The structure of the nuclear envelope. Cell nucleus: functions and structure

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study lesson and primary fixation new knowledge.

Lesson plan:

I. Organizing time

II. Updating of basic knowledge

III. Exploring a new topic

IV. Consolidation of the studied material

V. Homework

During the classes

I. Organizational moment. (Introductory speech of the teacher).

II. Updating of basic knowledge.

That. topic of our lesson The structure and functions of the nucleus”.

Goals and objectives of the lesson:

1. To summarize and study the material on the structure and functions of the nucleus as the most important component of the eukaryotic cell.

2. Features of eukaryotic cells. Prove that the nucleus is the control center of the cell's vital activity. The structure of nuclear pores. contents of the cell nucleus.

3.Activate cognitive activity using the technology of "key words": karyoplasm, chromatin, chromosomes, nucleolus (nucleolus). Develop test skills.

4. Analyze and establish connections and relationships between cell organelles, make comparisons, develop the ability for analytical thinking.

5. To continue the development of cognitive interest among high school students in the study of the structure of the cell, as a unit of structure and function of organisms.

6. Contribute to the development of value-semantic, general cultural, educational, cognitive, informational competencies. Competencies of personal self-improvement.

III. Explanation of new material.

Introductory word.

What organelles are shown on slide number 4? (mitochondria, chloroplasts).

Why are they considered semi-autonomous cell structures? (Contain their own DNA, ribosomes, can synthesize their own proteins).

Where else is DNA found? (In the core).

That. cell life processes will depend on the nucleus. Let's try to prove it.

Watch a fragment of the film "Cell nucleus". (Slide number 5).

The nucleus was discovered in a cell by the English botanist R. Brown in 1831.

Make a conclusion. The nucleus is the most important component of a eukaryotic cell.

The nucleus is most often located in the center of the cell, and only in plant cells with a central vacuole - in the parietal protoplasm. It can be of various forms:

• spherical;
• ovoid;
• lenticular;
• segmented (rare);
• elongated;
• spindle-shaped, as well as a different shape.

The diameter of the nucleus varies from 0.5 microns (in fungi) to 500 microns (in some eggs), in most cases it is less than 5 microns.

Most cells have one nucleus, but there are cells and organisms that contain 2 or more nuclei.

Let's remember. (Liver cells, cells of striated muscle tissue). Slide number 6.

From organisms: mushroom - mucor - several hundred, infusoria - shoe has two nuclei. Slide number 7.

Cells without nuclei: sieve tubes of the phloem of higher plants and mature mammalian erythrocytes. (Slide number 8).

Watch a fragment of the film “The structure of the nucleus” (slide No. 9, 58 sec.)

1. Formulate the functions of the kernel.
2. Describe the structure of the nuclear membrane and its functions.
3. The relationship between the nucleus and the cytoplasm.
4. Kernel content.

The nucleus in the cell is distinguishable only in the interphase (interphase nucleus) - the period between its divisions.

Functions:(slide number 10)

1. Stores genetic information contained in DNA and transfers it to daughter cells during cell division.

2. Controls the vital activity of the cell. Regulates metabolic processes occurring in the cell.

We consider Fig. “The structure of the nucleus” (slide 11)

We draw up a diagram: students draw up on their own, check slide 12.

Consider the nuclear envelope (slide 13)

The nuclear envelope consists of an outer and an inner membrane. The shell is pierced nuclear pores. We conclude that the nucleus is a two-membrane structure of the cell.

Working with fig. 93. p. 211. (Textbook by I.N. Ponomarev, O.A. Kornilov, L.V. Simonov, (slide 14), we analyze the structure and functions of the nuclear membrane.

Separates the nucleus from the cytoplasm of the cell;

The outer shell passes into the ER and carries ribosomes, can form protrusions.

The nuclear plate (lamina) underlies the inner membrane, takes part in the fixation of chromatin - terminal and other parts of chromosomes can be attached to it.

The perinuclear space is the space between the membranes.

The pores carry out selective transport of substances from the nucleus to the cytoplasm and from the cytoplasm to the nucleus. The number of pores is not constant and depends on the size of the nuclei and their functional activity.

Transport of substances through the pores (slide 15).

Passive transport: sugar molecules, salt ions.

Active and selective transport: proteins, ribosome subunits, RNA.

Getting acquainted with the pore complex, p. 212. Fig. 94 (slides 16,17).

We conclude: the function of the nuclear membrane is the regulation of the transport of substances from the nucleus to the cytoplasm and from the cytoplasm to the nucleus.

Core content (slide 18,19,20) .

Nuclear juice (nucleoplasm, or karyoplasm, karyolymph) is a structureless mass surrounding chromatin (chromosomes) and nucleoli. Similar to the cytosol (hyaloplasm) of the cytoplasm. Contains various RNA and enzyme proteins, unlike hyaloplasm contains a high concentration of Na, + K +, Cl - ions; lower content of SO 4 2- .

Functions of the nucleoplasm:

• fills the space between nuclear structures;
• participates in the transport of substances from the nucleus to the cytoplasm and from the cytoplasm to the nucleus;
• regulates DNA synthesis during replication, mRNA synthesis during transcription

Chromatin has the form of clumps, granules and filaments (slide 20.21).

The chemical composition of chromatin: 1) DNA (30–45%), 2) histone proteins (30–50%), 3) non-histone proteins (4–33%), therefore, chromatin is a deoxyribonucleoprotein complex (DNP).

Chromatin is a form of existence of genetic material in interphase cells. In a dividing cell, DNA strands coil (condensation of chromatin), forming chromosomes.

The chromosomes of the nucleus make up its chromosome set - karyotype.

