Features of the structure of the structure and function of viruses. Viruses have a cellular structure

The structure of viruses

1) Viruses do not have a cellular structure. Each viral particle consists of a centrally located carrier of genetic information and a shell. The genetic material is a short nucleic acid molecule, this forms the core of the virus. Nucleic acid in different viruses can be represented by DNA or RNA, and these molecules can have an unusual structure: single-stranded DNA and two-stranded RNA are found.

2) The shell is called capsid.

The capsid performs several functions.

Protection of the genetic material (DNA or RNA) of the virus from mechanical and chemical damage.

Determining the potential to infect a cell.

At the initial stages of cell infection: attachment to the cell membrane, rupture of the membrane and introduction of the genetic material of the virus into the cell.

particles of tobacco mosaic virus, wart-causing virus, and adenovirus

It is formed by subunits - capsomeres, each of which consists of one or two protein molecules. The number of capsomeres for each virus is constant (there are 60 of them in the capsid of the polio virus, and 2130 in the tobacco mosaic virus). Sometimes a nucleic acid together with a capsid is called a nucleocapsid. If the viral particle, apart from the capsid, no longer has an envelope, it is called a simple virus, if there is one more - external, the virus is called complex.

3) Outer sheath is also called supercapsid, genetically it does not belong to the virus, but originates from the plasma membrane of the host cell and is formed when the assembled viral particle leaves the infected cell. organized by a double layer of lipids and specific viral proteins, most often forming spike outgrowths penetrating the lipid bilayer. Such viruses are called "dressed". They perform protective functions in the virion, the function of attaching to a susceptible cell and penetrating into its cytoplasm, determines many characteristics of the virus (antigenic properties, sensitivity to damaging factors, etc.) .- influenza and herpes viruses

4) For each virus, the capsid capsomeres are arranged in a strictly defined order, due to which a certain type of symmetry arises. With helical symmetry, the capsid acquires a tubular (tobacco mosaic virus) or spherical (RNA-containing animal viruses) shape. With cubic symmetry, the capsid has the shape of an icosahedron (twenty-sided), isometric viruses have such symmetry. In the case of combined symmetry, the capsid has a cubic shape, and the nucleic acid located inside is spirally stacked. The correct geometry of the capsid even allows viral particles to form crystalline structures together.

In nature, viruses exist in two forms: extracellular and intracellular.

Extracellular form of the virus called Virion - it is an inert infectious particle that consists of a nucleic acid and a protein shell - capsid . Nucleic acid in the composition of the virion - the genetic apparatus or genome - can be of only one type - either DNA or RNA. The genome can be represented by one chain (single-component or integral genome) or there are several of them (fragmented genome). Most plant viruses are RNA-containing.

capsid composed of protein subunits Capsomeres. Capsids come in various shapes:

1). Isometric: spherical (fig.17 BUT) or polyhedral (“polyhedron” means polyhedron) with a cubic type of symmetry (Fig. 17 B).

2). Anisometric with a spiral type of symmetry - rod-shaped, filiform (Fig. 17 G). There are viruses with a combined type of symmetry, for example, in the form of a tadpole or Bacillus(fig.17 D).

The sizes of various viruses most often range from 20 to 300 nm, but there are filamentous viruses of greater length - up to 2000 nm.

Due to the presence of a protein shell in plant viruses, which contains a nucleic acid, the viruses have antigenic activity, or immunogenicity, that is, they are able to cause the formation of antibodies when they are introduced into the body of animals.

Virus manifestations.

Viruses reproduce only in living cells. Many viruses are capable of infecting any one host. So respiratory viruses multiply only in the cells of the mucous membranes respiratory tract. Others, such as the tobacco mosaic virus (TMV), have a wide range of hosts. Some plant viruses are able to replicate in the bodies of insect vectors.

a )spherical B)polyhedral

G ) rod-shaped D) bacillus

Fig.17. Types of morphology of virus capsids

Intracellular activity of viruses , probably consists of a number of the following steps:

1. The virus enters the cell entirely - completely the entire NC in the capsid shell - through damage in the membrane.

2. Dropping the capsid . When infected with TMV, the first symptoms appear several hours later than when infected with free RNA of this virus. This is a fact in favor of the statement that the virus that has entered the cell "undresses" - sheds the capsid.

3. Reproduction of viruses . Viral RNA is more often introduced into the nucleus of a plant cell, where a complementary RNA-(¾)-strand is synthesized and Double-stranded RNA - replicative form (RF). Then, probably, multiple replication of viral RNA occurs in the nucleoli.

