They have osmotic properties. Test paper in botany

Rice. 1. Scheme of a plant cell as an osmotic system:

π*-osmotic pressure, P - turgor pressure, -P - back pressure of the cell wall.

In terms of the chemical composition and concentration of substances, cell sap is very different from the protoplast of the cell, since the vacuolar membrane is tonoplast has selective permeability to different substances and mainly performs transport and barrier functions (it allows some substances to pass through and does not allow or only allows others to pass through with difficulty).

That is why the concentration of ions and organic matter in cell sap, vacuoles are usually higher than in the cell membrane, and therefore water will enter the vacuole by diffusion, trying to equalize the concentration of the environment and cell sap.

This one-way, unidirectional, passive penetration of water through a membrane semi-permeable to dissolved substances is called by osmosis.

As the cell vacuole becomes saturated with water, it is created vacuole pressure on the protoplast – osmotic pressure (π*) . The more concentrated the cell sap, the more active the diffusion of water into the cell, therefore the higher π* in a cage.

As the cell is saturated with water, the protoplast becomes elastic and hydrostatic (turgor) pressure of the protoplast on the cell membrane develops ( R).

The elastic state of a cell when it is maximally saturated with water is called turgor state , or turgor . When water is lost, the plant loses turgor and withers.

That is turgor pressure is the pressure developing in a plant cell as a result osmosis .

Turgor pressure is opposed by mechanical pressure equal in magnitude and opposite in sign, caused by elastic stretching of the cell membrane, directed into the cell. It's called back pressure of the cell membrane (-P).

The amount of water required by the cell, its supply depends on the difference in osmotic ( π* ) and turgor ( R) pressure.

π* - P = S - sucking force , - the force with which water enters the cell .

Its value is determined by the osmotic pressure of cell sap (π*) and turgor pressure in the cell ( R) (which is equal to the back pressure of the cell wall that occurs during its elastic stretching).

When the cell is completely saturated with water S = 0, A P = π* (turgor pressure is equal to osmotic pressure).

Full turgor occurs with sufficient air and soil humidity.

With prolonged lack of water P = 0(the plant withers), and S= π*.

Depending on the suction force, water enters the root hairs, since the cellular sap of root cells is more concentrated than the surrounding solutions of soil mineral salts.

If a cell is placed in a more concentrated solution, a condition is observed plasmolysis – lag of the protoplast from the cell walls (due to the loss of water from it). This process is reversible and when the cell is placed in a solution of the same concentration as the cell sap, deplasmolysis – restoration of the turgor state of the cell.

Electrolytes– substances whose molecules disintegrate in aqueous solutions and melts with the formation of charged particles – ions. Electrolytes include all salts, alkalis, and soluble acids. Real electrolyte solutions, in contrast to non-electrolyte solutions, differ in their properties from ideal ones. Thus, for electrolyte solutions, the experimentally found values ​​of colligative characteristics are always greater than those calculated according to the van’t Hoff and Raoult laws. That is, electrolyte solutions in practice behave as if they contain more solute particles than their analytical concentration indicates. Based on this, Van't Hoff proposed for electrolyte solutions in the theoretical calculation of Rosm., tboil., Δtzam., to use the correction factor i, which was called the Van't Hoff coefficient or isotonic coefficient:

Rosm. = iCRT; Δtboil. = iEm; Δtdeput. = iKm;

where C is the molar concentration of the dissolved substance, m is the molar concentration of the dissolved substance, E and K are the ebullioscopic and cryoscopic constants, respectively.

The isotonic coefficient shows how many times the actual number of particles of a dissolved substance is greater than the theoretically expected one (assuming that the substance is present in the solution only in the form of molecules).

For ideal electrolyte solutions i >1.

The isotonic coefficient also shows how many times the observed experimental value of Rosm., Δtboil., Δtdec. is greater than the theoretically calculated one. The reason for the deviation of electrolyte solutions from the laws of Raoult and Van't Hoff was first explained by the Swedish scientist S. Arrhenius. He showed that electrolytes, due to the action of solvent molecules, break down into ions. This process results in an increase in the actual number of solute particles.

The maximum value of the isotonic coefficient (i max) for any electrolyte will be equal to the number of ions that are formed during the complete dissociation of its molecule (or formula unit), because This is exactly how many times the number of electrolyte particles in the solution will increase.

So, for NaCl i max = 2, for Na 3 PO 4 i max = 4.

In real solutions, dissociation often does not occur completely, especially if the electrolyte is weak. In addition, interionic interactions are observed, leading to a decrease in the number of kinetically active particles. In this case, the value of i will be less than its possible maximum value and will depend on the degree of dissociation of the electrolyte:

i = 1 + α (m - 1)

where α is the degree of electrolyte dissociation (in fractions of unity); m is the number of ions formed during the complete breakdown of one molecule or one formula unit of the electrolyte.