Chromatin functions:

• Contains genetic material - DNA, consisting of genes that carry hereditary information;
• Carries out the synthesis of DNA (during the doubling of chromosomes in the S-period of the cell cycle), mRNA (transcription during protein biosynthesis);
• Regulates the synthesis of proteins and controls the vital activity of the cell;
• Histone proteins provide chromatin condensation.

Nucleus. The nucleus contains one or more nucleoli. They have a rounded structure (slide 22, 23)

It contains: protein - 70-80% (determines high density), RNA - 5-14%, DNA - 2-12%.

The nucleolus is a dependent structure of the nucleus. It is formed in the region of the chromosome that carries the rRNA genes. Such sections of chromosomes are called nucleolar organizers. The formation of the nucleolus of a human cell involves loops of ten individual chromosomes containing rRNA genes (nucleolar organizers). In the nucleoli, rRNA is synthesized, which, together with the protein received from the cytoplasm, forms ribosome subunits.

Secondary constriction - nucleolar organizer, contains rRNA genes, is present in one or two chromosomes in the genome.

The assembly of ribosomes in the cytoplasm is completed. During cell division, the nucleolus disintegrates, and re-forms in telophase.

Functions of the nucleolus:

Synthesis of rRNA and assembly of ribosome subunits (assembly of ribosomes from subunits in the cytoplasm is completed after they exit the nucleus);

To summarize:

The cell nucleus is the control center of the cell's vital activity.

1. Nucleus -> chromatin (DNP) -> chromosomes -> DNA molecule -> DNA section - the gene stores and transmits hereditary information.
2. The nucleus is in constant and close interaction with the cytoplasm; mRNA molecules are synthesized in it, which transfer information from DNA to the site of protein synthesis in the cytoplasm on ribosomes. However, the nucleus itself is also influenced by the cytoplasm, since the enzymes synthesized in it enter the nucleus and are necessary for its normal functioning.
3. The nucleus controls the synthesis of all proteins in the cell and through them - all physiological processes in the cell.

Even at the end of the last century, it was proved that fragments devoid of a nucleus, cut off from an amoeba or ciliate, die after a more or less short time.

In order to find out the role of the nucleus, one can remove it from the cell and observe the consequences of such an operation. If using a microneedle to remove the nucleus from a unicellular animal - an amoeba, then the cell continues to live and move, but cannot grow and dies after a few days. Therefore, the nucleus is necessary for metabolic processes (primarily for the synthesis nucleic acids and proteins) that ensure the growth and reproduction of cells.

It can be objected that it is not the loss of the nucleus that leads to death, but the operation itself. In order to find out this, it is necessary to set up an experiment with a control, i.e., subject two groups of amoebas to the same operation, with the difference that in one case the nucleus is actually removed, and in the other, a microneedle is inserted into the amoeba and moved in the cell just as it is done when the nucleus is removed, and they are removed, leaving the nucleus in the cell; this is called an “imaginary” operation. After such a procedure, amoebas recover, grow and divide; this shows that the death of amoebas of the first group was caused not by the operation as such, but by the removal of the nucleus.

Acetabularia is a single-celled organism, a giant single-nuclear cell with a complex structure (slide 26).

It consists of a rhizoid with a nucleus, a stalk and an umbrella (cap).

Amputation of the stem (rhizoid), which contains the plant's single cell nucleus. A new rhizoid is formed, which, however, does not have a nucleus. The cell can survive in favorable conditions for several months, but is no longer able to reproduce.

An enucleated (nucleated) plant is able to restore the lost parts: umbrella, rhizoid: everything except the nucleus. Such plants die after a few months. On the contrary, parts of this unicellular plant with a nucleus are able to repeatedly recover from damage.

Run the test (comment on the answer, slides 27-37 ).

1. What human cells lose their nucleus in the process of development, but continue to perform their functions for a long time?

a) nerve cells

b) cells of the inner layer of the skin

c) erythrocytes +

d) striated muscle fibers

(Erythrocyte cells. Young have a nucleus, mature ones lose it, continue to function for 120 days).

2. The main genetic information of the body is stored in:

3. The function of the nucleolus is to form:

(In the nucleolus, rRNA is synthesized, which, together with the protein coming from the cytoplasm, forms ribosomes).

4. The proteins that make up chromosomes are called:

(Histone proteins provide chromatin condensation).

5. Pores in the shell of the nucleus:

(The pores are formed by protein structures, through which the nucleus and cytoplasm are passively and selectively connected).

6. What is right?

a) in the process of cell division, the nucleoli in the nucleus disappear +

b) chromosomes are made up of DNA

c) in plant cells, the nucleus pushes the vacuole to the wall

d) histone proteins eliminate disturbances in DNA

(The nucleolus is a non-independent structure of the nucleus. It is formed on a section of the chromosome that carries rRNA genes. Such sections of chromosomes are called nucleolar organizers. Before dividing, the nucleolus disappears and then forms again).

7. Main kernel function: (2 answers)

a) management of intracellular metabolism +

b) isolation of DNA from the cytoplasm

c) storage of genetic information +

d) unification of chromosomes before spiralization

(The nucleus contains DNA, which stores and transmits genetic information, through mRNA, protein synthesis occurs on the ribosomes, the exchange of substances between the nucleus and the cytoplasm is carried out)

Choose three answers.

8. Specify the structures of eukaryotic cells in which DNA molecules are localized.

(Semi-autonomous organelles of the cell are mitochondria and chloroplasts. The nucleus that controls all life processes in the cell).

9. The nucleoli are made up of:

(protein - 70-80% (determines high density), RNA - 5-14%, DNA - 2-12%).

