Mitochondria.

Mitochondria- an organelle consisting of two membranes with a thickness of about 0.5 microns.

Energy station of the cell; the main function is the oxidation of organic compounds and the use of energy released during their breakdown in the synthesis of ATP molecules (a universal source of energy for all biochemical processes).

In their structure, they are cylindrical organelles, found in a eukaryotic cell in quantities from several hundred to 1-2 thousand and occupying 10-20% of its internal volume. The size (from 1 to 70 microns) and shape of mitochondria also vary greatly. Moreover, the width of these parts of the cell is relatively constant (0.5-1 µm). Capable of changing shape. Depending on which areas of the cell at any given moment there is increased energy consumption, mitochondria are able to move through the cytoplasm to areas of greatest energy consumption, using the structures of the cellular framework of the eukaryotic cell for movement.

Beautiful mitochondria in 3D representation)

An alternative to many scattered small mitochondria functioning independently of each other and supplying ATP to small areas of the cytoplasm is the existence of long and branched mitochondria, each of which can provide energy to distant areas of the cell. A variant of such an extended system can also be an ordered spatial association of many mitochondria (chondriomes or mitochondria), ensuring their cooperative work.

This type of chondrioma is especially complex in muscles, where groups of giant branched mitochondria are connected to each other using intermitochondrial contacts (MMK). The latter are formed by outer mitochondrial membranes tightly adjacent to each other, as a result of which the intermembrane space in this zone has an increased electron density (many negatively charged particles). MMC are especially abundant in cardiac muscle cells, where they link multiple individual mitochondria into a coordinated working cooperative system.

Structure.

Outer membrane.

The outer membrane of the mitochondria is about 7 nm thick, does not form invaginations or folds, and is closed on itself. The outer membrane accounts for about 7% of the surface area of ​​all membranes of cellular organelles. The main function is to separate mitochondria from the cytoplasm. The outer membrane of the mitochondrion consists of a double fatty layer (like a cell membrane) and proteins that penetrate it. Proteins and fats in equal proportions by weight.
Plays a special role porin - channel-forming protein.
It forms holes in the outer membrane with a diameter of 2-3 nm, through which small molecules and ions can penetrate. Large molecules can only cross the outer membrane through active transport through mitochondrial membrane transport proteins. The outer membrane of the mitochondrion can interact with the membrane of the endoplasmic reticulum; it plays an important role in the transport of lipids and calcium ions.

Inner membrane.

The inner membrane forms numerous comb-like folds - crista,
significantly increasing its surface area and, for example, in liver cells constitutes about a third of all cell membranes. a characteristic feature of the composition of the inner membrane of mitochondria is the presence in it cardiolopina - a special complex fat that contains four fatty acids at once and makes the membrane absolutely impermeable to protons (positively charged particles).

Another feature of the inner mitochondrial membrane is a very high protein content (up to 70% by weight), represented by transport proteins, respiratory chain enzymes, as well as large enzyme complexes that produce ATP. The inner membrane of the mitochondrion, unlike the outer one, does not have special openings for the transport of small molecules and ions; on it, on the side facing the matrix, there are special ATP-producing enzyme molecules, consisting of a head, a stalk and a base. When protons pass through them, atf is created.
At the base of the particles, filling the entire thickness of the membrane, are the components of the respiratory chain. The outer and inner membranes touch in some places; there is a special receptor protein that promotes the transport of mitochondrial proteins encoded in the nucleus into the mitochondrial matrix.

Matrix.

Matrix- space limited by the internal membrane. The matrix (pink substance) of the mitochondria contains enzyme systems for the oxidation of fatty acid pyruvate, as well as enzymes such as tricarboxylic acids (cell respiration cycle). In addition, mitochondrial DNA, RNA, and the mitochondria’s own protein-synthesizing apparatus are also located here.

pyruvates (salts of pyruvic acid)- important chemical compounds in biochemistry. They are the end product of glucose metabolism during its breakdown.

Mitochondrial DNA.

Several differences from nuclear DNA:

- Mitochondrial DNA is circular, unlike nuclear DNA, which is packaged into chromosomes.

- between different evolutionary variants of mitochondrial DNA of the same species, the exchange of similar sections is impossible.

And so the entire molecule changes only through slow mutation over thousands of years.

- Code mutations in mitochondrial DNA can occur independently of nuclear DNA.

Mutation of the nuclear DNA code occurs mainly during cell division, but mitochondria divide independently of the cell, and can receive a mutation of the code separately from the nuclear DNA.

- The structure of mitochondrial DNA itself is simplified, because many of the component DNA reading processes have been lost.

- transport RNAs have the same structure. but mitochondrial RNAs are involved only in the synthesis of mitochondrial proteins.

Having its own genetic apparatus, the mitochondrion also has its own protein synthesizing system, a feature of which in animal and fungal cells are very small ribosomes.

Functions.

Energy generation.

The main function of mitochondria is the synthesis of ATP, a universal form of chemical energy in any living cell.

This molecule can be formed in two ways:

- through a reaction in which the energy released at certain oxidative stages of fermentation is stored in the form of ATP.

- thanks to the energy released during the oxidation of organic substances in the process of cellular respiration.

Mitochondria implement both of these pathways, the first of which is characteristic of the initial processes of oxidation and occurs in the matrix, and the second completes the processes of energy generation and is associated with the cristae of mitochondria.
At the same time, the uniqueness of mitochondria as energy-producing organelles of a eukaryotic cell determines precisely the second pathway of ATP generation, called “chemiosmotic coupling.”
In general, the entire process of energy production in mitochondria can be divided into four main stages, the first two of which occur in the matrix, and the last two on the mitochondrial cristae:

1) Conversion of pyruvate (the final product of the breakdown of glucose) and fatty acids received from the cytoplasm into the mitochondria into acetyl cola;

acetyl coa– an important compound in metabolism, used in many biochemical reactions. its main function is to deliver carbon atoms (c) with an acetyl group (ch3 co) into the cellular respiration cycle so that they are oxidized to release energy.

cellular respiration - a set of biochemical reactions occurring in the cells of living organisms, during which the oxidation of carbohydrates, fats and amino acids to carbon dioxide and water occurs.

2) Oxidation of acetyl-coa in the cellular respiration cycle, leading to the formation of nadn;

NADH– coenzyme acts as a carrier of electrons and hydrogen, which it receives from oxidized substances.

3) Transfer of electrons from nadn to oxygen through the respiratory chain;

4) Formation of ATP as a result of the activity of the membrane ATP-creating complex.

ATP synthetase.

ATP synthetase– station for the production of ATP molecules.