Thus, of two solutions of the same type of electrolytes (i.e., disintegrating into the same number of ions) with the same molar (molal) concentration, the isotonic coefficient will be greater in the electrolyte solution with a higher degree of dissociation α. Accordingly, growth, Δtboil, Δtdc. for such a solution will also have large values. If the molar concentration and degree of dissociation of electrolytes of different types in solution are the same, then the value of i will be higher for the electrolyte that dissociates into a larger number of ions m.

5. Hypo-, hyper-, isotonic solutions. The concept of isosmia (electrolyte homeostasis). Osmolality and osmolarity of biological fluids.

Solutions whose osmotic pressure is equal to the osmotic pressure of a solution taken as a standard are called isotonic . In medicine, the osmotic pressure of solutions is compared with the osmotic pressure of blood. Isotonic with respect to blood are 0.9% (0.15 M) NaCl solution and 4.5-5% glucose solution. In these solutions, the concentration of solute particles is the same as in blood plasma. Solutions with a higher osmotic pressure than blood plasma are called hypertensive , and solutions having lower pressure - hypotonic . During various medical procedures, only isotonic solutions should be injected into the human blood in large quantities, so as not to cause an osmotic conflict due to a sharp discrepancy between the osmotic pressure of the biological fluid and the injected solution.

Human blood, lymph, and tissue fluids are aqueous solutions of molecules and ions of many substances and, as a result, have a certain osmotic pressure. Moreover, throughout the life of the body, biological fluids maintain their pressure at a constant level, regardless of the condition external environment. This phenomenon is called differently isoosmia human body and is integral part more general process- homeostasis or constancy of a number of physical and chemical indicators of the internal environment of a person in changing external conditions. Isoosmia is especially characteristic of biological fluids such as blood and lymph. Thus, the osmotic pressure of blood in humans is almost constant and at 37 o C varies within 740-780 kPa (i.e., almost 8 times more than atmospheric pressure). When the osmotic pressure of the blood changes, the body seeks to restore it by removing an excess amount of dissolved particles from the blood (if the pressure increases) or, conversely, by increasing the number of kinetically active particles (if the pressure decreases). The kidneys play the main role in regulating the osmotic pressure of the blood. Izoosmia regulated primarily by the central nervous system and the activity of the endocrine glands.

The composition of biological fluids includes a number of substances. Their total concentration is called osmolarity (isotonic concentration) and represents chemical quantity all kinetically active (i.e., capable of independent movement) particles (regardless of their shape, size and nature) contained in 1 liter of liquid and not penetrating through a semi-permeable membrane. Osmolarity expressed in milliosmoles per liter (mOsm/L). Normally, blood plasma osmolarity is 280-300 mOsm/L, for cerebrospinal fluid – 270-290 mOsm/L, for urine – 600-1200 mOsm/L. Osmolality - the concentration of the same particles dissolved in a kilogram of biological fluid, expressed in milliosmoles per kilogram (mOsm/kg). Normally, total intracellular osmolality depends mainly on the concentration of K + ions and associated anions and is equal to the osmolality of the extracellular fluid, determined by Na + ions and associated anions. Therefore, there is no general movement of water into or out of cells. Osmolar balance is maintained by several physiological mechanisms that can be disrupted in critical conditions: the movement of water towards increased ion concentrations, renal excretion of osmotically active substances (urea, salts), removal of CO2 through the lungs, and antidiuretic hormone.
6. The role of osmosis in biological systems. Plasmolysis and cytolysis. Dependence of the degree of hemolysis of erythrocytes on the concentration of NaCl solution.

The reason for the occurrence of osmotic phenomena in the body is that all biological fluids are aqueous solutions of electrolytes and non-electrolytes, and cell membranes can be considered semi-permeable. Osmosis plays a leading role in the distribution of water between intra- and extracellular contents, between various tissues and tissue systems that form organs. The cell membrane is semi-permeable and water passes through it quite freely. The shell allows electrolyte ions and molecules of other substances to pass through strictly selectively. From the outside, cells are washed by intercellular fluid, which is also aqueous solution. Moreover, the concentration of dissolved substances inside cells is greater than in the intercellular fluid. As a result of osmosis, a transition of solvent from the external environment into the cell is observed, which causes its partial swelling or turgor. In this case, the cell acquires appropriate firmness and elasticity. Turgor contributes to the preservation of a certain shape of organs in animal organisms, stems and leaves in plants.

