitrogen to organisms of the biosphere with different nutritional needs. Over 90% of total nitrogen fixation is due to the metabolic activity of certain bacteria.

Carbon cycle

The biological transformation of organic carbon into carbon dioxide, accompanied by the reduction of molecular oxygen, requires the joint metabolic activity of various microorganisms. Many aerobic bacteria carry out complete oxidation of organic substances. Under aerobic conditions, organic compounds are initially broken down by fermentation, and the organic end products of fermentation are further oxidized by anaerobic respiration if inorganic hydrogen acceptors (nitrate, sulfate, or CO 2 ) are present.

Sulfur cycle

Sulfur is available to living organisms mainly in the form of soluble sulfates or reduced organic sulfur compounds.

Iron cycle

Some freshwater bodies contain high concentrations of reduced iron salts. In such places, a specific bacterial microflora develops - iron bacteria, which oxidize reduced iron. They participate in the formation of bog iron ores and water sources rich in iron salts.

Bacteria are the most ancient organisms, appearing about 3.5 billion years ago in the Archean. For about 2.5 billion years they dominated the Earth, forming the biosphere, and participated in the formation of the oxygen atmosphere.

Bacteria are one of the most simply structured living organisms (except viruses). They are believed to be the first organisms to appear on Earth.

22. Bacterial nucleus. Types of bacterial cell division. Division process.

Types of division:

1. Equal area binary transverse division, leading to the formation of two identical daughter cells. With this method of division, there is symmetry in relation to the longitudinal and transverse axes. With equal binary fission, the mother cell, dividing, gives rise to two daughter cells and thus itself disappears.

2. Unequal binary fission, or budding. During budding, a small outgrowth (bud) is formed at one of the poles of the mother cell, which increases in size during growth. Gradually, the bud reaches the size of the mother cell, after which it separates from the latter. The cell wall of the kidney is completely synthesized anew. During the budding process, symmetry is observed in relation to only the longitudinal axis. During budding, the mother cell gives rise to a daughter cell, and in most cases morphological and physiological differences can be found between them: there is an old mother cell and a new daughter cell.

3. Reproduction by multiple fission, characteristic of one group of unicellular cyanobacteria, results in the formation of small cells called baeocytes (Greek. bae- small, cyto- cell), the number of which in different species ranges from 4 to 1000. The release of baeocytes occurs by rupture of the maternal cell wall. Multiple fission is based on the principle of equal-area binary fission. The difference is that in this case, after binary fission, the resulting daughter cells do not grow, but they undergo division again.

23. Bacterial nucleus. Forms of exchange of genetic information in bacteria. Variability of bacteria.

Forms of exchange of genetic material in bacteria:

1. horizontal

* transformation – transfer of genetic material, which consists in the fact that the recipient bacterium captures (absorbs) fragments of foreign DNA from the external environment.

A) Induced (artificially obtained) transformation occurs when purified DNA obtained from cultures of those bacteria whose genetic characteristics are sought to be transferred to the culture under study is added to a bacterial culture.

B) Spontaneous transformation occurs under natural conditions and manifests itself in the emergence of recombinants when genetically different cells are mixed. It occurs due to DNA released by cells into the environment as a result of their lysis or as a result of active DNA release by viable donor cells.

* sexduction

* transfection is a variant of transformation of bacterial cells lacking a cell wall, carried out by viral (phage) nucleic acid. Using transfection, it is possible to induce a viral infection in such bacteria (without a cell wall). Transfection can also be carried out with other (non-bacterial) cells by introducing into them foreign DNA that can recombine with the DNA of these cells, or reproduce virions, or replicate independently.

* conjugation is a process of exchange of genetic material (chromosomal and plasmid), carried out through direct contact of donor and recipient cells. This process is controlled only by conjugative plasmids that have a set of genes called the tra-operon (tra - from English, transfer - transfer).

