2.4.1. Opening
In 1852, 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 retention filter, the filtered liquid still retained infectious properties. In 1898, the Dutchman Beijerinck coined the new word “virus” (from the Latin word meaning “poison”) to describe the infectious nature of certain filtered plant fluids. Although significant success has been achieved in obtaining highly purified virus samples and it has been established that chemical nature These are nucleoproteins (complex compounds consisting of nucleic acids), the particles themselves were still elusive and mysterious because they were too small to be seen using light. That is why viruses were among the first biological structures that were examined in an electron microscope immediately after its invention in the thirties of the 20th century.
2.4.2. Properties of viruses
Viruses have the following properties.
Below we will look at these properties in more detail.
Dimensions
Viruses are the smallest living organisms, the sizes of which vary 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 do not allow bacteria to pass through.
Origin
Researchers often wonder, are viruses alive? If we consider any structure that has genetic material (DNA or RNA) and is capable of self-reproduction to be alive, then the answer must be affirmative: yes, viruses are alive. If the presence of a cellular structure is considered a sign of living things, then the answer will be negative: viruses are not alive. It should be added that outside the host cell, viruses are unable to reproduce themselves.
For a more complete understanding of viruses, it is necessary to know their origin in the process of evolution. There is an assumption, although unproven, 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 (genetic code) necessary to re-enter the cell and, subordinating it to their instructions, force it to produce many copies identical to itself (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:
- cores– genetic material, represented either by DNA or RNA; DNA or RNA can be single-stranded or double-stranded;
- capsid– protective protein shell surrounding the core;
- nucleocapsid– complex structure formed by the core and capsid;
- shell– some viruses, such as HIV and influenza viruses, have an additional lipoprotein layer derived from the plasma membrane of the host cell;
- capsomeres– identical repeating subunits, from which capsids are often built.
Rice. 2.16. Schematic cross-sectional representation of the virus.
The general shape of the capsid is characterized by a high degree of symmetry, determining the ability of viruses to crystallize. This makes it possible to study them using both X-ray crystallography and electron microscopy. Once viral subunits are formed in the host cell, they can immediately self-assemble into a complete viral particle. 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 negative contrast (it is not the preparation itself that is stained, but its background). Notice how clearly the details of the structure of the virus are visible. Individual capsomeres are visible exactly 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. In Fig. 2.17, And we see a regular icosahedron, and in Fig. 2.17, B – herpes virus, in a 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 based on the results of X-ray structural analysis, biochemical data and electron microscopic studies. B. Electron micrograph of the tobacco mosaic virus obtained by negative contrast (x 800,000). The capsid (shell) is formed by 2130 identical protein capsomeres. B. A tobacco plant infected with TMV. Pay attention to the characteristic spots in places where the leaf tissue is dying.
A visual 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 through smoke from cigarettes made from infected leaves.
Rice. 2.19. A. Structure of bacteriophage T2. B. Electron micrograph of a bacteriophage obtained by negative contrast.
Viruses that attack bacteria form a group called bacteriophages or simply phages. Some bacteriophages have a clearly defined icosahedral head and a tail with spiral symmetry (Fig. 2.19). In Fig. 2.20 and 2.21 provide schematic representations of some viruses, illustrating their relative sizes and general structure.
Rice. 2.20. Several simplified schematic images of viruses, reflecting the differences in their symmetry and size. Phage T2 is shown with the tail filaments that the phage releases before infecting a cell; at the phage? there are no filaments of the caudal process.
Rice. 2.21. The structure of the human immunodeficiency virus (HIV), a retrovirus. The cone-shaped capsid consists of capsomeres arranged in a spiral. The front of the capsid is cut away to expose 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 corresponding double-stranded DNA strands. The capsid is surrounded by a protein shell anchored in a lipid bilayer, an envelope derived from the plasma membrane of the host cell. This envelope contains viral glycoproteins built into it, which, by specifically binding to T-cell receptors, ensure the penetration of the virus into the host cell.
They are everywhere: in the air, water, soil and on the surfaces of objects. They are so small that not all of their types can be seen with a regular microscope. These are viruses, amazing natural formations, not fully understood and with amazing survival rates.