10. What is right?

a) nucleoli are "workshops" for the production of lysosomes

b) the outer membrane is covered with many ribosomes +

c) replication is the process of DNA self-copying +

d) ribosomal RNA is formed in the nucleoli +

Give an answer to a question.

• What is the structure and function of the kernel shell?

response elements.

1) 1. Restricts the contents of the nucleus from the cytoplasm

2) 2. Consists of outer and inner membranes, similar in structure to the plasma membrane. On the outer membrane - ribosomes, passes into the EPS.

3) 3. It has numerous pores through which the exchange of substances between the nucleus and the cytoplasm takes place.

Homework. Paragraph 46. Questions 2,4 p. 215.

Main literature.

1. I.N. Ponomareva, O.A. Kornilova, L.V. Simonova, Moscow Publishing Center "Ventana - Graf" 2013
2. V.V. Zakharov, S.G. Mamontov, I.I. Sonin General biology. Grade 10. Ed. "Drofa", Moscow 2007
3. A.A. Kamensky, E.A. Kriksunov, V.V. Pasechnik General biology grade 10-11 Ed. "Drofa" 2010
4. Krasnodembsky E.G., 2008. "General biology: A manual for high school students and university applicants"
5. Internet resources. Single collection educational resources. From Wikipedia, the free encyclopedia.

The role of the nucleus: The nucleus performs two groups of general functions: one related to the actual storage of genetic information, the other - with its implementation, with the provision of protein synthesis.

The first group includes processes associated with the maintenance of hereditary information in the form of an unchanged DNA structure. These processes are associated with the presence of so-called repair enzymes, which eliminate spontaneous damage to the DNA molecule (a break in one of the DNA chains, part of radiation damage), which keeps the structure of DNA molecules practically unchanged in a number of generations of cells or organisms. Further, reproduction or reduplication of DNA molecules takes place in the nucleus, which makes it possible for two cells to obtain exactly the same amounts of genetic information, both qualitatively and quantitatively. In the nuclei, the processes of change and recombination of genetic material occur, which is observed during meiosis (crossing over). Finally, nuclei are directly involved in the distribution of DNA molecules during cell division.

Another group of cellular processes provided by the activity of the nucleus is the creation of the actual apparatus of protein synthesis. This is not only synthesis, transcription on DNA molecules of various messenger RNA and ribosomal RNA. In the nucleus of eukaryotes, the formation of ribosome subunits also occurs by complexing ribosomal RNA synthesized in the nucleolus with ribosomal proteins that are synthesized in the cytoplasm and transferred to the nucleus.

Thus, the nucleus is not only a container of genetic material, but also a place where this material functions and reproduces. Therefore, the loss of lil, a violation of any of the functions listed above, is detrimental to the cell as a whole. Thus, a violation of repair processes will lead to a change in the primary structure of DNA and automatically to a change in the structure of proteins, which will certainly affect their specific activity, which may simply disappear or change in such a way that it will not provide cellular functions, as a result of which the cell dies. Violations of DNA replication will lead to a stop in cell reproduction or to the appearance of cells with an inferior set of genetic information, which is also detrimental to cells. The same result will lead to a violation of the distribution of genetic material (DNA molecules) during cell division. Loss as a result of damage to the nucleus or in the event of violations of any regulatory processes for the synthesis of any form of RNA will automatically lead to a halt in protein synthesis in the cell or to its gross violations.

The importance of the nucleus as a repository of genetic material and its the main role in determining phenotypic traits were established long ago. The German biologist Hammerling was one of the first to demonstrate the essential role of the nucleus. He chose the unusually large single-celled (or non-cellular) seaweed Acetabularia as the object of his experiments.

Hammerling showed that a nucleus is necessary for the normal development of the cap. In further experiments, in which the lower part containing the nucleus of one species was connected with the stalk devoid of the nucleus of another species, such chimeras always developed a cap typical of the species to which the nucleus belongs.

In evaluating this model of nuclear control, however, one should take into account the primitiveness of the organism used as an object. The transplantation method was later applied in experiments carried out in 1952 by two American researchers, Briggs and King, with cells from the frog Rana pipenis. These authors removed nuclei from unfertilized eggs and replaced them with nuclei from late blastula cells that already showed signs of differentiation. In many cases, normal adult frogs developed from recipient eggs.

Speaking of the cell nucleus, we mean the actual nuclei of eukaryotic cells. Their nuclei are built in a complex way and quite sharply differ from nuclear formations, nucleoids, prokaryotic organisms. In the latter, the nucleoids (nucleus-like structures) include a single circular DNA molecule, practically devoid of proteins. Sometimes such a DNA molecule of bacterial cells is called a bacterial chromosome, or a genophore (gene carrier). The bacterial chromosome is not separated by membranes from the main cytoplasm, but is assembled into a compact nuclear zone - a nucleoid that can be seen in a light microscope after special stains.

The term nucleus itself was first used by Brown in 1833 to refer to spherical permanent structures in plant cells. Later, the same structure was described in all cells of higher organisms.

The cell nucleus is usually one per cell (there are examples of multinucleated cells), consists of a nuclear envelope that separates it from the cytoplasm, chromatin, nucleolus, karyoplasm (or nuclear juice) (Fig.). These four main components are found in almost all non-dividing cells of eukaryotic unicellular and multicellular organisms.

The nuclei are usually spherical or ovoid; the diameter of the former is approximately 10 μm, and the length of the latter is 20 μm.

The nucleus is necessary for the life of the cell, since it regulates all its activity. This is due to the fact that the nucleus carries the genetic (hereditary) information contained in the DNA.

nuclear envelope

This structure is characteristic of all eukaryotic cells. The nuclear envelope consists of outer and inner membranes separated by a perinuclear space 20 to 60 nm wide. The nuclear envelope contains nuclear pores.