In structural and functional terms, ATP synthetase consists of two large fragments, designated by the symbols F1 and F0. The first of them (coupling factor F1) faces the mitochondrial matrix and protrudes noticeably from the membrane in the form of a spherical formation 8 nm high and 10 nm wide. It consists of nine subunits represented by five types of proteins. The polypeptide chains of three α subunits and the same number of β subunits are arranged in protein globules of similar structure, which together form a hexamer (αβ)3, which looks like a slightly flattened ball.

Subunit- is a structural and functional component of any particle
Polypeptides- organic compounds containing from 6 to 80-90 amino acid residues.
Globule– a state of macromolecules in which the vibration of the units is small.
Hexamer– a compound containing 6 subunits.

Like tightly packed orange slices, the successive α and β subunits form a structure characterized by symmetry around a 120° rotation angle. At the center of this hexamer is the γ subunit, which is formed by two extended polypeptide chains and resembles a slightly deformed curved rod about 9 nm long. In this case, the lower part of the γ subunit protrudes from the ball by 3 nm towards the membrane complex F0. Also located within the hexamer is a minor ε subunit associated with γ. The last (ninth) subunit is designated δ and is located on the outer side of F1.

Minor– single subunit.

The membrane part of ATP synthetase is a water-repellent protein complex that penetrates the membrane through and has two half-channels inside for the passage of hydrogen protons. In total, the F0 complex includes one protein subunit of the type A, two copies of the subunit b, as well as 9 to 12 copies of the small subunit c. Subunit A(molecular weight 20 kDa) is completely immersed in the membrane, where it forms six α-helical sections crossing it. Subunit b(molecular weight 30 kDa) contains only one relatively short α-helical region immersed in the membrane, and the rest of it protrudes noticeably from the membrane towards F1 and is attached to the δ subunit located on its surface. Each of 9-12 copies of a subunit c(molecular weight 6-11 kDa) is a relatively small protein of two water-repellent α-helices connected to each other by a short water-attracting loop oriented towards F1, and together they form a single ensemble having the shape of a cylinder immersed in the membrane . The γ subunit protruding from the F1 complex towards F0 is precisely immersed inside this cylinder and is quite firmly attached to it.
Thus, in the ATPase molecule, two groups of protein subunits can be distinguished, which can be likened to two parts of a motor: the rotor and the stator.

"Stator" is motionless relative to the membrane and includes a spherical hexamer (αβ)3 located on its surface and the δ subunit, as well as subunits a And b membrane complex F0.

Movable relative to this design "rotor" consists of subunits γ and ε, which, prominently protruding from the (αβ)3 complex, connect to a ring of subunits immersed in the membrane c.

The ability to synthesize ATP is a property of a single complex F0F1, combined with the transfer of hydrogen protons through F0 to F1, in the latter of which the reaction centers that convert ADP and phosphate into an ATP molecule are located. The driving force for the operation of ATP synthetase is the proton (positively charged) potential created on the inner mitochondrial membrane as a result of the operation of the electron (negatively charged) transport chain.
The force driving the “rotor” of ATP synthetase occurs when the potential difference between the outer and inner sides of the membrane reaches > 220 10−3 Volts and is provided by the flow of protons flowing through a special channel in F0, located at the boundary between subunits a And c. In this case, the proton transfer pathway includes the following structural elements:

1) Two “half-channels” located on different axes, the first of which ensures the supply of protons from the intermembrane space to the essential functional groups F0, and the other ensures their release into the mitochondrial matrix;

2) Ring of subunits c, each of which in its central part contains a protonated carboxyl group (COOH), capable of attaching H+ from the intermembrane space and releasing them through the corresponding proton channels. As a result of periodic displacements of subunits With, caused by the flow of protons through the proton channel, the γ subunit rotates, immersed in a ring of subunits With.

Thus, the unifying activity of ATP synthetase is directly related to the rotation of its “rotor”, in which the rotation of the γ subunit causes a simultaneous change in the conformation of all three unifying β subunits, which ultimately ensures the functioning of the enzyme. In this case, in the case of ATP formation, the “rotor” rotates clockwise at a speed of four revolutions per second, and such rotation itself occurs in precise jumps of 120°, each of which is accompanied by the formation of one ATP molecule.
The work of ATP synthetase is associated with the mechanical movements of its individual parts, which makes it possible to classify this process as a special type of phenomenon called “rotational catalysis.” Just as the electric current in the winding of an electric motor drives the rotor relative to the stator, the directed transfer of protons through ATP synthetase causes the rotation of individual subunits of the conjugation factor F1 relative to other subunits of the enzyme complex, as a result of which this unique energy-producing device performs chemical work - synthesizes ATP molecules . Subsequently, ATP enters the cell cytoplasm, where it is spent on a wide variety of energy-dependent processes. This transfer is carried out by a special enzyme, ATP/ADP translocase, built into the mitochondrial membrane.

ADP translocase- a protein that penetrates the inner membrane, which exchanges newly synthesized ATP for cytoplasmic ADP, which guarantees the safety of the fund inside the mitochondria.

Mitochondria and heredity.

Mitochondrial DNA is inherited almost exclusively through the maternal line. Each mitochondria has several sections of nucleotides in DNA that are identical in all mitochondria (that is, there are many copies of mitochondrial DNA in the cell), which is very important for mitochondria that are unable to repair DNA from damage (a high frequency of mutations is observed). Mutations in mitochondrial DNA are the cause of a number of hereditary human diseases.

3d model

Discovery

With English voice acting

A little about cell respiration and mitochondria in a foreign language

Building structure

Mitochondria of a plant cell. Their structure and functions

Form− round or dumbbell-shaped bodies.

Dimensions− length 1-5 microns, diameter 0.4-0.5 microns.

Quantity per cage− from tens to 5,000.

Structure. They consist mainly of protein (60-65%) and lipids (30%). These are double-membrane organelles. The thickness of the outer and inner membranes is 5-6 nm each. The perimitochondrial space (the space between the membranes) is filled with a fluid such as serum. The inner membrane forms folds of various shapes − cristas. On the inner surface of the inner membrane there are mushroom-shaped particles - oxisomes containing oxidative enzymes. Internal contents of mitochondria − matrix. The matrix contains ribosomes and mitochondrial DNA (0.5%), which has a ring structure and is responsible for the synthesis of mitochondrial proteins. Mitochondria have all types of RNA (1%), divide independently of nuclear division, and in the cell are formed from preexisting mitochondria by fission or budding. The half-life of mitochondria is 5-10 days.