Meet: poisonous and dangerous
The virus absolutely lives up to its name, if translated from Latin: poison. Previously, this word was used indiscriminately in relation to all pathogens. But at the end of the 19th century the situation changed.
Two centuries ago, the Russian scientist Ivanovsky, during experiments with tobacco leaves affected by a specific disease, found out that if the bacterial contents are separated from the squeezed juice using a filter, the resulting biomaterial still retains the ability to infect healthy plants. Next, scientists began to isolate new types of aggressive agents using filtration, for example, foot-and-mouth disease or yellow fever virus. Gradually, the word "filtered" disappeared, and at this stage In the development of science, what causes most diseases around the world is usually called viruses.
Neither alive nor dead
This question is still the subject of scientific debate. The fact is that since the structure of viruses (primarily the one that causes tobacco mosaic) and their behavioral patterns have been studied, important details have emerged that make us think: is he more likely alive than dead, or vice versa?
Arguments for:
- molecular structure;
- contain a genome;
- inside the cells they behave quite actively.
Arguments against:
- outside the cell cavity are completely inert;
- They do not synthesize protein on their own, therefore they are not able to share genetic material without the presence of a host cell.
Structural features
The structure of viruses that cause many diseases varies in detail, but has many common features. First of all, the extracellular form of the virus is called the virion. It consists of the following elements:
- a nucleus that contains from 1 to 3 molecules of nucleic acid;
- capsid - a cover made of protein that protects the acid from influences;
- shell consisting of protein-lipid compounds (not always available).
Nucleic acid is the genetic code of the virus. Interestingly, deoxyribonucleic acid and ribonucleic acid are never found together. While microorganisms, the “liveness” of which no one doubts, for example, chlamydia, contain both acids. As for genetic information, it can be limited to 1-3 genes, and sometimes contains up to 100 units.
The virions borrowed an additional shell from the occupied organism, making changes to the structure of the cell. A virus that has such an addition is interested in the cytoplasmic or nuclear membrane in order to form a secondary protective layer from its fragments. Moreover, such a shell is characteristic only of relatively large specimens, such as herpes or the influenza virus.
The components of virions perform not only the functions of protection and information storage, but are also responsible for viral reproduction and the necessary mutations.
Shaped virus
The structural features of viruses are such that their classification depends on the shape of the capsid.
The simplest viruses have a structure that is distinguished by the presence of one type of protein molecules in the capsids. These are so-called naked viruses, that is, completely devoid of an envelope.
But there are virions covered with capsomeres - this is a combination of several molecules that forms a certain geometric shape. The structure of viruses, as well as their capsomeres, plays a role important role in identifying an aggressive agent. The shape varies significantly: head with a tail, rectangle (smallpox), ball (influenza), stick (tobacco mosaic), thread (potato tuber diseases), polyhedron (poliomyelitis), bullet-shaped (rabies).
Nanosize
Viruses are so small that most of them can only be examined in detail with an electron microscope. Whatever the shape and structure of the virus, bacteria will always be larger in size (about 50 times). The size of virions varies from small (20-30 nm) to large (400 nm).
Cellular occupation
Viral invasion of a cell cannot be compared to any other - in nature, a similar mechanism is not found anywhere else. Outside the cell, the virion is in a dormant, crystallized state. But as soon as he gets into the desired cavity, active actions begin.
- Adsorption. In other words, this is the attachment of virions (sometimes hundreds) to the walls of a selected cell.
- Viropexis. The process of direct immersion into a cell, occurring through the site of attachment of the virus. An interesting point: the cell does not prevent the invasion in any way, because the virus particle, or rather its protein, is identified by the cell as “its own”.
- Reduplication. Infectious invasion begins when viruses multiply in a cell. They synthesize new molecules similar to themselves, forming numerous capsids.
- Exit. At the moment of oversaturation, the cellular structure is disrupted, the viruses are no longer restrained, and they break out to infect new cells. This process can happen in several ways.
Surprisingly, microorganisms hundreds of times smaller than a cell confidently and quickly destroy its work, destructively affecting metabolic processes and often destroying the victim.