This operon controls the synthesis of the transport apparatus, conjugative replication and the phenomenon of surface exclusion. The transfer apparatus is special donor villi, with the help of which contact is established between conjugating cells. Donor villi are long (1-20 µm) thin tubular structures of a protein nature with an internal diameter of about 3 nm.

establishing contact between donor and recipient

pulling a strand of DNA from the donor to the recipient

completion of the transferred DNA strand by a complementary strand in the recipient cell

recombination between the transferred chromosome (its fragments) and the chromosome of the recipient cell

merozygote reproduction

the formation of cells bearing characteristics of the donor and recipient

Conjugative replication of the transferred strand of chromosomal or plasmid DNA is also carried out under the control of plasmid genes. A classic example of a conjugative plasmid is the sex factor, or F-plasmid (from the English . fertility– fertility). The F-plasmid can be either in an autonomous state or integrated into the cell chromosome. Being in an autonomous state, it controls only its own transfer, in which a P~-cell (a cell lacking an F-plasmid) turns into a P+-cell (a cell containing an F-plasmid). The F-plasmid can integrate into certain areas of the bacterial chromosome, in which case it will control the conjugative transfer of the cell chromosome.

Thus, conjugation begins with the establishment of contact between the donor and the recipient using the donor villus. The latter connects with the receptor of the cell membrane of the recipient cell. Often such contact is established not only between two cells, but between many cells, forming mating aggregates. It is assumed that the DNA strand is pulled through the donor villus channel during conjugation. Since the donor bridge is fragile, the conjugation process can be interrupted at any time. Therefore, during conjugation, either a part of a chromosome or, less commonly, a complete chromosome can be transferred. With the help of F-plasmids, the frequency of gene transfer between bacteria increases significantly.

* transduction - transfer of genetic material from a donor cell to a recipient cell using bacteriophages. A distinction is made between nonspecific and specific transduction.

A) Nonspecific transduction - random transfer of DNA fragments from one bacterial cell to another.

B) Specific transduction is carried out only by temperate phages, capable of inserting into strictly defined regions of the chromosome of a bacterial cell and transferring certain genes.

Molecular mechanisms of bacterial variability

Bacteria, due to the relative simplicity of their organization and short life span, undergo variability faster than many other organisms. Their variability is based on mutations and genetic recombinations, especially those occurring with the participation of transposable elements.

*Mutations are changes in the genotype that are stably inherited. Mutations can be spontaneous or induced.

a) Spontaneous mutations occur without any special influence; they occur as a result of errors during replication and repair. The average frequency of spontaneous mutations is about 1,106 (one mutant per 1 million cells).

b) Induced mutations occur with a much higher frequency; they arise as a result of exposure to various mutagens - physical and chemical factors that damage DNA: ionizing radiation, UV irradiation, various analogues of DNA bases, alkylating compounds, acridines, antibiotics

c) Point mutations can be caused by: base replacement, loss (deletion) of a base, or the appearance of an additional base (insertion). Point mutations can have three consequences:

1) replacing one codon with another, and therefore one amino acid with another;

2) a reading frame shift, which will lead to a change in a whole series of sequences of amino acid residues;

3) the appearance of a “senseless” codon, which will lead to the cessation of translation at a given point

protein synthesis may be completely blocked. Changed protein will be synthesized

All this will lead either to the loss of some phenotypic trait in the mutant, or, less often, to the appearance of a new trait.

Genome disruption may result from:

*extended deletions

*inversions (rotation of a chromosome segment by 180°)

*translocation (movement of a chromosome section from one position to another)

All this will also lead to changes and disruption of various functions of the cell (organism).

A large role in the variability of bacteria and other organisms belongs to the so-called transposable genetic elements, that is, genetic structures capable of moving in an intact form within a given genome or moving from one genome to another, for example, from a plasmid genome to a bacterial one and vice versa. There are three classes of transposable elements: IS elements, transposons and episomes.