The membranes of the nuclear membrane do not differ morphologically from other intracellular membranes: they are about 7 nm thick and consist of two osmiophilic layers.

IN general view the nuclear membrane can be represented as a hollow two-layer bag that separates the contents of the nucleus from the cytoplasm. Of all the intracellular membrane components, only the nucleus, mitochondria, and plastids have this type of membrane arrangement. However, the nuclear membrane has a characteristic feature that distinguishes it from other membrane structures of the cell. This is the presence of special pores in the nuclear membrane, which are formed due to numerous fusion zones of two nuclear membranes and are, as it were, rounded perforations of the entire nuclear membrane.

The structure of the nuclear envelope

The outer membrane of the nuclear envelope, which is in direct contact with the cytoplasm of the cell, has a number of structural features that allow it to be attributed to the proper membrane system of the endoplasmic reticulum. Thus, a large number of ribosomes are usually located on the outer nuclear membrane. In most animal and plant cells, the outer membrane of the nuclear membrane does not represent a perfectly flat surface - it can form protrusions or outgrowths of various sizes towards the cytoplasm.

The inner membrane is in contact with the chromosomal material of the nucleus (see below).

The most characteristic and conspicuous structure in the nuclear envelope is the nuclear pore. The pores in the shell are formed by the fusion of two nuclear membranes in the form of rounded through holes or perforations with a diameter of 80-90 nm. The rounded through hole in the nuclear envelope is filled with intricately organized globular and fibrillar structures. The combination of membrane perforations and these structures is called the core pore complex. Thus, it is emphasized that the nuclear pore is not just a through hole in the nuclear membrane through which the substances of the nucleus and cytoplasm can communicate directly.

The complex complex of pores has octagonal symmetry. Along the border of the rounded hole in the nuclear membrane there are three rows of granules, 8 pieces each: one row lies on the side of the nucleus, the other on the side of the cytoplasm, the third is located in the central part of the pores. The granule size is about 25 nm. Fibrillar processes extend from these granules. Such fibrils extending from the peripheral granules can converge in the center and create, as it were, a partition, a diaphragm, across the pore. In the center of the hole, one can often see the so-called central granule.

The number of nuclear pores depends on the metabolic activity of the cells: the higher the synthetic processes in the cells, the more pores per unit surface of the cell nucleus.

Number of nuclear pores in various objects

Nuclear envelope chemistry

In the composition of the nuclear membranes, small amounts of DNA (0-8%), RNA (3-9%) are found, but the main chemical components are lipids (13-35%) and proteins (50-75%), which is for all cell membranes.

The composition of lipids is similar to that in the membranes of microsomes or membranes of the endoplasmic reticulum. The nuclear membranes are characterized by a relatively low content of cholesterol and a high content of phospholipids enriched in saturated fatty acids.

The protein composition of membrane fractions is very complex. Among proteins, a number of enzymes common with ER were found (for example, glucose-6-phosphatase, Mg-dependent ATPase, glutamate dehydrogenase, etc.), RNA polymerase was not found. Here, the activities of many oxidative enzymes (cytochrome oxidase, NADH-cytochrome-c-reductase) and various cytochromes were revealed.

Among the protein fractions of nuclear membranes, there are basic histone-type proteins, which is explained by the connection of chromatin regions with the nuclear envelope.

Nuclear envelope and nuclear-cytoplasmic exchange

The nuclear membrane is a system that delimits the two main cell compartments: the cytoplasm and the nucleus. The nuclear membranes are completely permeable to ions, to substances of small molecular weight, such as sugars, amino acids, nucleotides. It is believed that proteins with a molecular weight of up to 70 thousand and a size of no more than 4.5 nm can freely diffuse through the shell.

The reverse process is also known - the transfer of substances from the nucleus to the cytoplasm. This primarily concerns the transport of RNA synthesized exclusively in the nucleus.

Another way of transporting substances from the nucleus to the cytoplasm is associated with the formation of outgrowths of the nuclear membrane, which can be separated from the nucleus in the form of vacuoles, their contents are then poured out or thrown into the cytoplasm.

Thus, from the numerous properties and functional loads of the nuclear membrane, it should be emphasized its role as a barrier separating the contents of the nucleus from the cytoplasm, limiting Free access into the nucleus of large aggregates of biopolymers, a barrier that actively regulates the transport of macromolecules between the nucleus and the cytoplasm.

One of the main functions of the nuclear envelope should also be considered its participation in the creation of intranuclear order, in the fixation of chromosomal material in the three-dimensional space of the nucleus.

nuclear matrix

This complex does not represent some pure fraction, it includes components of the nuclear envelope, the nucleolus, and the karyoplasm. Both heterogeneous RNA and part of DNA turned out to be associated with the nuclear matrix. These observations gave grounds to believe that the nuclear matrix plays an important role not only in maintaining the overall structure of the interphase nucleus, but may also be involved in the regulation of nucleic acid synthesis.

Lecture no.

Number of hours: 2

CellularCORE

1. General characteristics of the interphase nucleus. Kernel functions

2.

3.

4.