4. Biosynthesis of the structural protein of the virus . After increased replication of viral RNA in the cell, the amount of capsid protein increases. The synthesis of these proteins takes place on the ribosomes of the host cell.

5. Aggregation of viral RNA and capsid . The appearance of mature viral particles.

6. The release of viruses from the cell in plants occurs through plasmodesmata, in animals through damage to the membrane.

In the centuries-old history of our planet, invisible invaders constantly intervened in the development of all flora and fauna -viruses(lat. virus - poison).
Due to their microscopic size, viruses do not have such a complex internal multicellular structure as in living organisms, since they are several times smaller than any living cell and even much smaller than any bacteria. All known living organisms are affected by viruses, not only people, animals, reptiles and fish, but also all kinds of plants.
Only at the beginning of the 20th century, after the invention of the electron microscope, scientists were able to see with their own eyes the tiny pathogens about which a great many theories had already been expressed until that moment. Certain human viruses differed in shape and size. Depending on the type of disease, the symptoms various diseases manifest themselves in different ways: skin becomes inflamed, internal organs or joints.

Viral infection

In 1852, Dmitry Iosifovich Ivanovsky (a Russian botanist) managed to obtain an infectious extract from tobacco plants that had been infected with mosaic disease. This structure is called the tobacco mosaic virus.

The structure of the virus

In the very center of the viral particle is the genome (hereditary information, which is represented by DNA or RNA structure - position 1). Around the genome is a capsid (position 2), which is represented by a protein shell. On the surface of the protein shell of the capsid is the lipoprotein shell (position 3). Inside the shell are capsomeres (position 4). Each capsomere consists of one or two protein filaments. The number of capsomeres for each virus is strictly constant. Each virus contains a certain number of capsomeres, so their number different types virus
is significantly different. Some viruses do not have a protein shell (capsid) in their structure. Such viruses are called simple. Conversely, viruses that in their structure have one more outer (additional lipoprotein) shell are called complex. Viruses have two life forms. The extracellular life form of a virus is called varion(state of rest, waiting). The intracellular life form of the virus, which actively reproduces, is called vegetative.

Properties of viruses

Viruses do not have a cellular structure, they are classified as the smallest living organisms, reproduce inside cells, have a simple structure, most of them cause various diseases, each type of virus recognizes and infects only certain types of cells, contain only one type of nucleic acid (DNA or RNA) .

Virus classification

How body cells absorb substances

Unlike other living organisms, a virus needs living cells to reproduce. By itself, it does not know how to reproduce. For example, the cells of the human body consist of a nucleus (it contains DNA - a genetic map, a plan of action for the cell to maintain its vital activity). The cell nucleus is surrounded by cytoplasm, in which mitochondria are located (they produce energy for chemical reactions, lysosomes (they break down materials that come from outside), polysomes and ribosomes (they produce proteins and enzymes to carry out chemical reactions that occur in the cell). the cytoplasm of the cell, or rather its space, is permeated with a network of tubules through which the necessary substances are absorbed, as well as unnecessary ones are removed.The cell is also surrounded by a membrane that protects it and acts as a two-way filter.The cell membrane constantly vibrates.If there is a protein corpuscle on the membrane surface, it bends and encloses it in a digestive vesicle, which draws it into the cell.Next, the brain center of the cell (nucleus) recognizes the substance that has come from outside and gives a series of commands to the centers that are located in the cytoplasm.They decompose the incoming substance into simpler compounds.Some of the useful compounds are used to maintain life and execution for programmed functions, and unnecessary connections are brought out of the cell. This is how the process of absorption, digestion, assimilation of substances in the cell and the removal of unnecessary outwards is carried out.

Reproduction of viruses

As noted above, a virus needs living cells to reproduce its own kind, because by itself it cannot reproduce. The process of penetration of the virus into the cell consists of several stages.

The first stage of penetration of the virus into the cell is its deposition (adsorption through electrical interaction) on the surface of the target cell. The target cell must, in turn, possess the corresponding surface receptors. Without the presence of appropriate surface receptors, the virus cannot attach itself to the cell. Therefore, such a virus that has joined the cell as a result of electrical interaction can be removed by shaking. The second stage of penetration of the virus into the cell is called irreversible. In the presence of appropriate receptors, the virus attaches to the cell and protein spikes or threads begin to interact with the cell's receptors. A protein or glycoprotein, which is usually specific for each virus, acts as a cell receptor.