#Insertion sequences (from English, insertion sequence) usually have sizes not exceeding 2 thousand base pairs, or 2 kb. (kilobase – thousand base pairs). IS elements carry only one gene encoding a protein transposase, with the help of which IS elements are integrated into various parts of the chromosome. They are designated by numbers: IS1, IS2, IS3, etc.

#Transposons are larger segments of DNA flanked by inverted IS elements. They are able to integrate into various parts of the chromosome or move from one genome to another, i.e. they behave like IS elements. In addition to the genes that allow them to move, they also contain other genes, such as drug resistance genes. Transposons are found in the genomes of plasmids, viruses, prokaryotes and eukaryotes and, like IS elements, they are designated by a serial number: Tn1, Tn2, Tn3, etc.

# Episomes include even larger and more complex self-regulating systems containing IS elements and transposons and capable of replicating in any of their two alternative states - autonomous or integrated - into the chromosome of the host cell. Episomes include various temperate lysogenic phages; they differ from all other transposable elements by the presence of their own protein shell and a more complex reproduction cycle. Episomes themselves are viruses that, like other transposable elements, have the ability to move from one genome to another in an intact form.

Typically, bacterial cell division is described as “binary”: after duplication, the nucleoids associated with the plasma membrane separate due to stretching of the membrane between the nucleoids, and then a constriction or septum is formed, dividing the cell in two. This type of division results in a very precise distribution of genetic material, with virtually no errors (less than 0.03% of defective cells). Let us recall that the nuclear apparatus of bacteria, the nucleoid, is a cyclic giant (1.6 mm) DNA molecule that forms numerous loop domains in a state of supercoiling; the order of folding of the loop domains is unknown.

The average time between bacterial cell divisions is 20-30 minutes. And during this period, a whole series of events must occur: replication of the nucleoid DNA, segregation, separation of sister nucleoids, their further divergence, cytotomy due to the formation of a septum dividing the original cell exactly in half.

The entire range of these processes has received intense attention from researchers in recent years, resulting in important and unexpected observations. It turned out that at the beginning of DNA synthesis, which begins at the point of replication (origin), both growing DNA molecules initially remain associated with the plasma membrane. Simultaneously with DNA synthesis, the process of removing supercoiling of both old and replicating loop domains occurs due to a number of enzymes (topoisomerase, gyrase, ligase, etc.), which leads to the physical separation of two daughter (or sister) chromosome-nucleoids that are still in close contact with each other. After such segregation of nucleoids, they diverge from the center of the cell, from the place of their former location. Moreover, this discrepancy is very precise: a quarter of the length of the cell in two opposite directions. As a result, two new nucleoids are located in the cell. What is the mechanism for this discrepancy? Assumptions were made (Delamater, 1953) that the division of bacterial cells is similar to the mitosis of eukaryotes, but data in favor of this assumption did not appear for a long time.

New information about the mechanisms of bacterial cell division was obtained by studying mutants in which cell division was impaired.

It was discovered that several groups of special proteins take part in the process of nucleoid divergence. One of them, the Muk B protein, is a giant homodimer (mol. mass about 180 kDa, length 60 nm), consisting of a central helical section and terminal globular sections, reminiscent in structure of eukaryotic filamentous proteins (myosin II chain, kinesin). At the N-terminus, Muk B binds to GTP and ATP, and at the C-terminus to a DNA molecule. These properties of Muk B give grounds to consider it a motor protein involved in the divergence of nucleoids. Mutations of this protein lead to disturbances in nucleoid segregation: a large number of anucleate cells appear in the mutant population.

In addition to the Muk B protein, nucleoid divergence apparently involves bundles of fibrils containing the Caf A protein, which can bind to myosin heavy chains, like actin.