1. General characteristics of the interphase nucleus

The nucleus is the most important component of the cell, which is present in almost all cells of multicellular organisms. Most cells have a single nucleus, but there are binucleated and multinucleated cells (for example, striated muscle fibers). Binuclear and multinuclear are due to the functional characteristics or pathological state of the cells. The shape and size of the nucleus are very variable and depend on the type of organism, type, age and functional state of the cell. On average, the volume of the nucleus is approximately 10% of the total volume of the cell. Most often, the nucleus has a round or oval shape ranging in size from 3 to 10 microns in diameter. Minimum size the nucleus is 1 micron (in some protozoa), the maximum is 1 mm (eggs of some fish and amphibians). In some cases, there is a dependence of the shape of the nucleus on the shape of the cell. The nucleus usually occupies a central position, but in differentiated cells it can be displaced to the peripheral part of the cell. The nucleus contains almost all of the DNA of a eukaryotic cell.

The main functions of the kernel are:

1) Storage and transfer of genetic information;

2) Regulation of protein synthesis, metabolism and energy in the cell.

Thus, the nucleus is not only a receptacle for genetic material, but also a place where this material functions and reproduces. Therefore, violation of any of these functions will lead to cell death. All this points to leading value nuclear structures in the processes of synthesis of nucleic acids and proteins.

One of the first scientists to demonstrate the role of the nucleus in the life of the cell was the German biologist Hammerling. As an experimental object, Hammerling used large unicellular algae. Acetobulariamediterranea and A.crenulata. These closely related species are well distinguished from each other by the shape of the "cap". At the base of the stem is the nucleus. In some experiments, the cap was separated from the lower part of the stem. As a result, it was found that the core is necessary for the normal development of the cap. In other experiments, a stalk with a nucleus of one species of algae was connected with a stalk without a nucleus of another species. The resulting chimeras always developed a cap typical of the species to which the nucleus belonged.

Overall plan The structure of the interphase nucleus is the same in all cells. The core is made up of nuclear membrane, chromatin, nucleoli, nuclear protein matrix and karyoplasm (nucleoplasm). These components are found in almost all nondividing cells of eukaryotic unicellular and multicellular organisms.

2. Nuclear envelope, structure and functional significance

Nuclear envelope (karyolemma, karyotheca) consists of outer and inner nuclear membranes with a thickness of 7 nm. Between them is perinuclear space width from 20 to 40 nm. The main chemical components of the nuclear membrane are lipids (13-35%) and proteins (50-75%). Small amounts of DNA (0-8%) and RNA (3-9%) are also found in the composition of the nuclear membranes. The nuclear membranes are characterized by a relatively low content of cholesterol and a high content of phospholipids. The nuclear membrane is directly connected with the endoplasmic reticulum and the contents of the nucleus. Network-like structures adjoin to it on both sides. The network-like structure lining the inner nuclear membrane looks like a thin shell and is called nuclear lamina. The nuclear lamina supports the membrane and is in contact with chromosomes and nuclear RNA. The network-like structure surrounding the outer nuclear membrane is much less compact. The outer nuclear membrane is littered with ribosomes involved in protein synthesis. The nuclear envelope contains numerous pores with a diameter of about 30-100 nm. The number of nuclear pores depends on the cell type, the stage of the cell cycle, and the specific hormonal situation. So the more intense the synthetic processes in the cell, the more pores there are in the nuclear envelope. Nuclear pores are rather labile structures, i.e., depending on external influences, they are able to change their radius and conductivity. The pore opening is filled with complexly organized globular and fibrillar structures. The combination of membrane perforations and these structures is called the nuclear pore complex. The complex complex of pores has octagonal symmetry. Three rows of granules, 8 pieces in each, are located along the border of the rounded hole in the nuclear membrane: one row is a tool for constructing conceptual models of the side of the nucleus, the other is a tool for constructing conceptual models of the side of the cytoplasm, the third is located in the central part of the pores. The granule size is about 25 nm. Fibrillar processes extend from the granules. Such fibrils extending from the peripheral granules can converge in the center and create, as it were, a partition, a diaphragm, across the pore. In the center of the hole, one can often see the so-called central granule.

Nuclear cytoplasmic transport

The process of substrate translocation through the nuclear pore (for the case of import) consists of several stages. At the first stage, the transported complex anchors on the fibril facing the cytoplasm. Then the fibril bends and moves the complex to the entrance to the nuclear pore channel. The actual translocation and release of the complex into the karyoplasm takes place. The reverse process is also known - the transfer of substances from the nucleus to the cytoplasm. This primarily concerns the transport of RNA synthesized exclusively in the nucleus. There is also another way of transferring substances from the nucleus to the cytoplasm. It is associated with the formation of outgrowths of the nuclear membrane, which can be separated from the nucleus in the form of vacuoles, and then their contents are poured out or ejected into the cytoplasm.

Thus, the exchange of substances between the nucleus and the cytoplasm is carried out in two main ways: through the pores and by lacing.

Functions of the nuclear envelope:

1. Barrier.This function is to separate the contents of the nucleus from the cytoplasm. As a result, the processes of RNA/DNA synthesis from protein synthesis turn out to be spatially separated.

2. Transport.The nuclear envelope actively regulates the transport of macromolecules between the nucleus and the cytoplasm.

3. Organizing.One of the main functions of the nuclear envelope is its participation in the creation of intranuclear order.

3. The structure and functions of chromatin and chromosomes

Hereditary material can be in the cell nucleus in two structural and functional states:

1. Chromatin.This is a decondensed, metabolically active state designed to provide transcription and reduplication processes in the interphase.

2. Chromosomes.This is the most condensed, compact, metabolically inactive state, designed to distribute and transport genetic material to daughter cells.