During the third stage, the virus is absorbed (moves) in the cell membrane with the help of intracellular membrane vesicles.

In the fourth stage, cell enzymes cleave viral proteins, and thus are released from the "imprisonment" of the virus genome, which contains hereditary information, which is represented by a DNA or RNA structure. The RNA helix then quickly unfolds and rushes into the cell nucleus. In the cell nucleus, the virus genome changes the cell's genetic information and implements its own. As a result of such changes, the work of the cell is completely disorganized and instead of the proteins and enzymes it needs, the cell begins to synthesize viral (modified) proteins and enzymes.

The time elapsed from the moment the virus enters the cell until the release of new varions is called the latent or latent period. It can vary from several hours (smallpox, influenza) to several days (measles, adenovirus).

2.4.1. Opening

In 1852, the Russian botanist D.I. Ivanovsky was the first to obtain an infectious extract from tobacco plants affected by mosaic disease. When such an extract was passed through a retaining filter, the filtered liquid still retained infectious properties. In 1898, the Dutchman Beijerinck coined the new word "virus" (from the Latin word for "poison") to denote the infectious nature of certain filtered plant fluids. Although considerable progress had been made in obtaining highly purified samples of viruses and it was found that they were chemically nucleoproteins (complex compounds consisting of and nucleic acids), the particles themselves were still elusive and mysterious, because they were too small to be detected. was to be seen with the help of a light. That is why viruses were among the first biological structures that were studied in the electron microscope immediately after its invention in the thirties of the XX century.

2.4.2. Properties of viruses

Viruses have the following properties.

We'll look at these properties in more detail below.

Dimensions

Viruses are the smallest living organisms, the size of which varies from 20 to 300 nm; on average they are fifty times smaller. They cannot be seen with a light microscope and pass through filters that keep bacteria out.

Origin

Researchers often wonder if viruses are alive? If any structure that has genetic material (DNA or RNA) and is capable of self-replication is considered alive, then the answer should be in the affirmative: yes, viruses are alive. If, however, the presence of a cellular structure is considered a sign of the living, then the answer will be negative: viruses are not alive. To this it should be added that outside the host cell, viruses are incapable of self-replication.

For a more complete understanding of viruses, it is necessary to know their origin in the evolutionary process. There is an assumption, although not proven, that viruses are genetic material that once “escaped” from prokaryotic and eukaryotic cells and retained the ability to reproduce when returned to the cellular environment. Outside the cell, viruses are in a completely inert state, but they have a set of instructions (the genetic code) necessary to re-enter the cell and, having subordinated it to their instructions, make many copies identical to themselves (the virus). Therefore, it is logical to assume that in the process of evolution, viruses appeared later than cells.

Structure

The structure of viruses is very simple. They consist of the following structures:

1. core- genetic material represented by either DNA or RNA; DNA or RNA may be single or double stranded;
2. capsid- a protective protein shell surrounding the core;
3. nucleocapsid- a complex structure formed by the core and capsid;
4. shells- some viruses, such as HIV and influenza viruses, have an additional lipoprotein layer derived from the plasma membrane of the host cell;
5. capsomeres- identical repeating subunits, from which capsids are often built.

Rice. 2.16. Schematic representation of a virus in a section.

The general shape of the capsid is characterized by a high degree of symmetry, which determines the ability of viruses to crystallize. This makes it possible to study them both by X-ray crystallography and by electron microscopy. As soon as the virus subunits are formed in the host cell, they can immediately self-assemble into a complete viral particle by self-assembly. A simplified diagram of the structure of the virus is shown in Fig. 2.16.



Rice. 2.17. A. Icosahedron. B. Electron micrograph of the herpes simplex virus, obtained by the method of negative contrast (not the preparation itself is stained, but its background). Pay attention to how clearly the details of the structure of the virus are visible. Individual capsomeres are visible just where the dye has penetrated between them.

The structure of the capsid is characterized by certain types of symmetry, especially polyhedral and helical. A polyhedron is a polyhedron. The most common polyhedral shape in viruses is the icosahedron, which has 20 triangular faces, 12 corners, and 30 edges. On fig. 2.17, And we see a regular icosahedron, and in fig. 2.17, B - herpes virus, in the particle of which 162 capsomeres are organized into an icosahedron.



Rice. 2.18. A. The structure of the tobacco mosaic virus (TMV); the helical symmetry of the capsid is visible. Only part of the rod-shaped virus is shown. The figure is built on the basis of the results of X-ray structural analysis, biochemical data and electron microscopic studies. B. Electron micrograph of tobacco mosaic virus, obtained by the method of negative contrast (x 800,000). The capsid (shell) is formed by 2130 identical protein capsomeres. B. Tobacco plant infected with TMV. Pay attention to the characteristic spots in those places where the leaf tissue dies off.