Functions. Mitochondria are the centers of energy activity of cells. Aerobic respiration and oxidative phosphorelation systems function in mitochondria. The components of the electron transport chain and ATP synthetase complexes, which carry out the transport of electrons and protons and the synthesis of ATP, are localized in the inner membrane of mitochondria. The matrix contains systems for the oxidation of di- and tricarboxylic acids, a number of systems for the synthesis of lipids, amino acids, etc.

Mitochondria are able to move to places of increased energy consumption. They can associate with each other by close proximity or with the help of cords. During anaerobic respiration, mitochondria disappear.

Mitochondria have a round and oblong shape with a diameter of 0.4–0.5 μm and a length of 1–5 μm (Fig. 1.3).

The number of mitochondria varies from a few to 1,500–2,000 per plant cell.

Mitochondria are bounded by two membranes: outer and inner, the thickness of each of them is 5–6 nm. The outer membrane appears stretched, and the inner one forms folds called ridges (cristae) of various shapes. The space between the membranes, which also includes the internal space of the cristae, is called the intermembrane (perimitochondrial) space. It serves as a medium for the inner membrane and matrix of mitochondria.

Mitochondria generally contain 65–70% protein, 25–30% lipids, and small amounts of nucleic acids. Phospholipids (phosphatidylcholine and phosphatidylethanolamine) account for 70% of the total lipid content. The fatty acid composition is characterized by a high content of saturated fatty acids, which ensure the “rigidity” of the membrane.

The systems of aerobic respiration and oxidative phosphorylation are localized in mitochondria. As a result of respiration, organic molecules are broken down and energy is released and transferred to the ATP molecule.

Mitochondria contain proteins, RNA, DNA strands, ribosomes similar to bacterial ones, and various solutes. DNA exists in the form of circular molecules located in one or more nucleotides.

plastids, along with vacuoles and the cell membrane, they are characteristic components of plant cells. Each plastid is surrounded by its own shell, consisting of two elementary membranes. Inside plastids, a membrane system and a more or less homogeneous substance, the stroma, are distinguished. The internal structure of the chloroplast is quite complex. The stroma is permeated by a developed system of membranes in the form of flat vesicles called thylakoids. Thylakoids are collected in stacks - grana, resembling columns of coins.

Chloroplasts, in which photosynthesis occurs, contain chlorophylls and carotenoids. Size – 4–5 microns. One leaf mesophyll cell can contain 40–50 chloroplasts, and about 500,000 per 2 mm of leaf. In the cytoplasm, chloroplasts are usually located parallel to the cell wall.

Chlorophylls and carotenoids are embedded in thylakoid membranes. The chloroplasts of green plants and algae often contain starch grains and small lipid (fat) droplets. Starch grains are temporary storage facilities for photosynthesis products. They can disappear from chloroplasts kept in the dark for only 24 hours and reappear within 3–4 hours after the plants are transferred to the light.

In isolated chloroplasts, RNA synthesis occurs, which is usually controlled only by chromosomal DNA. The formation of chloroplasts and the synthesis of the pigments contained in them are largely controlled by chromosomal DNA, which interacts with the DNA of chloroplasts in a poorly understood way. However, in the absence of their own DNA, chloroplasts do not form.

Chloroplasts can be considered the main cellular organelles, since they are the first in the chain of conversion of solar energy, as a result of which we obtain food and fuel. Not only photosynthesis occurs in chloroplasts. They participate in the synthesis of amino acids and fatty acids and serve as a storage facility for temporary starch reserves.

Chromoplasts(from the Greek chroma - color) - pigmented plastids. Chromoplasts, varied in shape, do not contain chlorophyll, but synthesize and accumulate carotenoids, which give yellow, orange and other colors. Carrot roots and tomato fruits are colored by pigments that are found in chromoplasts.

Leukoplasts are a place of accumulation of a reserve substance - starch. There are especially many leukoplasts in the cells of potato tubers. In the light, leucoplasts can transform into chloroplasts (potato tubers turn green). In autumn, the chloroplasts transform into chromoplasts and green leaves, and the fruits turn yellow and red.

The structure and function of mitochondria is a rather complex issue. The presence of an organelle is characteristic of almost all nuclear organisms - both autotrophs (plants capable of photosynthesis) and heterotrophs, which are almost all animals, some plants and fungi.

The main purpose of mitochondria is the oxidation of organic substances and the subsequent use of the energy released as a result of this process. For this reason, organelles also have a second (unofficial) name - the energy stations of the cell. They are sometimes called "catabolism plastids".

What are mitochondria

The term is of Greek origin. Translated, this word means “thread” (mitos), “grain” (chondrion). Mitochondria are permanent organelles that are of great importance for the normal functioning of cells and make the existence of the entire organism possible.

“Stations” have a specific internal structure, which changes depending on the functional state of the mitochondria. Their shape can be of two types - oval or oblong. The latter often has a branching appearance. The number of organelles in one cell ranges from 150 to 1500.

A special case is germ cells. Sperm contain only one spiral organelle, while female gametes contain hundreds of thousands more mitochondria. In a cell, organelles are not fixed in one place, but can move throughout the cytoplasm and combine with each other. Their size is 0.5 microns, their length can reach 60 microns, while the minimum is 7 microns.

Determining the size of one “energy station” is not an easy task. The fact is that when examined under an electron microscope, only part of the organelle gets into the section. It happens that a spiral mitochondrion has several sections that can be mistaken for separate, independent structures.

Only a three-dimensional image will make it possible to find out the exact cellular structure and understand whether we are talking about 2-5 separate organelles or one mitochondria with a complex shape.

Structural features

The mitochondrial shell consists of two layers: outer and inner. The latter includes various outgrowths and folds, which have a leaf-like and tubular shape.

Each membrane has a special chemical composition, a certain amount of certain enzymes and a specific purpose. The outer shell is separated from the inner shell by an intermembrane space 10-20 nm thick.

The structure of the organelle looks very clearly in the figure with captions.

Mitochondria structure diagram

Looking at the structure diagram, we can make the following description. The viscous space inside the mitochondrion is called the matrix. Its composition creates a favorable environment for the necessary chemical processes to occur in it. It contains microscopic granules that promote reactions and biochemical processes (for example, they accumulate glycogen ions and other substances).

The matrix contains DNA, coenzymes, ribosomes, t-RNA, and inorganic ions. ATP synthase and cytochromes are located on the surface of the inner layer of the shell. Enzymes contribute to processes such as the Krebs cycle (TCA cycle), oxidative phosphorylation, etc.

Thus, the main task of the organelle is performed by both the matrix and the inner side of the shell.