Chromatin.In the nucleus of cells, zones of dense substance are revealed, which are well stained with basic dyes. These structures are called "chromatin" (from the Greek "chromo"– color, paint). The chromatin of the interphase nuclei is the chromosomes that are in a decondensed state. The degree of decondensation of chromosomes can be different. The zones of complete decondensation are called euchromatin. With incomplete decondensation, areas of condensed chromatin, called heterochromatin. The degree of chromatin decondensation in the interphase reflects the functional load of this structure. The more "diffuse" the chromatin is distributed in the interphase nucleus, the more intense the synthetic processes in it. DecreaseRNA synthesis in cells is usually accompanied by an increase in condensed chromatin zones.Maximum condensation of condensed chromatin is achieved during mitotic cell division. During this period, the chromosomes do not perform any synthetic functions.

Chemically, chromatin consists of DNA (30-45%), histones (30-50%), non-histone proteins (4-33%) and a small amount of RNA.The DNA of eukaryotic chromosomes is a linear molecule consisting of tandem (one after the other) arranged replicons. different size. The average replicon size is about 30 µm. Replicons are sections of DNA that are synthesized as independent units. Replicons have start and end points for DNA synthesis. RNA is all known cellular types of RNA in the process of synthesis or maturation. Histones are synthesized on polysomes in the cytoplasm, and this synthesis begins somewhat earlier than DNA replication. Synthesized histones migrate from the cytoplasm to the nucleus, where they bind to DNA regions.

Structurally, chromatin is a filamentous complex molecules of deoxyribonucleoprotein (DNP), which consist of DNA associated with histones. The chromatin filament is a double helix of DNA surrounding the histone core. It is made up of repeating units called nucleosomes. The number of nucleosomes is huge.

Chromosomes(from the Greek chromo and soma) are the organelles of the cell nucleus, which are carriers of genes and determine the hereditary properties of cells and organisms.

Chromosomes are rod-shaped structures of varying length with a fairly constant thickness. They have a zone of primary constriction, which divides the chromosome into two arms.Chromosomes with equal numbers are called metacentric, with arms of unequal length - submetacentric. Chromosomes with a very short, almost imperceptible second arm are called acrocentric.

In the region of the primary constriction, there is a centromere, which is a lamellar structure in the form of a disk. Bundles of microtubules of the mitotic spindle are attached to the centromere and run towards the centrioles. These bundles of microtubules are involved in the movement of chromosomes to the poles of the cell during mitosis. Some chromosomes have a secondary constriction. The latter is usually located near the distal end of the chromosome and separates a small area, the satellite. Secondary constrictions are called nucleolar organizers. The DNA responsible for the synthesis of rRNA is localized here. The arms of chromosomes terminate in telomeres, the end segments. The telomeric ends of chromosomes are not able to connect with other chromosomes or their fragments. In contrast, the broken ends of chromosomes can join the same broken ends of other chromosomes.

The size of chromosomes in different organisms varies widely. So, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi. The longest are in some orthopteran insects, in amphibians and in lilies. The length of human chromosomes is in the range of 1.5-10 microns.

The number of chromosomes in different objects also varies significantly, but is typical for each species of animal or plant. In some radiolarians, the number of chromosomes reaches 1000-1600. The record holder among plants in terms of the number of chromosomes (about 500) is the grass fern, 308 chromosomes in the mulberry tree. The smallest number of chromosomes (2 per diploid set) is observed in the malarial plasmodium, horse roundworm. Humans have 46 chromosomesin chimpanzee, cockroach and pepper– 48, fruit fly Drosophila - 8, house fly - 12, carp - 104, spruce and pine - 24, pigeon - 80.

Karyotype (from the Greek. Karion - kernel, nut kernel, operators - sample, shape) - a set of features of the chromosome set (number, size, shape of chromosomes) characteristic of a particular species.

Individuals of different sexes (especially in animals) of the same species can differ in the number of chromosomes (the difference is most often on one chromosome). Even in closely related species, chromosome sets differ from each other either in the number of chromosomes or in the size of at least one or more chromosomes.Therefore, the structure of the karyotype can be a taxonomic trait.

In the second half of the 20th century, the practice of chromosome analysis began to be introduced methods of differential staining of chromosomes. It is believed that the ability of individual sections of chromosomes to stain is associated with their chemical differences.

4. Nucleus. Karyoplasm. Nuclear protein matrix

The nucleolus (nucleolus) is an essential component of the cell nucleus of eukaryotic organisms. However, there are some exceptions. Thus, nucleoli are absent in highly specialized cells, in particular, in some blood cells. The nucleolus is a dense round body with a size of 1-5 microns. Unlike cytoplasmic organelles, the nucleolus does not have a membrane that would surround its contents. The size of the nucleolus reflects the degree of its functional activity, which varies widely in different cells. The nucleolus is a derivative of the chromosome. The nucleolus consists of protein, RNA and DNA. The concentration of RNA in the nucleoli is always higher than the concentration of RNA in other components of the cell. Thus, the concentration of RNA in the nucleolus can be 2-8 times higher than in the nucleus, and 1-3 times higher than in the cytoplasm. Due to the high content of RNA, the nucleoli stain well with basic dyes. The DNA in the nucleolus forms large loops called nucleolar organizers. The formation and number of nucleoli in the cells depend on them. The nucleolus is heterogeneous in its structure. It has two main components: granular and fibrillar. The diameter of the granules is about 15-20 nm, the thickness of the fibrils– 6-8 nm. The fibrillar component can be concentrated in the central part of the nucleolus, and the granular component - along the periphery. Often the granular component forms filamentous structures - nucleolonemes with a thickness of about 0.2 μm. The fibrillar component of the nucleoli is the ribonucleoprotein strands of ribosome precursors, and the granules are the maturing ribosome subunits. The function of the nucleolus is to form ribosomal RNA (rRNA) and ribosomes, on which polypeptide chains are synthesized in the cytoplasm. The mechanism of ribosome formation is as follows: a rRNA precursor is formed on the DNA of the nucleolar organizer, which is dressed with a protein in the nucleolus zone. Ribosome subunits are assembled in the nucleolus. In actively functioning nucleoli, 1500-3000 ribosomes are synthesized per minute. Ribosomes from the nucleolus through pores in the nuclear envelope enter the membranes of the endoplasmic reticulum. The number and formation of nucleoli is associated with the activity of nucleolar organizers. Changes in the number of nucleoli can occur due to the fusion of nucleoli or due to shifts in the chromosomal balance of the cell. Nuclei usually contain several nucleoli. The nuclei of some cells (newt oocytes) contain a large number of nucleoli. This phenomenon has been named amplification. It lies in the organization of quality management systems, which occurs overreplication of the nucleolar organizer zone, numerous copies move away from the chromosomes and become additionally working nucleoli. Such a process is necessary for the accumulation of a huge number of ribosomes per egg. This ensures the development of the embryo in the early stages, even in the absence of the synthesis of new ribosomes. The supernumerary nucleoli disappear after maturation of the egg cell.