If a cell enters a solution environment with a high concentration of salts and other soluble substances (hypertonic solution), this leads to osmosis, in which water diffuses from the cell into the solution. If a cell with a strong cellulose shell gets into such a hypertonic solution, then the phenomenon occurs plasmolysis – compression of the protoplast and its separation from the cell walls. In the case of animal cells that have a plastic membrane (for example, red blood cells), a general contraction and wrinkling of the cell occurs. The use of high concentrations of salts or sugar for canning is widely known. food products. Under these conditions, microorganisms are exposed plasmolysis and become unviable. If a cell enters a solution environment with a reduced concentration of substances (hypotonic solution), this leads to osmosis, in which water diffuses from the solution into the cell, which leads to its swelling. If the difference in the concentrations of intra- and extracellular fluids is large enough and the cell does not have strong walls, the cell membrane is destroyed and its contents are released into the surrounding solution - cytolysis. In case of destruction of the erythrocyte membrane and release into environment contents of the red blood cell, a phenomenon called osmotic shock ( hemolysis ).

An indicator of the strength of red blood cells is their osmotic resistance, i.e. ability to resist a decrease in osmotic pressure. A measure of the osmotic resistance of red blood cells is the NaCl concentration at which hemolysis begins. In humans, this occurs in a 0.4% NaCl solution (minimal osmotic resistance), and in a 0.34% NaCl solution solution, all red blood cells are destroyed and complete hemolysis of the blood occurs (maximum osmotic resistance).

The red blood cells in the blood of each individual, according to the criterion of osmotic resistance, are distributed according to the Gaussian law. Therefore, one of the main parameters characterizing the osmotic properties of erythrocytes in suspension is the average value of the so-called. osmotic fragility, numerically equal to the NaCl concentration at which lysis of 50% of cells occurs (Fig.).

Osmotic properties of the cell

Osmosis- this is the one-way penetration of water through a semi-permeable membrane from an area with a lower concentration of a solution to an area with a higher concentration. The resulting pressure on the membrane is called osmotic. Osmotic pressure is determined by solutions of salts and other low molecular weight substances (sugar, urea) contained in the cells. One-way diffusion of solutes is called dialysis.

Solutions in which the osmotic pressure is the same as in cells are called isotonic. When cells are immersed in isotonic solutions, their volume remains unchanged. Isotonic solutions salts are called physiological. For different organisms, the concentration of sodium chloride in physiological solution is not the same. Thus, for mammals it is 0.9%, for amphibians - 0.75%, for marine invertebrates - 3%.

Saline solutions and other isotonic liquids are used in medicine. They are used for severe dehydration and blood loss in patients.

A solution whose osmotic pressure is higher than that in the cells is called hypertensive. Cells immersed in a hypertonic solution begin to lose water and shrink, i.e. wrinkled.

Hypertonic solution is widely used in surgery for the treatment of purulent wounds. A gauze bandage moistened with a hypertonic solution absorbs pus well, which helps cleanse and heal the wound.

The opposite picture is observed when cells are immersed in hypotonic a solution in which the concentration of salts is lower than in the cells. In these cases, water rushes into the cell, the cell swells, the pressure on the membranes becomes greater, and the cell turgor increases. Underturgor refers to the tense state of cell membranes caused by pressure on them from the inside. Human skin, in the cells of which turgor is reduced, becomes flabby. If there is a significant difference in osmotic pressure, a cell in a hypotonic solution may burst, i.e. lyse.

Living cells actively regulate osmotic pressure. In single-celled animals living in fresh water, the function of osmoregulation is performed by pulsating (excretory) vacuoles. In three-layered animals, osmotic pressure is generally regulated by the system of excretory organs.

II. StructureAndchemical composition of eukaryotic cell chromosomes

One of the key questions of genetics is the question of the structure and characteristics of the functioning of the material carriers of heredity. The latter have three main levels of organization: gene, chromosome, genomic.

The branch of genetics that studies the chemical organization, structure, significance and functioning of chromosomes is called cytogenetics.

For medical and biological education, human cytogenetics is of particular interest, the object of study of which is human chromosomes. In the history of the development of this section of genetics, three periods can be distinguished, passing into each other

Start of the first period falls at the end of the last century. It can be said that human cytogenetics began with the work of Arnold (1879) and Flemming (1882), who were the first to observe human chromosomes.

Start of the second period was founded by Swedish cytologists Tio and Levan (1956), who, using colchicine, modified the method for obtaining metaphase plates of chromosomes and convincingly proved that a normal human cell contains 46 chromosomes. Soon these data were confirmed by other cytogeneticists.

Since 1956, human cytogenetics has undergone rapid development. During this period, all the main methods of chromosome analysis were developed, and fundamental work on the human karyotype appeared.

Third period in the development of cytogenetics begins in the 70s. It can rightfully be considered the beginning modern stage in the development of the science of the cytological bases of human heredity. By this period, it became possible to study the individual characteristics of human chromosomes and their individual sections. Information appeared about the supramolecular organization of chromosomes, and their genetic maps began to be created.

The structure of chromosomes at the microscopic level

Chromosomes like separate structures, become available for research only after significant chromatin condensation, which occurs during mitosis (in somatic cells) or during meiosis (during the formation of germ cells). Chromatin condensation, which began in prophase, ends in metaphase; therefore, as a rule, chromosomes are studied at the stage metaphase plate.