Functions of mitochondria

The purpose of “energy stations” can be characterized by two main tasks:

• energy production: oxidative processes are carried out in them with the subsequent release of ATP molecules;
• storage of genetic information;
• participation in the synthesis of hormones, amino acids and other structures.

The process of oxidation and energy production takes place in several stages:

Schematic drawing of ATP synthesis

It is worth noting: As a result of the Krebs cycle (citric acid cycle), ATP molecules are not formed, the molecules are oxidized and carbon dioxide is released. This is an intermediate step between glycolysis and the electron transport chain.

Table “Functions and structure of mitochondria”

What determines the number of mitochondria in a cell?

The prevailing number of organelles accumulates near those areas of the cell where the need for energy resources arises. In particular, a large number of organelles gather in the area where myofibrils are located, which are part of the muscle cells that ensure their contraction.

In male germ cells, the structures are localized around the axis of the flagellum - it is assumed that the need for ATP is due to the constant movement of the gamete tail. The arrangement of mitochondria in protozoa, which use special cilia for movement, looks exactly the same - the organelles accumulate under the membrane at their base.

As for nerve cells, the localization of mitochondria is observed near the synapses through which signals from the nervous system are transmitted. In cells that synthesize proteins, organelles accumulate in zones of ergastoplasm - they supply the energy that powers this process.

Who discovered mitochondria

The cellular structure acquired its name in 1897-1898 thanks to K. Brand. Otto Wagburg was able to prove the connection between the processes of cellular respiration and mitochondria in 1920.

Conclusion

Mitochondria are the most important component of a living cell, acting as an energy station that produces ATP molecules, thereby ensuring cellular vital processes.

The work of mitochondria is based on the oxidation of organic compounds, resulting in the generation of energy potential.

Mitochondria is a spiral, round, elongated or branched organelle.

The concept of mitochondria was first proposed by Benda in 1897. Mitochondria can be detected in living cells using phase contrast and interference microscopy in the form of grains, granules or filaments. These are quite mobile structures that can move, merge with each other, and divide. When stained using special methods in dead cells under light microscopy, mitochondria have the appearance of small grains (granules), diffusely distributed in the cytoplasm or concentrated in some specific zones of it.

As a result of the destruction of glucose and fats in the presence of oxygen, energy is generated in the mitochondria, and organic substances are converted into water and carbon dioxide. This is how animal organisms obtain the basic energy necessary for life. Energy is stored in adenosine triphosphate (ATP), or more precisely, in its high-energy bonds. The function of mitochondria is closely related to the oxidation of organic compounds and the use of energy released during their breakdown for the synthesis of ATP molecules. Therefore, mitochondria are often called the energy stations of the cell, or the organelles of cellular respiration. ATP functions as an energy supplier by transferring one of its energy-rich terminal phosphate groups to another molecule and converting it into ADP.

It is believed that in evolution, mitochondria were prokaryotic microorganisms that became symbiotes in the body of an ancient cell. Subsequently, they became vitally necessary, which was associated with an increase in the oxygen content in the Earth’s atmosphere. On the one hand, mitochondria removed excess oxygen, which is toxic to the cell, and on the other, they provided energy.

Without mitochondria, a cell is virtually unable to use oxygen as a substance to supply energy and can only meet its energy needs through anaerobic processes. Thus, oxygen is poison, but the poison is vital for the cell, and excess oxygen is just as harmful as its deficiency.

Mitochondria can change their shape and move to those areas of the cell where the need for them is greatest. Thus, in cardiomyocytes, mitochondria are located near the myofibrils, in the cells of the renal tubules near the basal invaginations, etc. The cell contains up to a thousand mitochondria, and their number depends on the activity of the cell.

Mitochondria have an average transverse size of 0.5...3 µm. Depending on the size, small, medium, large and giant mitochondria are distinguished (they form a branched network - the mitochondrial reticulum). The size and number of mitochondria are closely related to cell activity and energy consumption. They are extremely variable and, depending on the activity of the cell, oxygen content, hormonal influences, can swell, change the number and structure of cristae, vary in number, shape and size, as well as enzymatic activity.

The volume density of mitochondria, the degree of development of their internal surface and other indicators depend on the energy needs of the cell. Lymphocytes have only a few mitochondria, while liver cells have 2-3 thousand.

Mitochondria consist of a matrix, an inner membrane, a perimitochondrial space, and an outer membrane. The outer mitochondrial membrane separates the organelle from the hyaloplasm. Usually it has smooth contours and is closed so that it represents a membrane sac.

The outer membrane is separated from the inner membrane by a perimitochondrial space about 10...20 nm wide. The inner mitochondrial membrane limits the actual internal contents of the mitochondrion - the matrix. The inner membrane forms numerous protrusions into the mitochondria, which look like flat ridges, or cristae.

The shape of the cristae can look like plates (trabecular) and tubes (multivesicular on a section), and they are directed longitudinally or transversely in relation to the mitochondria.

Each mitochondria is filled with a matrix that appears denser in electron micrographs than the surrounding cytoplasm. The mitochondrial matrix is ​​uniform (homogeneous), sometimes fine-grained, with varying electron densities. It reveals thin threads with a thickness of about 2...3 nm and granules with a size of about 15...20 nm. The matrix threads are DNA molecules, and the small granules are mitochondrial ribosomes. The matrix contains enzymes, one single-stranded, cyclic DNA, mitochondrial ribosomes, and many Ca 2+ ions.

The autonomous system of mitochondrial protein synthesis is represented by DNA molecules free of histones. The DNA is short, ring-shaped (cyclic) and contains 37 genes. Unlike nuclear DNA, it contains virtually no non-coding nucleotide sequences. Features of structure and organization bring mitochondrial DNA closer to the DNA of bacterial cells. On mitochondrial DNA, the synthesis of RNA molecules of different types occurs: informational, transfer (transport) and ribosomal. The messenger RNA of mitochondria is not subject to splicing (cutting out areas that do not carry an information load). The small size of mitochondrial DNA molecules cannot determine the synthesis of all mitochondrial proteins. Most mitochondrial proteins are under the genetic control of the cell nucleus and are synthesized in the cytoplasm, since mitochondrial DNA is weakly expressed and can provide the formation of only part of the enzymes of the oxidative phosphorylation chain. Mitochondrial DNA encodes no more than ten proteins that are localized in membranes and are structural proteins responsible for the correct integration of individual functional protein complexes of mitochondrial membranes. Proteins that perform transport functions are also synthesized. Such a system of protein synthesis does not provide all the functions of the mitochondrion, therefore the autonomy of the mitochondria is limited and relative.