The fate of the nucleolus during cell division. As rRNA synthesis decays in prophase, the nucleolus loosens and ready-made ribosomes emerge into the karyoplasm and then into the cytoplasm. During chromosome condensation, the fibrillar component of the nucleolus and part of the granules are closely associated with their surface, forming the basis of the matrix of mitotic chromosomes. This fibrillar-granular material is transferred by chromosomes to daughter cells. In early telophase, as chromosomes decondense, matrix components are released. Its fibrillar part begins to assemble into numerous small associates - prenucleoli, which can combine with each other. As RNA synthesis resumes, the prenucleoli transform into normally functioning nucleoli.

Karyoplasm(from Greek.< карион > – walnut, nut kernel), or nuclear juice, in the form of a structureless semi-liquid mass, surrounds chromatin and nucleoli. Nuclear sap contains proteins and various RNAs.

Nuclear protein matrix (nuclear skeleton) - frame intranuclear system, which serves to maintain the overall structure of the interphase core of the union of all nuclear components. It is an insoluble material remaining in the nucleus after biochemical extractions. It does not have a clear morphological structure and consists of 98% proteins.

The nucleus is found in every eukaryotic cell. There may be one nucleus, or there may be several nuclei in a cell (depending on its activity and function).

The cell nucleus consists of a membrane, nuclear juice, nucleolus and chromatin. The nuclear envelope consists of two membranes separated by a perinuclear (perinuclear) space, between which there is a liquid. The main functions of the nuclear membrane are the separation of genetic material (chromosomes) from the cytoplasm, as well as the regulation of bilateral relationships between the nucleus and the cytoplasm.

The nuclear envelope is permeated with pores that have a diameter of about 90 nm. The pore area (pore complex) has a complex structure (this indicates the complexity of the mechanism for regulating the relationship between the nucleus and the cytoplasm). The number of pores depends on the functional activity of the cell: the higher it is, the more pores (there are more pores in immature cells).

The basis of nuclear juice (matrix, nucleoplasm) is proteins. Juice forms the internal environment of the nucleus, plays an important role in the work of the genetic material of cells. Proteins: filamentous or fibrillar (support function), heteronuclear RNA (products of primary transcription of genetic information) and mRNA (processing result).

The nucleolus is the structure where the formation and maturation of ribosomal RNA (rRNA) takes place. rRNA genes occupy certain areas several chromosomes (in humans, these are 13–15 and 21–22 pairs), where nucleolar organizers are formed, in the region of which the nucleoli themselves are formed. In metaphase chromosomes, these areas are called secondary constrictions and look like constrictions. Electron microscopy revealed filamentous and granular components of the nucleoli. Filamentous (fibrillar) is a complex of proteins and giant rRNA precursor molecules, which subsequently give rise to smaller molecules of mature rRNA. During maturation, the fibrils are transformed into ribonucleoprotein granules (granular component).

Chromatin got its name for its ability to stain well with basic dyes; in the form of clumps, it is scattered in the nucleoplasm of the nucleus and is an interphase form of the existence of chromosomes.

Chromatin consists mainly of DNA strands (40% of the mass of the chromosome) and proteins (about 60%), which together form the nucleoprotein complex. There are histone (five classes) and non-histone proteins.

Histones (40%) have regulatory (strongly connected to DNA and prevent reading information from it) and structural functions (organization of the spatial structure of the DNA molecule). Non-histone proteins (more than 100 fractions, 20% of the chromosome mass): enzymes of RNA synthesis and processing, DNA replication repair, structural and regulatory functions. In addition, RNA, fats, polysaccharides, and metal molecules were found in the composition of chromosomes.

Depending on the state of chromatin, euchromatic and heterochromatic regions of chromosomes are distinguished. Euchromatin is less dense and genetic information can be read from it. Heterochromatin is more compact, and information cannot be read within it. There are constitutive (structural) and facultative heterochromatin.

5. Structure and functions of semi-autonomous cell structures: mitochondria and plastids

Mitochondria (from Gr. mitos - “thread”, chondrion - “grain, grain”) are permanent membrane organelles of a round or rod-shaped (often branching) shape. Thickness - 0.5 microns, length - 5-7 microns. The number of mitochondria in most animal cells is 150-1500; in female eggs - up to several hundred thousand, in spermatozoa - one helical mitochondria twisted around the axial part of the flagellum.

The main functions of mitochondria:

1) play the role of energy stations of cells. The processes of oxidative phosphorylation (enzymatic oxidation of various substances with subsequent accumulation of energy in the form of molecules of adenosine triphosphate - ATP) proceed in them;

2) store hereditary material in the form of mitochondrial DNA. Mitochondria require the proteins encoded in the nuclear DNA genes to function, since their own mitochondrial DNA can provide the mitochondria with only a few proteins.