A clear illustration of spiral symmetry can be seen in Fig. 2.18, B RNA-containing tobacco mosaic virus (TMV). The capsid of this virus is formed by 2130 identical protein capsomeres. TMV was the first virus isolated in its pure form. When infected with this virus, yellow specks appear on the leaves of a diseased plant - the so-called leaf mosaic (Fig. 2.18, B). Viruses spread very quickly, either mechanically when diseased plants or plant parts come into contact with healthy plants, or through the air via cigarette smoke made from infected leaves.



Rice. 2.19. A. The structure of bacteriophage T2. B. Electron micrograph of a bacteriophage obtained by negative contrasting.

Viruses that attack bacteria form a group called bacteriophages or just phages. Some bacteriophages have a distinct icosahedral head and tail with helical symmetry (Fig. 2.19). On fig. 2.20 and 2.21 are schematic representations of some viruses, illustrating their relative size and general structure.



Rice. 2.20. Several simplified schematic representations of viruses, reflecting the difference in their symmetry and size. The T2 phage is shown with tail filaments that the phage releases before infecting the cell; at the phage? there are no threads of the caudal process.



Rice. 2.21. The structure of the human immunodeficiency virus (HIV), which is a retrovirus. The cone-shaped capsid consists of capsomeres arranged in a spiral. The capsid has been cut off in front to show two copies of the RNA genomes. Under the action of an enzyme called reverse transcriptase, the information encoded in these single-stranded RNA strands is transcribed into the corresponding double-stranded DNA strands. The capsid is surrounded by a protein coat anchored in the lipid bilayer, a coat derived from the plasma membrane of the host cell. This envelope contains viral glycoproteins embedded in it, which, by specifically binding to T-cell receptors, ensure the penetration of the virus into the host cell.

Viruses are made up of various components:

• a) core genetic material (DNA or RNA). The genetic apparatus of the virus carries information about several types of proteins that are necessary for the formation of a new virus: the gene encoding reverse transcriptase and others.
• b) a protein shell, which is called a capsid. The shell is often built from identical repeating subunits - capsomeres. Capsomeres form structures with a high degree of symmetry.
• c) additional lipoprotein shell. It is formed from the plasma membrane of the host cell. It occurs only in relatively large viruses (flu, herpes).

A fully formed infectious particle is called a virion.

Schematic structure of the virus: 1 - core (single-stranded RNA); 2 - protein coat (capsid); 3 - additional lipoprotein shell; 4 - capsomeres (structural parts of the capsid).

Viruses cannot be seen with an optical microscope because they are smaller than the wavelength of light. They can only be seen with an electron microscope. Viruses do not have a cellular structure. Each viral particle has a very simple structure - it consists of a carrier of genetic information located in the center and a shell. The genetic material is a short nucleic acid molecule, this forms core virus. Nucleic acid in different viruses can be represented by DNA or RNA, and these molecules can have an unusual structure: single-stranded DNA and double-stranded RNA are found. The shell is called capsid. It is made up of subunits capsomeres, each of which consists of one or two protein molecules. The number of capsomeres for each virus is strictly constant (for example, in the capsid of the polio virus there are 60 of them - no more and no less, and for the tobacco mosaic virus - 2130, and not 2129 and not 2131). Sometimes the nucleic acid together with the capsid is called nucleocapsid. If the virus particle, except for the capsid, no longer has an envelope, it is called simple virus, if there is one more - external, the virus is called complex . The outer shell is also called supercapsid , genetically, it does not belong to the virus, but originates from the plasma membrane of the host cell and is formed when the assembled viral particle leaves the infected cell. Thus, a viral particle consists of only two classes of biopolymers: nucleic acids and proteins, while polysaccharides and lipids must also be present in any cell.

In each virus, the capsid capsomeres are arranged in a strictly defined order, due to which a certain type of symmetry arises. With spiral symmetry the capsid acquires a tubular (tobacco mosaic virus) or spherical (RNA-containing animal viruses) shape. With cubic symmetry the capsid has the shape of an icosahedron (twenty-sided), isometric viruses have such symmetry. In the case of combined symmetry, the capsid has a cubic shape, and the nucleic acid located inside is spirally stacked. The correct geometry of the capsid even allows viral particles to form crystalline structures together.