The formation of a constriction, or septum, also generally resembles the cytotomy of animal cells. In this case, proteins of the Fts family (fibrillar thermosensitive) take part in the formation of septa. This is a group of several proteins, among which the FtsZ protein is the most studied. This protein is similar in most bacteria, archbacteria, and is found in mycoplasmas and chloroplasts. It is a globular protein similar in its amino acid sequence to tubulin. When interacting with GTP in vitro, it is capable of forming long filamentous protofilaments. In interphase, FtsZ is diffusely localized in the cytoplasm, its quantity is very large (5-20 thousand monomers per cell). During cell division, all this protein is localized in the septal zone, forming a contractile ring, very reminiscent of the acto-myosin ring during cell division of animal origin.

22. Bacterial nucleus. Types of bacterial cell division. Division process.

Mutations in this protein lead to the cessation of cell division: long cells containing many nucleoids appear. These observations show a direct dependence of bacterial cell division on the presence of Fts proteins.

Regarding the mechanism of septa formation, there are several hypotheses that postulate contraction of the ring in the septal zone, leading to the division of the original cell in two. According to one of them, protofilaments should slide relative to each other with the help of still unknown motor proteins; according to the other, a reduction in the diameter of the septum can occur due to the depolymerization of FtsZ anchored on the plasma membrane.

Phases of bacterial culture propagation under stationary conditions

The last phase of growth is the stationary phase, which is caused by nutrient depletion. Cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress state, which is characterized by an increase in the expression of genes that are involved in DNA repair and antioxidant metabolism.

When bacteria multiply not in flow, but in stationary conditions, the nutrient medium changes and the waste products of bacteria accumulate in it, as a result of which their physiological characteristics also change. Thus, young cells of Clostridium acetobutylicum are not able to form acetone; they acquire this property in an older culture. If spore-bearing bacteria are grown under flow culture conditions, they will divide but will not produce spores. When bacteria are grown on solid nutrient media, they form clusters of cells of different sizes, shapes, and colors, called colonies.

Bacteria strain, pure culture
Features of sexual reproduction
Types of asexual reproduction, terms
Gloxinia, types, diseases
The effect of hormones on the body
Stages of glycolysis
Pentose monophosphate pathway of glucose oxidation
Krebs cycle

Bacterial division

Bacterial division occurs as a result of the formation of an intercellular septum, which occurs as follows. In the region of the CM to which a DNA molecule (chromosome, plasmid) is associated with a special receptor, events occur that initiate the replication process, as a result of which the newly formed daughter DNA molecule is also attached to the receptor on the CM.

The region of the latter between two receptors, to one of which the parent DNA is attached, and to the other - the daughter DNA, begins to lengthen, as a result of which the distance between them constantly increases over time. Upon completion of the replication process, strictly along the equator, an intercellular partition begins to form between the separated chromosomes through counter-invagination (growing towards each other) of the CM and the associated cell wall region.

As a result of the fusion of the invaginating sections of the CM and CS, an intercellular septum is formed, and the parent cell is divided into two daughter cells of equal length. The function of the mitosis apparatus in bacteria is performed by the CM through its elongation, which pushes the chromosomes (and plasmids) apart in such a way that they end up in the same position. the other side of the developing intercellular septum in equal proportions.

There can be at least two results from a violation of the genetic control of cell division. If the formation of the intercellular septum does not occur, long filamentous forms appear. However, when the damaged mechanism of such control is restored, the threads are divided into fragments equal in length to normal cells. In some cases, a violation of control mechanisms leads to the fact that instead of one intercellular septum forming along the equator, one or two septa are formed, each of which is localized closer to its pole.

Since in this case the formation of the septum is not associated with chromosome segregation, so-called mini-cells are formed, devoid of chromosomes, which remain in the parent cell. Mini-cells can carry out various biochemical processes because they contain enzymes, but they are not capable of reproduction because they lack chromosomes.

In addition to mini-cells, due to various adverse effects, so-called nano-cells, i.e., tiny cells measuring 0.2-0.3 microns in size, can be formed from bacteria. They were described under various names: filterable forms of bacteria, elementary bodies, ultramicrobacteria.