In mammals, mitochondria are transferred during fertilization only through the egg, and the sperm introduces nuclear DNA into the new organism.

Ribosomes are formed in the mitochondrial matrix, which differ from the ribosomes of the cytoplasm. They are involved in the synthesis of a number of mitochondrial proteins that are not encoded by the nucleus. Mitochondrial ribosomes have a sedimentation number of 60 (in contrast to cytoplasmic ribosomes with a sedimentation number of 80). The sedimentation number is the rate of sedimentation during centrifugation and ultracentrifugation. In structure, mitochondrial ribosomes are close to the ribosomes of prokaryotic organisms, but are smaller in size and are sensitive to certain antibiotics (chloramphenicol, tetracycline, etc.).

The inner membrane of the mitochondrion has a high degree of selectivity in the transport of substances. Closely adjacent enzymes of the oxidative phosphorylation chain, electron carrier proteins, transport systems ATP, ADP, pyruvate, etc. are attached to its inner surface. As a result of the close arrangement of enzymes on the inner membrane, high conjugacy (interconnectedness) of biochemical processes is ensured, increasing the speed and efficiency of catalytic processes.

Electron microscopy reveals mushroom-shaped particles protruding into the lumen of the matrix. They have ATP-synthetic (forms ATP from ADP) activity. Electron transport occurs along the respiratory chain, localized in the inner membrane, which contains four large enzyme complexes (cytochromes). As electrons pass through the respiratory chain, hydrogen ions are pumped out of the matrix into the perimitochondrial space, which ensures the formation of a proton gradient (pump). The energy of this gradient (differences in the concentration of substances and the formation of membrane potential) is used for the synthesis of ATP and the transport of metabolites and inorganic ions. Carrier proteins contained on the inner membrane transport organic phosphates, ATP, ADP, amino acids, fatty acids, tri- and dicarboxylic acids through it.

The outer membrane of the mitochondria is more permeable to low molecular weight substances, since it contains many hydrophilic protein channels. On the outer membrane there are specific receptor complexes through which proteins from the matrix are transported into the perimitochondria space.

In its chemical composition and properties, the outer membrane is close to other intracellular membranes and the plasmalemma. It contains enzymes that metabolize fats, activate (catalyze) the transformation of amines, amine oxidase. If the enzymes of the outer membrane remain active, then this is an indicator of the functional safety of mitochondria.

Mitochondria have two autonomous subcompartments. While the permitochondrial space, or outer chamber of the mitochondrion (external subcompartment), is formed due to the penetration of protein complexes of the hyaloplasm, the internal subcompartment (mitochondrial matrix) is partially formed due to the synthetic activity of mitochondrial DNA. The internal subcompartment (matrix) contains DNA, RNA and ribosomes. It is characterized by a high level of Ca 2+ ions in comparison with hyaloplasm. Hydrogen ions accumulate in the outer subcompartment. The enzymatic activity of the external and internal subcompartments and the composition of proteins differ greatly. The inner subcompartment has a higher electron density than the outer one.

Specific markers of mitochondria are the enzymes cytochrome oxidase and succinate dehydrogenase, the identification of which makes it possible to quantitatively characterize energy processes in mitochondria.

Main function of mitochondria- ATP synthesis. First, sugars (glucose) are broken down in the hyaloplasm to lactic and pyruvic acids (pyruvate), with the simultaneous synthesis of a small amount of ATP. As a result of glycolysis of one glucose molecule, two ATP molecules are used and four are produced. Thus, the positive balance is made up of only two ATP molecules. These processes occur without oxygen (anaerobic glycolysis).

All subsequent stages of energy production occur through the process of aerobic oxidation, which ensures the synthesis of large amounts of ATP. In this case, organic substances are destroyed to CO 2 and water. Oxidation is accompanied by the transfer of protons to their acceptors. These reactions are carried out using a number of enzymes of the tricarboxylic acid cycle, which are located in the mitochondrial matrix.

Systems for electron transfer and associated ADP phosphorylation (oxidative phosphorylation) are built into the cristae membranes. In this case, electrons are transferred from one electron acceptor protein to another and, finally, they bind with oxygen, resulting in the formation of water. At the same time, part of the energy released during such oxidation in the electron transport chain is stored in the form of a high-energy bond during the phosphorylation of ADP, which leads to the formation of a large number of ATP molecules - the main intracellular energy equivalent. On the membranes of the mitochondrial cristae, the process of oxidative phosphorylation occurs with the help of the oxidation chain proteins and the phosphorylation enzyme ADP ATP synthetase located here. As a result of oxidative phosphorylation, 36 ATP molecules are formed from one glucose molecule.

For some hormones and substances, mitochondria have specialized (affinity) receptors. Triiodothyronine normally accelerates the synthetic activity of mitochondria. Interleukin-1 and high concentrations of triiodothyronine uncouple the chains of oxidative phosphorylation and cause mitochondrial swelling, which is accompanied by an increase in the production of thermal energy.

New mitochondria are formed by fission, constriction or budding. In the latter case, a protomitochondrion is formed, gradually increasing in size.

Protomitochondrion is a small organelle with outer and inner membranes. The inner membrane does not have or contains poorly developed cristae. The organelle is characterized by a low level of aerobic phosphorylation. When a constriction is formed, the contents of the mitochondrion are distributed between two new rather large organelles. With any method of reproduction, each of the newly formed mitochondria has its own genome.

Old mitochondria are destroyed by autolysis (self-digestion by the cell using lysosomes) to form autolysosomes. A residual body is formed from the autolysosome. Upon complete digestion, the contents of the residual body, consisting of low molecular weight organic substances, are excreted by exocytosis. If digestion is incomplete, mitochondrial remnants can accumulate in the cell in the form of layered bodies or granules with nipofuscin. In some mitochondria, insoluble calcium salts accumulate with the formation of crystals - calcifications. The accumulation of mitochondrial degeneration products can lead to cell degeneration.

Mitochondria

Mitochondria were discovered in animal cells in 1882, and in plants only in 1904 (in the anthers of water lilies). Biological functions were established after isolation and purification of the fraction by fractional centrifugation. They contain 70% protein and about 30% lipids, a small amount of RNA and DNA, vitamins A, B6, B12, K, E, folic and pantothenic acids, riboflavin, and various enzymes. Mitochondria have a double membrane, the outer one isolates the organelle from the cytoplasm, and the inner one forms cristae. The entire space between the membranes is filled with matrix (Fig. 13).