During interphase, chromosomes are in a decondensed state, and it is not possible to identify them as separate structures.

In metaphase, each chromosome has an X-shaped shape and consists of two identical halves - chromatid(sister chromosomes), closely adjacent to each other only in the region primary constriction (centromeres), and along the rest of the length a large gap is visible between the chromatids. Centromere- this is the area where the chromosome is in a decondensed state, and the filaments of the spindle are attached to it. The centromere divides the chromosomes into arms. Based on the position of the centromere, there are three types of chromosomes.

1. Metacentric, in which the arms are approximately the same length (i.e., the centromere is located in the middle of the chromosome).

2. Submetacentric, in which the centromere is displaced from the middle, is located submedially and divides the chromosome into two arms of unequal length. The top is always smaller.

Z. Acrocentric, in which the centromere is located almost at the end of the chromosome, separating a very short upper arm from the long arm.

Upper short shoulders usually denoted by the letter ";r";, A lower ones are long letter "; q"; . Characteristic feature for some chromosomes is the presence secondary constrictions, they arise in areas of incomplete condensation of chromosomes and are located in the pericentromeric regions of the 1st, 9th and 16th chromosomes. Secondary constrictions are also present in chromosomes 13-15 and 21-22, but here they occupy a position remote from the centromere, separating a small terminal section of the short arm of the chromosomes in the form of a satellite. These chromosomes are called satellite. In these chromosomes, in the region of the secondary constriction, genes encoding r-RNA are concentrated, and in the adjacent areas of the karyoplasm they are formed nucleoli. Therefore, this kind of secondary constriction is called nucleolar organizers. In the chromosome sets of some people, these chromosomes have a secondary constriction, while in the same chromosomes in other people they may not have it.

Chemical composition of chromosomes

Molecular biological studies have provided insight not only into the chemical structure of chromosomes, but also into their supramolecular organization and functional features. It is now known that chromosomes are nucleoprotein formations, consisting of DNA and protein. In addition, the chromosomes contain a certain amount of RNA formed during transcription, as well as Ca + and Mg + ions.

Each chromatid, and in the period of time anaphase-S-period of interphase and chromosome, contains one DNA molecule, which determines all the functions of the chromosome associated with storage of hereditary information, its transmission and implementation.

The DNA molecule in chromosomes is closely associated with two classes of proteins - histones (basic proteins) and non-histones (acidic proteins).

Histones- These are small proteins with a high content of charged amino acids (lysine and arginine).

Total positive charge allows histones to bind to DNA regardless of nucleotide composition. They own it mostly structural function. These are very stable proteins, the molecules of which can persist throughout the life of the cell.

In a eukaryotic cell there are 5 types of histones, which are divided into two main groups: the first group (they are designated as H2A, H2B, NZ, H4), is responsible for the formation of specific deoxyribonucleoprotein complexes - nucleosomes. The second group of histones (HI) is located between nucleosomes and fixes the folding of the nucleosomal chain into more high level structural organization (supernucleosomal thread).

Among histone proteins, in addition to structural ones, there are those that are capable of limiting the availability of DNA for DNA binding regulatory proteins and thereby participate in the regulation of gene activity.

Non-histonesquirrels very diverse. The number of their fractions exceeds 100. They are present in smaller quantities in chromosomes compared to histones and mainly perform regulatoryfunction. Participate in the regulation of transcriptional activity of genes, in ensuring DNA replication and repair.

Most non-histone chromatin proteins are presentin cells in small quantities (minor) - these are regulatory proteins that recognize specific DNA sequences and bind to them. They are involved in many genetic processes, but little is known about them yet. Non-histone proteins (major) are quantitatively predominant, highly mobile, relatively small in size, with a large electric charge- they always bind to nucleosomes containing active genes. In addition, the group of non-histone proteins includes many enzymes.

Supramolecular organization of chromosomes

The supramolecular organization of chromosomes is also called spiralization, or condensation, or compactization.

Currently, three levels of supramolecular organization of chromosomes are accepted: primary, secondary, tertiary.

DNA compaction is important for a eukaryotic cell for two reasons: it allows very long DNA molecules to be placed in an orderly manner in a small volume of the cell nucleus and, in addition, it is one of the ways of functional control of genes - the nature of DNA packaging affects the activity of certain regions of the genome.