Bacterial cell division

Most often they are formed during L-transformation of bacteria.

Since it is more convenient to express the sizes of such cells in nanometers rather than in fractions of a micrometer, they began to be called nanocells. The formation of nanocells is a universal response of bacteria to unfavorable living conditions.

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1. What methods of division are characteristic of eukaryotic cells? For prokaryotic cells?

Mitosis, amitosis, simple binary fission, meiosis.

Eukaryotic cells are characterized by the following division methods: mitosis, amitosis, meiosis.

Prokaryotic cells are characterized by simple binary fission.

2. What is simple binary fission?

Simple binary fission is characteristic only of prokaryotic cells. Bacterial cells contain one chromosome, a circular DNA molecule. Before cell division, replication occurs and two identical DNA molecules are formed, each of them attached to the cytoplasmic membrane. During division, the plasmalemma grows between two DNA molecules in such a way that it ultimately divides the cell in two. Each resulting cell contains one identical DNA molecule.

3. What is mitosis? Describe the phases of mitosis.

Mitosis is the main method of division of eukaryotic cells, as a result of which two daughter cells with the same set of chromosomes are formed from one mother cell. For convenience, mitosis is divided into four phases:

● Prophase. In the cell, the volume of the nucleus increases, chromatin begins to spiral, resulting in the formation of chromosomes. Each chromosome consists of two sister chromatids connected at the centromere (in a diploid cell - set 2n4c). The nucleoli dissolve and the nuclear membrane disintegrates. Chromosomes end up in the hyaloplasm and are arranged randomly (chaotically) in it. Centrioles diverge in pairs to the cell poles, where they initiate the formation of spindle microtubules. Some of the spindle threads go from pole to pole, other threads are attached to the centromeres of chromosomes and contribute to their movement to the equatorial plane of the cell. Most plant cells lack centrioles. In this case, the centers for the formation of spindle microtubules are special structures consisting of small vacuoles.

● Metaphase. The formation of the fission spindle is completed. Chromosomes reach maximum spiralization and are arranged in an orderly manner in the equatorial plane of the cell. A so-called metaphase plate is formed, consisting of two-chromatid chromosomes.

● Anaphase. The spindle strands shorten, causing the sister chromatids of each chromosome to separate from each other and stretch toward opposite poles of the cell. From this moment on, the separated chromatids are called daughter chromosomes. The cell poles have the same genetic material (each pole has 2n2c).

● Telophase. Daughter chromosomes despiral (unwind) at the cell poles to form chromatin. Nuclear shells form around the nuclear material of each pole. Nucleoli appear in the two formed nuclei. The spindle filaments are destroyed. At this point, nuclear division ends and the cell begins to divide into two. In animal cells, a ring constriction appears in the equatorial plane, which deepens until the separation of two daughter cells occurs. Plant cells cannot divide by constriction, because have a rigid cell wall. In the equatorial plane of the plant cell, the so-called median plate is formed from the contents of the vesicles of the Golgi complex, which separates the two daughter cells.

4. How do daughter cells receive identical hereditary information as a result of mitosis? What is the biological significance of mitosis?

In metaphase, bichromatid chromosomes are located in the equatorial plane of the cell. The DNA molecules in sister chromatids are identical to each other, because formed as a result of replication of the original maternal DNA molecule (this occurred in the S-period of interphase preceding mitosis).

In anaphase, with the help of spindle threads, the sister chromatids of each chromosome are separated from each other and stretched to opposite poles of the cell. Thus, the two poles of the cell have the same genetic material (2n2c at each pole), which, upon completion of mitosis, becomes the genetic material of the two daughter cells.

The biological significance of mitosis is that it ensures the transmission of hereditary characteristics and properties over a series of cell generations. This is necessary for the normal development of a multicellular organism. Due to the precise and uniform distribution of chromosomes during mitosis, all cells in the body are genetically identical. Mitosis determines the growth and development of organisms, restoration of damaged tissues and organs (regeneration). Mitotic cell division underlies asexual reproduction in many organisms.