The main function of mitochondria is to participate in cellular respiration. The role of mitochondria in respiration was established in 1950-1951. The complex enzyme system of the Krebs cycle is concentrated on the outer membranes. When the substrates of respiration are oxidized, energy is released, which is immediately accumulated in the resulting molecules of ADP and mainly ATP during the process of oxidative phosphorylation occurring in the cristae. The energy stored in high-energy compounds is subsequently used to satisfy all the needs of the cell.

The formation of mitochondria in a cell occurs continuously from microbodies; more often, their occurrence is associated with the differentiation of membrane structures of the cell. They can be restored in the cell by dividing and budding. Mitochondria are not long-lived; their lifespan is 5-10 days.

Mitochondria are the “power” stations of the cell. They concentrate energy, which is stored in energy “accumulators” - ATP molecules, and is not dissipated in the cell. Violation of the mitochondrial structure leads to disruption of the respiration process and, ultimately, to pathology of the body.

Golgi apparatus.Golgi apparatus(synonym - dictyosomes) are stacks of 3-12 flattened, closed disks surrounded by a double membrane, called cisternae, from the edges of which numerous vesicles (300-500) are laced. The width of the tanks is 6-90 A, the thickness of the membranes is 60-70 A.

The Golgi apparatus is the center for the synthesis, accumulation and release of polysaccharides, in particular cellulose, and is involved in the distribution and intracellular transport of proteins, as well as in the formation of vacuoles and lysosomes. In plant cells, it was possible to trace the participation of the Golgi apparatus in the emergence of the middle plate and the growth of the cell pecto-cellulose membrane.

The Golgi apparatus is most developed during the period of active cell life. As she ages, it gradually atrophies and then disappears.

Lysosomes.Lysosomes- rather small (about 0.5 microns in diameter) rounded bodies. They are covered with a protein-lipoid membrane. Lysosomes contain numerous hydrolytic enzymes that perform the function of intracellular digestion (lysis) of protein macromolecules, nucleic acids, and polysaccharides. Their main function is the digestion of individual sections of the cell protoplast (autophagy - self-devouring). This process occurs through phagocytosis or pinocytosis. The biological role of this process is twofold. Firstly, it is protective, since during a temporary lack of reserve products, the cell maintains life due to constitutional proteins and other substances, and secondly, there is a release from excess or worn-out organelles (plastids, mitochondria, etc.) The lysosome membrane prevents the release of enzymes into the cytoplasm , otherwise it would all be digested by these enzymes.

In a dead cell, lysosomes are destroyed, enzymes end up in the cell and all its contents are digested. All that remains is the pecto-cellulose shell.

Lysosomes are products of the activity of the Golgi apparatus, vesicles detached from it, in which this organelle accumulated digestive enzymes.

Spherosomes- round protein-lipoid bodies 0.3-0.4 microns. In all likelihood, they are derivatives of the Golgi apparatus or endoplasmic reticulum. They resemble lysosomes in shape and size. Since spherosomes contain acid phosphatase, they are probably related to lysosomes. Some authors believe that spherosomes and lysosomes are equivalent to each other, but most likely only in origin and form. There is an assumption about their participation in the synthesis of fats (A. Frey-Wissling).

Ribosomes- very small organelles, their diameter is about 250A, they are almost spherical in shape. Some of them are attached to the outer membranes of the endoplasmic reticulum, some of them are in a free state in the cytoplasm. A cell can contain up to 5 million ribosomes. Ribosomes are found in chloroplasts and mitochondria, where they synthesize part of the proteins from which these organelles are built, and the enzymes that function in them.

The main function is the synthesis of specific proteins according to information coming from the nucleus. Their composition: protein and ribosomal ribonucleic acid (RNA) in equal proportions. Their structure is small and large subunits formed from ribonucleotide.

Microtubules.Microtubules- peculiar derivatives of the endoplasmic reticulum. Found in many cells. Their very name speaks of their shape - one or two parallel tubes with a cavity inside. External diameter within 250A. The walls of microtubules are made of protein molecules. Microtubules form spindle filaments during cell division.

Core

The nucleus was discovered in a plant cell by R. Brown in 1831. It is located in the center of the cell or near the cell membrane, but is surrounded on all sides by the cytoplasm. In most cases, there is one nucleus per cell; several nuclei are found in the cells of some algae and fungi. Green algae with a noncellular structure have hundreds of nuclei. Multinucleated cells of unarticulated laticifers. There are no nuclei in the cells of bacteria and blue-green algae.

The shape of the nucleus is most often close to the shape of a sphere or an ellipse. Depends on the shape, age and function of the cell. In a meristematic cell, the nucleus is large, round in shape and occupies 3/4 of the cell volume. In parenchymal cells of the epidermis, which have a large central vacuole, the nucleus has a lenticular shape and is moved along with the cytoplasm to the periphery of the cell. This is a sign of a specialized, but already aging cell. A cell lacking a nucleus can live only for a short time. Nucleated sieve tube cells are living cells, but they do not live long. In all other cases, anucleated cells are dead.

The core has a double shell, through the pores in which the contents
the nuclei (nucleoplasm) can communicate with the contents of the cytoplasm. The membranes of the nuclear membrane are equipped with ribosomes and communicate with the membranes of the endoplasmic reticulum of the cell. The nucleoplasm contains one or two nucleoli and chromosomes. Nucleoplasm is a colloidal sol system, reminiscent of thickened gelatin in consistency. The nucleus, according to domestic biochemists (Zbarsky I.B. et al.), contains four fractions of proteins: simple proteins - globulins 20%, deoxyribonucleoproteins - 70%, acidic proteins - 6% and residual proteins 4%. They are localized in the following nuclear structures: DNA proteins (alkaline proteins) - in chromosomes, RNA proteins (acidic proteins) - in nucleoli, partially in chromosomes (during the synthesis of messenger RNA) and in the nuclear membrane. Globulins form the basis of the nucleoplasm. Residual proteins (nature not specified) form the nuclear membrane.

The bulk of nuclear proteins are complex alkaline deoxyribonucleoproteins, which are based on DNA.