Primary level supramolecular organization - nucleosomal. Elementary structure The chromosome, distinguished using an electron microscope, is a thread with a diameter of 10-13 nm, which is a complex of DNA and histone proteins. This thread consists of a histone backbone (in the form of a chain of disk-shaped protein bodies located one after another), on top of which a DNA strand is spirally twisted. The complex of DNA and histones at the level of one disc-shaped body is called nucleosome. It contains two molecules of each of the 4 types of histone (H2A, H2B, NZ, H4), connected in the form of an octamer. The DNA in the nucleosome lies on top of the octamer, spiraling onto the histone backbone. At the level of each nucleosome, DNA forms 2.3 turns of the helix, which corresponds to approximately 200 nucleotide pairs. Communication between neighboring nucleosomes is carried out by histone HI. There are 60 nucleotide pairs in this binding region. A thread with a diameter of approximately 11 nm is formed.

Nucleosome is a universal particle that is found in both euchromatin and heterochromatin, in the interphase nucleus and metaphase chromosomes.

In the case of linear rectifier, which is hardly present in a living cell, the structure formed by nucleosomes resembles a string of "beads" and is called a nucleosome filament. Due to the nucleosomal organization of chromosomes, the original length of DNA is shortened by 7 times, i.e. compaction occurs. This is apparently the state of the interphase chromosome and its euchromatic regions.

Further compaction of DNA within chromosomes is associated with the formation supranucleosomal structures. So, secondary level chromosomal arrangement of DNA is expressed in the formation supercoil thread (solenoid), in which the original DNA molecule is shortened by 40 times. The thickness reaches 30-40 nm. When a superhelix is ​​formed, the nucleosomal thread becomes helically twisted due to the interaction of histones HI and NZ. It is also possible that non-histone proteins are involved in this. This level of DNA folding appears to correspond to prophase mitotic and meiotic chromosomes observed under a light microscope. Or interphase, but not transcribed, possibly, chromosome regions, i.e. heterochromatic.

Third level chromosomal arrangement is the least studied.

There are two models: the basis of the first based on the principle of spiral laying, based on the second- structure based on the principle of folding loops. IN recent years Numerous material has been accumulated indicating the reality of loop-like structures in the chromosome and their dense packing in the metaphase chromosome around an axial framework built from non-histone proteins. Loop structures, but not tightly packed, are also present in the interphase chromosome. Around the frame, as in a brush, there are loops of superspiral thread. Moreover, the ends of each loop are localized at the same point of the protein framework. It is also assumed that the loops can twist around their own axis, i.e. The metaphase chromosome can be depicted as tightly packed solenoid loops, coiled into a tight spiral. A typical human chromosome can contain up to 2,600 loops.

Third level of installation - Thiscondensation of a prophase chromosome into a metaphase one. The thickness of such a structure reaches 1400 nm (two chromatids), and the DNA molecule is shortened by I0 4 times, i.e. from 5 cm stretched DNA to 5 µm. This supercoiling is accompanied by phosphorylation of all HI molecules in the cell. In any case, DNA in the nuclei of eukaryotic cells forms a hierarchical system of helices and loops, the basic unit of which is the nucleosome. Nucleosomes, in turn, are not located exactly the same everywhere. These subtle and little-studied differences are biologically very important, because... they appear to occur preferentially in those regions of chromatin where active genes are located. During the S-period of interphase, the replication process somehow, unknown, passes through the nucleosomes of the parent chromatin chain, which move to one of the daughter DNA helices. Then all new histone octamers are attached to the second daughter DNA helix, free of nucleosomes.

The nucleosome structure is maintained during DNA transcription, although it is difficult to imagine how RNA polymerase could transcribe histone-associated DNA without any noticeable changes in nucleosome organization. But in the cells of insect embryos, in the region of activated genes for r-RNA, there are apparently no nucleosomes. And biochemical differences between transcribed active and inactive chromatin were discovered. In particular, HI is bound to nucleosomes much less tightly in active chromatin, and in general histones in these regions exhibit a higher degree of acetylation.

Longitudinal organization of chromosomes

The longitudinal organization of chromosomes of higher organisms, which is based on the interrelation of morphological, chemical and functional patterns, is characterized by linear heterogeneity. Already interphase chromosomes turn out to be deeply differentiated in the degree of chromatin condensation, which was initially discovered using light microscopy.

Some of their areas are becoming decondensed (euchromatin), others remain condensed (heterochromatin). In metaphase chromosomes, the division into these two types of chromatin does not disappear. It manifests itself in the natural course of mitotic condensation: in early prophase, areas of heterochromatin are ahead of euchromatic areas in their condensation. Residual phenomena of unequal condensation of the interphase chromosome are detected morphologically and in metaphase (in the area of ​​the secondary constriction).

The concept of "heterochromatin" and "euchromatin" As a result of cytogenetic studies, genetic content was obtained. Heterochromatin unlike euchromatin, it does not contain structural genes or is depleted in them. At the same time euchromatin is functionally active, transcribed chromatin, i.e. chromatin structure influences the regulation of eukaryotic gene expression. Like mitotic chromatin, heterochromatin is not involved in transcription; DNA within heterochromatin is replicated in the late S period of the cell cycle. The biochemical basis for the observed differences between hetero- and euchromatin is unknown.