5. Number of chromosomes - n, chromatids - c. What will be the ratio of n and c for human somatic cells in the following periods of interphase and mitosis. Match:

1) In the G1 period, each chromosome consists of one chromatid, i.e. somatic cells contain a set of 2n2c, which for humans is 46 chromosomes, 46 chromatids.

2) In the G2 period, each chromosome consists of two chromatids, i.e. somatic cells contain a set of 2n4c (46 chromosomes, 92 chromatids).

3) In prophase of mitosis, the set of chromosomes and chromatids is 2n4c, (46 chromosomes, 92 chromatids).

4) In the metaphase of mitosis, the set of chromosomes and chromatids is 2n4c (46 chromosomes, 92 chromatids).

5) At the end of anaphase of mitosis, due to the separation of sister chromatids from each other and their divergence to opposite poles of the cell, each pole has a set of 2n2c (46 chromosomes, 46 chromatids).

6) At the end of the telophase of mitosis, two daughter cells are formed, each containing a set of 2n2c (46 chromosomes, 46 chromatids).

Answer: 1 - B, 2 - G, 3 - G, 4 - G, 5 - V, 6 - V.

6. How does amitosis differ from mitosis?

Prokaryotic cell division

Why do you think amitosis is called direct cell division, and mitosis is called indirect?

In contrast to mitosis, amitosis:

● The nucleus divides by constriction without chromatin spiralization and spindle formation; all four phases characteristic of mitosis are absent.

● Hereditary material is distributed unevenly and randomly among daughter nuclei.

● Often only nuclear division is observed without further division of the cell into two daughter cells. In this case, binucleate and even multinucleate cells appear.

● Less energy is wasted.

Mitosis is called indirect division, because. Compared to amitosis, it is a rather complex and precise process, consisting of four phases and requiring preliminary preparation (replication, doubling of centrioles, energy storage, synthesis of special proteins, etc.). During direct (i.e. simple, primitive) division - amitosis, the cell nucleus, without any special preparation, is quickly divided by a constriction, and the hereditary material is randomly distributed between the daughter nuclei.

7. In the nucleus of a non-dividing cell, hereditary material (DNA) is in the form of an amorphous dispersed substance - chromatin. Before division, chromatin spirals and forms compact structures - chromosomes, and after division it returns to its original state. Why do cells make such complex modifications of their hereditary material?

DNA in the composition of amorphous and dispersed chromatin during division would be impossible to accurately and evenly distribute between daughter cells (this is exactly the picture that is observed during amitosis - the hereditary material is distributed unevenly, randomly).

On the other hand, if cellular DNA were always in a compacted state (i.e., as part of spiralized chromosomes), it would be impossible to read all the necessary information from it.

Therefore, at the beginning of division, the cell transfers DNA to the most compact state, and after division is completed, it returns it to its original state, convenient for reading.

8*. It has been established that in diurnal animals the maximum mitotic activity of cells is observed in the evening, and the minimum - during the day. In animals that are nocturnal, cells divide most intensively in the morning, while mitotic activity is weakened at night. What do you think is the reason for this?

Diurnal animals are active during daylight hours. During the day, they spend a lot of energy moving and searching for food, while their cells “wear out” faster and die more often. In the evening, when the body has digested food, absorbed nutrients and accumulated a sufficient amount of energy, regeneration processes and, above all, mitosis are activated. Accordingly, in nocturnal animals the maximum mitotic activity of cells is observed in the morning, when their body is resting after an active night period.

*Tasks marked with an asterisk require students to put forward various hypotheses. Therefore, when marking, the teacher should focus not only on the answer given here, but take into account each hypothesis, assessing the biological thinking of students, the logic of their reasoning, the originality of ideas, etc. After this, it is advisable to familiarize students with the answer given.

Dashkov M.L.