DNA molecule.DNA molecule– polynucleotide and consists of nucleotides. A nucleotide consists of three components: a sugar molecule (deoxyribose), a nitrogenous base molecule, and phosphoric acid molecules. Deoxyribose is connected to a nitrogenous base by a glycosidic bond, and to phosphoric acid by an ester bond. In DNA there are only 4 types of nucleotides in different combinations, differing from each other in nitrogenous bases. Two of them (adenine and guanine) belong to purine nitrogenous compounds, and cytosine and thymine belong to pyrimidine compounds. DNA molecules are not located in one plane, but consist of two helical strands, i.e. two parallel chains twisted around one another form one DNA molecule. They are held together by hydrogen bonds between nitrogenous bases, with the purine bases of one chain attaching the pyrimidine bases of the other (Fig. 14). The structure and chemistry of the DNA molecule was discovered by English (Crick) and American (Watson) scientists and made public in 1953. This moment is considered to be the beginning of the development of molecular genetics. The molecular weight of DNA is 4-8 million. The number of nucleotides (various variants) is up to 100 thousand. The DNA molecule is very stable, its stability is ensured by the fact that throughout it has the same thickness - 20A (8A - the width of the pyrimidine base + 12A - the width of the purine base). If radioactive phosphorus is introduced into the body, the label will be detected in all phosphorus-containing compounds except DNA (Levi, Sikewitz).

DNA molecules are carriers of heredity, because their structure encodes information about the synthesis of specific proteins that determine the properties of the organism. Changes can occur under the influence of mutagenic factors (radioactive radiation, potent chemical agents - alkaloids, alcohols, etc.).

RNA molecule.Ribonucleic acid (RNA) molecules significantly fewer DNA molecules. These are single chains of nucleotides. There are three types of RNA: ribosomal, the longest, forming numerous loops, information (template) and transport, the shortest. Ribosomal RNA is localized in the ribosomes of the endoplasmic reticulum and makes up 85% of the total RNA of the cell.

Messenger RNA in its structure resembles a clover leaf. Its amount is 5% of the total RNA in the cell. It is synthesized in the nucleoli. Its assembly occurs in chromosomes during interphase. Its main function is the transfer of information from DNA to ribosomes, where protein synthesis occurs.

Transfer RNA, as has now been established, is a whole family of compounds related in structure and biological function. Each living cell, according to a rough estimate, contains 40-50 individual transfer RNAs, and their total number in nature, taking into account species differences, is enormous. (Academician V. Engelhardt). They are called transport because their molecules are involved in transport services for the intracellular process of protein synthesis. By combining with free amino acids, they deliver them to the ribosomes in the protein chain being built. These are the smallest RNA molecules, consisting of an average of 80 nucleotides. Localized in the cytoplasmic matrix and make up about 10% of cellular RNA

RNA contains four nitrogenous bases, but unlike DNA, the RNA molecule contains uracil instead of thymine.

Structure of chromosomes. Chromosomes were first discovered at the end of the 19th century by the classics of cytology Fleming and Strasburger (1882, 1884), and the Russian cell researcher I.D. Chistyakov discovered them in 1874.

The main structural element of a chromosis is the nucleus. They have different shapes. These are either straight or curved rods, oval bodies, balls, the sizes of which vary.

Depending on the location of the centromere, straight, equal-armed and unequal-armed chromosomes are distinguished. The internal structure of chromosomes is shown in Fig. 15, 16. It should be noted that deoxyribonucleoprotein is a monomer of the chromosome.

The chromosome contains 90-92% deoxyribonucleoproteins, of which 45% is DNA and 55% is protein (histone). The chromosome also contains small amounts of RNA (messenger).

Chromosomes also have a clearly defined transverse structure - the presence of thickened areas - disks, which back in 1909. were called genes. This term was proposed by the Danish scientist Johansen. In 1911, the American scientist Morgan proved that genes are the main hereditary units and they are distributed in chromosomes in a linear order and, therefore, the chromosome has qualitatively different sections. In 1934, the American scientist Paynter proved the discontinuity of the morphological structure of chromosomes and the presence of disks in chromosomes, and disks are places where DNA accumulates. This served as the beginning of the creation of chromosomal maps, which indicated the location (locus) of the gene that determines a particular trait of the organism. A gene is a section of a DNA double helix that contains information about the structure of a single protein. This is a section of the DNA molecule that determines the synthesis of one protein molecule. DNA is not directly involved in protein synthesis. It only contains and stores information about the structure of the protein.

The DNA structure, consisting of several thousand sequentially located 4 nucleotides, is the code of heredity.

Heredity code. Protein synthesis. The first message on the DNA code was made by the American biochemist Nirenberg in 1961 in Moscow at the international biochemical congress. The essence of the DNA code is as follows. Each amino acid corresponds to a section of a DNA chain consisting of three adjacent nucleotides (triplet). So, for example, a section consisting of T-T-T (a triplet of 3 thymine-containing nucleotides) corresponds to the amino acid lysine, a triplet A (adenine) - C (cytosine) - A (adenine) - cysteine, etc. Let us assume that a gene is represented by a chain of nucleotides arranged in the following order: A-C-A-T-T-T-A-A-C-C-A-A-G-G-G. By breaking this series into triplets, we can immediately decipher which amino acids and in what order will be located in the synthesized protein.

The number of possible combinations of 4 available nucleotides in threes is 4×64. Based on these relationships, the number of different triplets is more than enough to provide information on the synthesis of numerous proteins that determine both the structure and functions of the body. For protein synthesis, an exact copy of this information is sent to the ribosomes in the form of messenger RNA. In addition to mRNA, decoding and synthesis involve a large number of molecules of various transport ribonucleic acids (tRNA), ribosomes and a number of enzymes. Each of the 20 amino acids binds to T-RNA - molecule to molecule. Each of the 20 amino acids has its own tRNA. tRNA has chemical groups that can “recognize” their amino acid, choosing it from the available amino acids. This happens with the help of special enzymes. Having recognized its amino acid, t-RNA enters into a connection with it. A ribosome is attached to the beginning of the chain (molecule) of i-RNA, which, moving along the i-RNA, connects with each other into a polypeptide chain exactly those amino acids, the order of which is encrypted by the nucleotide sequence of this I-RNA. As a result, a protein molecule is formed, the composition of which is encoded in one of the genes.

Nucleoli- an integral structural part of the core. These are spherical bodies. They are very changeable, changing their shape and structure, appearing and disappearing. There are one or two of them. For each plant a certain number. The nucleoli disappear as the cell prepares to divide and then reappear; they appear to be involved in the synthesis of ribonucleic acids. If the nucleolus is destroyed by a focused beam of X-rays or ultraviolet rays, cell division is inhibited.

The role of the nucleus in the life of a cell. The nucleus serves as the control center of the cell; it directs cellular activity and contains carriers of heredity (genes) that determine the characteristics of a given organism. The role of the nucleus can be revealed if, using microsurgical techniques, it is removed from the cell and the consequences of this are observed. A series of experiments proving its important role in the regulation of cell growth were carried out by Gemmerling on the single-celled green alga Acetobularia. This seaweed reaches a height of 5 cm, looks like a mushroom, and has something like “roots” and “legs”. It ends at the top with a large disc-shaped “hat”. The cell of this algae has one nucleus, located in the basal part of the cell.