Side functions - participation in the synthesis of steroid hormones, some amino acids (for example, glutamine). The structure of mitochondria

Mitochondria have two membranes: outer (smooth) and inner (forming outgrowths - leaf-shaped (cristae) and tubular (tubules)). Membranes differ in chemical composition, set of enzymes and functions.

In mitochondria, the internal content is a matrix - a colloidal substance in which grains with a diameter of 20–30 nm were found using an electron microscope (they accumulate calcium and magnesium ions, reserves of nutrients, for example, glycogen).

The matrix houses the organelle protein biosynthesis apparatus: 2–6 copies of circular DNA devoid of histone proteins (like in prokaryotes), ribosomes, a set of t-RNA, enzymes of reduplication, transcription, translation of hereditary information. This apparatus as a whole is very similar to that of prokaryotes (in terms of the number, structure and size of ribosomes, the organization of its own hereditary apparatus, etc.), which confirms the symbiotic concept of the origin of the eukaryotic cell.

Both the matrix and the surface of the inner membrane are actively involved in the implementation of the energy function of mitochondria, on which the electron transport chain (cytochromes) and ATP synthase are located, which catalyzes the phosphorylation of ADP coupled with oxidation, which converts it into ATP.

Mitochondria multiply by ligation, so during cell division they are more or less evenly distributed between daughter cells. Thus, succession is carried out between the mitochondria of cells of successive generations.

Thus, mitochondria are characterized by relative autonomy within the cell (unlike other organelles). They arise during the division of maternal mitochondria, have their own DNA, which differs from the nuclear system of protein synthesis and energy storage.

plastids

These are semi-autonomous structures (they can exist relatively autonomously from the cell's nuclear DNA) that are present in plant cells. They are formed from proplastids, which are present in the embryo of the plant. Delimited by two membranes.

There are three groups of plastids:

1) leukoplasts. They are round, not colored and contain nutrients (starch);

2) chromoplasts. They contain molecules of coloring substances and are present in the cells of colored plant organs (fruits of cherries, apricots, tomatoes);

3) chloroplasts. These are the plastids of the green parts of the plant (leaves, stems). In structure, they are in many ways similar to the mitochondria of animal cells. The outer membrane is smooth, the inner one has outgrowths - lamelosomes, which end in thickenings - thylakoids containing chlorophyll. The stroma (liquid part of the chloroplast) contains a circular DNA molecule, ribosomes, reserve nutrients (starch grains, fat drops).

The nucleus is surrounded by a membrane consisting of two membranes

The outer nuclear membrane is a continuation of the ER membranes, and the perinuclear space (lumen) passes into the lumen of the ER

Numerous NPCs are present in the nuclear envelope, which are the only channels for the exchange of molecules and macromolecules between the nucleus and the cytoplasm

Core surrounded by a shell consisting of two concentrically arranged outer and inner nuclear membranes. Each membrane contains a specific set of proteins and a continuous double layer of phospholipids. With the exception of some unicellular eukaryotes, the inner nuclear membrane is supported by a network of filaments anchored in a reticulate structure. This network of filaments is called the nuclear lamina.

Outdoor nuclear membrane passes into the ER membranes and, like most of its membranes, is covered with ribosomes involved in protein synthesis. The figure below shows the connection of the outer membrane with the EPR.

Space between outside and inside nuclear membranes is the perinuclear space (PP). Just as the outer membrane is connected to the membrane, the nuclear envelope PP contacts the inner space of the ER. The thickness of each of the two membranes is 7-8 nm (nm), and the width of the nuclear envelope PP is 20-40 nm.

In the study of preparations of the nuclear envelope in the electronic microscope, the most noticeable feature of the structure is the NPCs (nuclear pore complexes), which serve as channels for the transport of most molecules between the nucleus and the cytoplasm. The shell of the nuclei of most cells contains about 10-20 NPCs per square micron of surface. Thus, yeast cells contain 150-250 NPCs, and mammalian somatic cells 2000-4000.

However, some cells have a much higher pore density, probably because they are characterized by a high intensity of transcription and translation processes, which implies the transport of a large number of macromolecules in and out of the nucleus. For example, the surface of the nucleus of amphibian oocytes is almost completely covered with NPCs.

How could there be double nuclear membrane? In a eukaryotic cell, mitochondria and chloroplast also have a double membrane. According to the endosymbiosis hypothesis, these organelles were formed during evolution, when some cells captured others in the process of endocytosis. Then the absorbed cells were surrounded by two membranes: their own and the membrane of the host cell. It turned out that some of the absorbed cells exhibit metabolic activity, for example, unlike host cells, they are able to carry out photosynthesis.

The most compelling evidence for endosymbiotic origin of mitochondria and chloroplasts lies in the fact that the ribosomes of both organelles are more reminiscent of the ribosomes of modern prokaryotes, and to a lesser extent, these same microstructures of the cytoplasm of a eukaryotic cell. Much less clear is the origin of the nucleus. However, the existence of a double nuclear membrane, like that of mitochondria and chloroplasts, suggests that the captured prokaryotic cell has evolved into a nucleus containing all of the cellular DNA.

The nuclear envelope is connected to endoplasmic reticulum(EPR). The surface of the nuclear membrane of the Xenopus laevis oocyte is covered with complexes of nuclear pores.
The nucleus could have formed as a result of endosymbiosis, a process
in which one prokaryotic cell captures another cell; then the captured cell becomes a primitive nucleus.