Some sections of chromosomes condense into heterochromatin in all cells of the body - this is constitutive heterochromatin. Other regions of chromosomes form heterochromatin only in certain cells - facultative heterochromatin.

Constitutive heterochromatin containsDNA, which appears to benever transcribed in any cell.

In human chromosomes, it is localized around centromeres and is easily detected in mitotic chromosomes using special staining, although it can also be detected in other regions of some chromosomes (1.9, 16, U). A similar state is characteristic of satellite DNA and DNA with highly repetitive sequences. Consequently, most constitutive heterochromatin contains a series of relatively simple, repeatedly repeated DNA sequences. Overall, the function of constitutive heterochromatin remains unclear. It is believed that some segments of this chromatin play a role in the pairing of chromosomes in meiosis. Perhaps it affects the stabilization of chromatin structure and protects genetically significant sequences of euchromatic regions from external influences, but there are most likely no classical Mendelian genes here.

In an interphase cell, sections of constitutive chromatin aggregate to form chromocenters, which we see in a light microscope in the form of tiny “lumps of chromatin.” In mammals, their number and distribution pattern vary depending on the type of cell and the stage of development of the organism.

Facultative heterochromatin has a more distinct functional value . There is almost no doubt that it reflects persistent differences in the nature of the genetic activity of cells of different types, and the amount of this chromatin in different cells varies: in embryonic cells there is very little of it, while highly specialized cells contain it in extremely large quantities, i.e. some genes are switched off from transcription. Facultative heterochromatin contains unique regions of DNA, rather than highly repetitive ones, and does not show up in anything when staining mitotic chromosomes. This type of genetic regulation is not available to bacteria.

A special case of facultative heterochromatization- this is the inactivation of one of the two X chromosomes in the cells of female mammals, which occurs in the early stages of embryonic development (in the human trophoblast on the 12th day of development, and in the embryo itself on the 16th day). At the same time, in all cells of the female embryo, with equal probability, one or the other X chromosome condenses and forms heterochromatin. This chromosome state is stably inherited in all subsequent replication cycles. Because of this, every female body has a sort of mosaic structure, because formed by clonal groups of cells, approximately half of which have a heterochromatic X chromosome inherited through the maternal line, and the other half have an X chromosome inherited through the paternal line.

In interphase, heterochromatized X chromosomes are clearly defined structural formations called Barr bodies, which are closely adjacent to the inner membrane of the nucleus and are clearly visible in a light microscope. Barr bodies are also called clumps of sexual X-chromatin.

Polytene chromosomes

To capture changes in chromatin structure at the level of individual genes, it is necessary to study stretched interphase chromosomes. This is not possible in normal cells because the strands of interphase chromatin are too thin and tangled. Thanks to the phenomenon of polyteny, numerous transverse stripes are clearly visible on interphase chromosomes, the frequency of alternation of which suggests that they correspond to individual genes.

Polytene chromosomes (giant chromosomes) contain many times more DNA than normal chromosomes. They do not change their shape throughout the entire mitotic cycle and reach a length of up to 0.5 mm and a thickness of up to 25 microns. They are found, for example, in the salivary glands of dipterans (flies, mosquitoes), in the macronucleus of ciliates and in the tissues of the ovary of beans. Most often they are visible in the haploid number, because. Homologous chromosomes are closely paired. Cells with these chromosomes grow to an unusually large size.

Polytene chromosomes arise due to repeated process of DNA reduplication. In this case, different sections of DNA are reduplicated to varying degrees. Most genetically informative regions are replicated 1000 times, and some more than 30 thousand times. In this case, DNA replication cycles are not accompanied by cell division. Essentially, polytene chromosomes are bundles of many incompletely separated individual chromatin strands closely adjacent to each other. In particular, the polytene chromosomes of the salivary glands of the Drosophila larva contain 1024 such strands. So, interphase polytene chromosomes are clearly visible in a light microscope, the chromatin loops in them are arranged in a linear order, and when these chromosomes are stained, alternating transverse stripes are noticeable: dark - disks and light - interdisk areas. It is assumed that the discs contain 1024 densely packed homologous loops of the individual loop region and the genes located there. The structural organization and function of DNA in the interband regions is still unknown.

With the beginning of gene transcription, the disks in which they are contained become decompacted, become swollen, and are called puffs. The DNA that forms them is packed much less densely. Apparently, such structural modifications of chromatin, when its partial decondensation occurs, are the first stage of activation of eukaryotic genes. Biochemically, puffs contain less histone HI, much RNA polymerase, and at least one common non-histone protein.

Perhaps the functional unit of the genome in higher karyotes, including humans, is structured and functions in the same way.