Hammerling found that if the stem is cut, the lower part continues to live and the cap is completely regenerated after the operation. The upper part, deprived of the nucleus, survives for some time, but eventually dies without being able to restore the lower part. Therefore, the acetobularia nucleus is essential for the metabolic reactions underlying growth.

The nucleus contributes to the formation of the cell membrane. This can be illustrated by experiments with the algae Voucheria and Spyrogyra. By releasing the contents of the cells from the cut threads into the water, we can obtain lumps of cytoplasm with one, several nuclei, or without nuclei. In the first two cases, the cell membrane formed normally. In the absence of a core, the shell was not formed.

In experiments by I.I. Gerasimov (1890) with spirogyra, it was found that cells with a double nucleus double the length and thickness of the chloroplast. In nuclear-free cells, the process of photosynthesis continues, assimilation starch is formed, but at the same time the process of its hydrolysis is damped, which is explained by the absence of hydrolytic enzymes, which can be synthesized in ribosomes only according to the information from the DNA of the nucleus. The life of a protoplast without a nucleus is incomplete and short-lived. In the experiments of I.I. Gerasimov, the nuclear-free cells of Spirogyra lived for 42 days and died. One of the most important functions of the nucleus is to supply the cytoplasm with ribonucleic acid necessary for protein synthesis in the cell. Removal of the nucleus from the cell leads to a gradual decrease in the RNA content in the cytoplasm and a slowdown in protein synthesis in it.

The most important role of the nucleus is in transmitting characteristics from cell to cell, from organism to organism, and does this during the process of division of the nucleus and the cell as a whole.

Cell division. Cells reproduce by division. In this case, from one cell two daughter cells are formed with the same set of hereditary material contained in the chromosomes as the mother cell. In somatic cells, chromosomes are represented by two, so-called homologous chromosomes, which contain allelic genes (carriers of opposite characteristics, for example, white and red color of pea petals, etc.), characteristics of two parental pairs. In this regard, in the somatic cells of the plant body there is always a double set of chromosomes, designated 2p. Chromosomes have distinct individuality. The quantity and quality of chromosomes is a characteristic feature of each species. Thus, in strawberry cells the diploid set of chromosomes is 14, (2n), in apple cells - 34, in Jerusalem artichoke - 102, etc.

Mitosis (karyokinesis)– division of somatic cells was first described by E. Russov (1872) and I.D. Chistyakov (1874). Its essence lies in the fact that from the mother cell, by division, two daughter cells with the same set of chromosomes are formed. The cell cycle consists of interphase and mitosis itself. Using the microautoradiography method, it was established that the longest and most complex is the interphase - the period of the “resting” nucleus, because During this period, nuclear material doubles. Interphase is divided into three phases:

Q1 - presynthetic (its duration is 4-6 hours);

S - synthetic (10-20 hours);

Q2 - postsynthetic (2-5 hours).

During the Q1 phase, preparations are made for DNA reduplication. And in the S phase, DNA reduplication occurs; the cell doubles its DNA supply. In the Q2 phase, enzymes and structures necessary to initiate mitosis are formed. Thus, in interphase, DNA molecules in chromosomes are split into two identical strands, and messenger RNAs are assembled on their matrix. The latter carries information about the structure of specific proteins into the cytoplasm, and in the nucleus, each of the DNA strands completes the missing half of its molecule. This process of duplication (reduplication) reveals a unique feature of DNA, which is the ability of DNA to accurately reproduce itself. The resulting daughter DNA molecules are automatically obtained as exact copies of the parent molecule, because during reduplication, complementary (A-T; G-C; etc.) bases from the environment are added to each half.

During the prophase of mitotic division, the duplicated chromosomes become noticeable. In metaphase, they are all located in the equatorial zone, arranged in one row. Spindle filaments (from microtubules connecting to each other) are formed. The nuclear membrane and nucleolus disappear. Thickened chromosomes are split lengthwise into two daughter chromosomes. This is the essence of mitosis. It ensures precise distribution of duplicated DNA molecules between daughter cells. Thus, it ensures the transmission of hereditary information encrypted in DNA.

In anaphase, the daughter chromosomes begin to move to opposite poles. The first fragments of the cell membrane (phragmoblast) appear in the center.

During telophase, the formation of nuclei in daughter cells occurs. The contents of the mother cell (organelle) are distributed among the resulting daughter cells. The cell membrane is fully formed. This ends cytokinesis (Fig. 17).

Meiosis - reduction division was discovered and described in the 90s of the last century by V.I. Belyaev. The essence of division is that from a somatic cell containing a 2n (double, diploid) set of chromosomes, four haploid cells are formed, with “n”, a half set of chromosomes. This type of division is complex and consists of two stages. The first is reduction by chromosis. Duplicated chromosomes are located in the equatorial zone in pairs (two parallel homologous chromosomes). At this moment, conjugation (coupling) with chromosis, crossing over (crossover) can occur and, as a result, an exchange of sections of chromosis can occur. As a result of this, some of the genes of paternal chromosomes pass into the composition of maternal chromosomes and vice versa. The appearance of both chromosomes does not change as a result of this, but their qualitative composition becomes different. Paternal and maternal heredity are redistributed and mixed.

In the anaphase of meiosis, homologous chromosomes, with the help of spindle threads, disperse to the poles, at which, after a short period of rest (the threads disappear, but the partition between new nuclei is not formed), the process of mitosis begins - metaphase, in which all the chromosomes are located in the same plane and their longitudinal splitting occurs to daughter chromosomes. During anaphase of mitosis, with the help of a spindle, they disperse to the poles, where four nuclei are formed and, as a result, four haploid cells. In the cells of some tissues, during their development, under the influence of certain factors, incomplete mitosis occurs and the number of chromosomes in the nuclei doubles due to the fact that they do not diverge to the poles. As a result of such disturbances of a natural or artificial nature, tetraploid and polyploid organisms arise. With the help of meiosis, sex cells are formed - gametes, as well as spores, elements of sexual and asexual reproduction of plants (Fig. 18).

Amitosis is direct division of the nucleus. During amitosis, the spindle does not form and the nuclear membrane does not disintegrate, as during mitosis. Previously, amitosis was considered as a primitive form of division. It has now been established that it is associated with the degradation of the body. It is a simplified version of a more complex nuclear fission. Amitosis occurs in the cells and tissues of the nucellus, endosperm, tuber parenchyma, leaf petioles, etc.