Chromosomestypelamp brushes

Another example of cells in which transcriptionally active chromosomes are clearly visible are immature eggs, or oocytes. Enhanced RNA synthesis in them is accompanied by stretching of long chromatin loops, to which numerous newly formed transcripts packaged in RNA complexes are attached. These so-called lampbrush chromosomes are clearly visible in a light microscope, although they are not very condensed.

Lampbrush chromosomes appear in Diplonema meiosis time during the formation of germ cells in most vertebrates, invertebrates and green algae. The DNA content in such chromosomes is normal; they are not polytene (each chromosome contains two DNA molecules).

In lampbrush chromosomes, in addition to the loop-like arrangement of the superhelix in the form of a ruff, there are individual significantly elongated symmetrical loops protruding above the surface of the main structure of the chromosome arrangement.

Normally, no RNA is synthesized during cell division, and lampbrush chromosomes appear to be create a reserve of RNA for subsequent stages of development. The observed lampbrush structures represent transcriptionally active chromatin and are not typical for somatic cells.

Human chromosomes

Everything stated above regarding the chemical composition and structure of eukaryotic chromosomes is typical for human chromosomes. Some detail is required for information that allows any human chromosome to be identified with a greater degree of accuracy.

1956 - the Swedes Tio and Levan, the Englishmen Ford and Hamerton established that the nucleus of a diploid human cell contains 46 chromosomes - this is the chromosome set or human karyotype; in 1960 - Moorehead et al. (USA) developed a method for preparing chromosome preparations from a short-term culture of lymphocytes; in 1968-70 methods for differential staining of chromosomes have been developed, which made it possible to unambiguously identify all human chromosomes - all these manipulations were and are carried out only on metaphase chromosomes, because they are best distinguished, because they are maximally shortened and thickened, lie free from one another, and are all located in the same plane of the cell (equatorial); in addition, only those metaphase chromosomes are examined whose chromatids have separated from each other in the region of the arms, but are still connected in the centromeric part.

The totality of all metaphase chromosomes located relatively randomly in the equatorial plane of the cell, called the metaphase plate or simply the chromosome set. After preparing preparations of chromosomes, which can be prepared from all tissues and cell suspensions containing dividing cells (depending on the purpose, the number of metaphases is important, of course), the chromosomes are stained, because only after this can they be distinguished in a light microscope, obtain a microphotography, identify and , arranging them in a certain order, i.e. By drawing up a karyogram, get a holistic picture of the karyotype of a particular person. Karyogram- these are the same chromosomes of the metaphase plate, but arranged in an orderly manner. The principle of orderliness is common to the entire species and is determined by the ideogram. Idiogram- This graphic image haploid set of chromosomes (diploid is also possible) and their arrangement in groups depending on shape and size. The groups are arranged in order of decreasing size of the chromosomes included in them.

Many solutions enter the cell osmotically. Osmosis (from the Greek osmos - pressure) is the one-way penetration of water through a semi-permeable shell. It can be observed if two solutions of different concentrations are separated by a semi-permeable partition that is accessible to water, but does not allow the dissolved substance to pass through. Water with a certain force, depending on the difference in concentration, will be attracted by a more saturated solution from a more dilute one. The resulting pressure on the semipermeable membrane is called osmotic pressure. The study of the osmotic properties of cells began with the great Dutch botanist G. de Vries (1848-1935), who discovered turgor in plants. De Vries' experiments, according to Van Hoff, formed the basis for the theory of osmotic pressure developed by this famous physicist.

Animal and plant cells contain solutions of salts and other osmotically active substances (sugars, urea). This determines a certain osmotic pressure. In the cells of terrestrial animals it is about 8 atm; in marine invertebrates it increases to 38 atm. Plant cells usually have an osmotic pressure of 5 to 20 atm, but in some cases it can reach 100 or even 140 atm. Here the conditions of existence are of primary importance, not the systematic position. Representatives of the same species growing in different conditions have different osmotic pressure of cell sap.

Solutions in which the osmotic pressure is the same as in the cells are called isotonic. When cells are immersed in isotonic solutions, their volume remains unchanged. Isotonic salt solutions are called physiological. For different objects, the concentration of table salt in physiological solution is not the same. Thus, for animals from the class of amphibians it is equal to a 0.75% NaCl solution, for mammals - 0.9%, for insects - 1%, and for marine invertebrates it corresponds to the salt concentration in sea water - 3% NaCl. Saline solutions and other isotonic liquids are used in medicine. They are used for severe dehydration and blood loss in patients.

A solution whose osmotic pressure is higher than that in the cells is called hypertonic. Plant cells immersed in such a solution begin to lose water, the protoplasm of the cell shrinks and peels off from the shell. This phenomenon is called plasmolysis (Figure 20). When osmotic pressure decreases in plant cells, turgor decreases. Turgor refers to the tense state of cell membranes caused by pressure on them from the inside. A plant whose cells have reduced turgor becomes flabby. This phenomenon can be easily observed in plucked and withering plants and their fruits. In some diseases, such as cholera, as a result of severe dehydration of the patient's cells, his entire body becomes flabby and his skin becomes “doughy.”

In surgery, a hypertonic solution of steamed salt is widely used in the treatment of infected wounds. A gauze bandage moistened with a Hypertonic solution absorbs pus well, which promotes wound healing.

The opposite of plasmolysis is observed when plant cells are immersed in a hypotonic solution. In this solution the osmotic pressure is lower than in the cells. Water begins to rush into the cell, the cell swells, the pressure on the membranes becomes greater, and turgor increases. If there is a significant difference in osmotic pressure, the cell may burst.

Isolated animal cells in hypotonic solutions are destroyed. The same thing will happen to red blood cells if a hypotonic solution is introduced into the blood of a person or animal. They first swell, and then their outer membrane ruptures (Fig. 21).

The outer layer of the cell, i.e. its membrane, allows not only water to pass through, but also, to some extent, substances dissolved in it. Living cell actively regulates osmotic pressure, changing the concentrations of osmotically active substances. Single-celled animals living in fresh water have developed special devices (pulsating vacuoles) that remove excess water from cells. Unicellular organisms that do not have pulsating vacuoles, excess water is removed through the cell membrane. In higher animals, osmotic pressure in the whole organism is regulated by the system of excretory organs (kidneys).

A plant cell differs from an animal cell mainly in the structure of the cell wall, the presence of chloroplasts that provide photosynthesis and vacuoles filled with cell sap (Fig. 2-13).

Cell membrane consists of two layers. The inner layer is adjacent to the cytoplasm and is called cytoplasmic or plasma membrane, over which an outer thick layer of cellulose is formed, called the cell wall. The cell membrane is easily permeable to liquids and gases, and is penetrated by the thinnest tubules (plasmodesmata) connecting neighboring cells.

o Plasmodesmata are pores through which substances are exchanged between neighboring cells and cells are organized into a single whole. An analogue of gap intercellular junctions between animal cells.

Plastids (chloroplasts)- double-membrane formations with their own DNA; presumably arose from cyanobacteria as a result of fusion with a plant cell. They provide photosynthesis of ATP and organic compounds with the participation of solar energy.

The vacuole is a single-membrane sac-like structure filled with cell sap that takes part in maintaining osmotic homeostasis and cell shape. Vacuoles develop from cisterns of the endoplasmic reticulum. The membrane that encloses the vacuole is called the tonoplast. In a young plant cell, cell sap accumulates in small vacuoles; in an adult cell, the vacuoles merge, the nucleus and other organelles move to the periphery, and the vacuole occupies almost the entire volume of the cell. The composition of the cell sap includes water in which organic acids (oxalic, malic, citric, etc.), sugars (glucose, sucrose, fructose), and mineral salts (calcium nitrate, magnesium sulfate, potassium phosphate, iron salts) are dissolved. One of the important functions of vacuoles is the accumulation of ions and maintenance of turgor (turgor pressure).

Rice. 2-13. The structure of a plant cell. 1 - Golgi complex; 2 - freely located ribosomes; 3 - chloroplasts; 4 - intercellular spaces; 5 - polyribosomes (several ribosomes interconnected); 6 - mitochondria; 7 - lysosomes; 8 - granular endoplasmic reticulum; 9 - smooth endoplasmic reticulum; 10 - microtubules; 11 - plasmodesmata; 12 - cell membrane; 13 - nucleolus; 14 - nuclear membrane; 15 - pores in the nuclear envelope; 16 - cellulose shell; 17 - hyaloplasm; 18 - tonoplast; 19 - vacuole; 20 - core.

2. Osmotic properties of a plant cell

1. Place fragments of the leaves of the aquatic Vallisneria plant on a glass slide and apply a few drops of distilled water so that the Vallisneria leaves remain in the aqueous environment. Cover the object with a cover glass and examine the turgor state of the cells under a microscope. At high magnification of the microscope, rectangular cells are visible, having a colorless double-contour shell and adjacent protoplasm with green chloroplasts (Fig. 2-14).

2. Replace the water in which the plant cells are located with a hypertonic solution (8% sodium chloride). To do this, use filter paper to absorb water. from under the cover glass. Then, using a pipette, drop a hypertonic solution under the coverslip. In a hypertonic solution, cells lose water and move from a turgor state to a state of plasmolysis. The preparation shows cells in which, as a result of loss of water from the vacuoles, the protoplasm with chloroplasts is separated from the cell membrane. The contents of the cell are compressed.

3. Next, you should again replace the hypertonic solution with distilled water using the above method. When the solution is replaced, the cells are saturated with water and return to their previous turgor state, which after plasmolysis is called deplasmolysis.

Rice. 2-14. Movement of water through the cell wall of a plant cell. A - turgor; B -

plasmolysis; B - deplasmolysis.