Types of Virus Intrusions
Such a classification depends on the nature of cellular destruction, as well as on the duration of stay of the aggressive agent. In this regard, three types of infection are distinguished:
- destructive: this type of infection is called lytic, in which viruses break out en masse from the cellular space, and, destroying everything in their path, strive to conquer new cells;
- persistent or persistent: characterized by the gradual flow of viral masses outward without disrupting the functioning of the cell;
- hidden: the latent type is distinguished by the integration of the viral genome into cellular chromosomes and later, during division, the cell transmits the virus to its daughter structures.
In conclusion, it is worth noting the amazing variety of these microscopic substances, which explains the difference in the observed symptoms. There are viruses with DNA - herpes, smallpox, and also containing RNA - foot-and-mouth disease, several bacteriophages. Among other things, these virions contain lipids.
Other options: lipid-free viruses such as adenoviruses and the vast majority of bacteriophages.
It is encouraging that sooner or later the scientific world will learn to subjugate these forms of life and turn them to the benefit of humanity.
Contents of the article
VIRUSES, the smallest pathogens of infectious diseases. Translated from Latin virus means “poison, poisonous beginning.” Until the end of the 19th century. the term "virus" was used in medicine to refer to any infectious agent that causes disease. This word acquired its modern meaning after 1892, when the Russian botanist D.I. Ivanovsky established the “filterability” of the causative agent of tobacco mosaic disease (tobacco mosaic). He showed that cell sap from plants infected with this disease, passed through special filters that retain bacteria, retains the ability to cause the same disease in healthy plants. Five years later, another filterable agent - the causative agent of foot-and-mouth disease in cattle - was discovered by the German bacteriologist F. Loeffler. In 1898, the Dutch botanist M. Beijerinck repeated these experiments in an expanded version and confirmed Ivanovsky’s conclusions. He called the “filterable poisonous principle” that causes tobacco mosaic a “filterable virus.” This term has been used for many years and has gradually been shortened to one word - “virus”.
In 1901, the American military surgeon W. Reed and his colleagues established that the causative agent of yellow fever is also a filterable virus. Yellow fever was the first human disease identified as viral, but it took another 26 years for its viral origin to be definitively proven.
Properties and origin of viruses.
It is generally accepted that viruses originated as a result of the isolation (autonomization) of individual genetic elements of the cell, which, in addition, received the ability to be transmitted from organism to organism. In a normal cell, movements of several types of genetic structures occur, for example, matrix, or information, RNA (mRNA), transposons, introns, and plasmids. Such mobile elements may have been the predecessors, or progenitors, of viruses.
Are viruses living organisms?
REPLICATION OF VIRUSES
The genetic information encoded in a single gene can generally be thought of as instructions for producing a specific protein in a cell. Such an instruction is perceived by the cell only if it is sent in the form of mRNA. Therefore, cells whose genetic material is represented by DNA must “rewrite” (transcribe) this information into a complementary copy of mRNA. DNA viruses differ in their method of replication from RNA viruses.
DNA usually exists in the form of double-stranded structures: two polynucleotide chains are connected by hydrogen bonds and twisted in such a way that a double helix is formed. RNA, on the other hand, usually exists as single-stranded structures. However, the genome of some viruses is single-stranded DNA or double-stranded RNA. The strands (chains) of viral nucleic acid, double or single, can be linear or closed in a ring.
The first stage of viral replication is associated with the penetration of viral nucleic acid into the host cell. This process can be facilitated by special enzymes that are part of the capsid or outer shell of the virion, with the shell remaining outside the cell or the virion losing it immediately after penetration into the cell. The virus finds a cell suitable for its reproduction by contacting individual sections of its capsid (or outer shell) with specific receptors on the cell surface in a “key-lock” manner. If there are no specific (“recognizing”) receptors on the cell surface, then the cell is not sensitive to viral infection: the virus does not penetrate it.
In order to realize its genetic information, the viral DNA that has entered the cell is transcribed by special enzymes into mRNA. The resulting mRNA moves to the cellular “factories” of protein synthesis – ribosomes, where it replaces the cellular “messages” with its own “instructions” and is translated (read), resulting in the synthesis of viral proteins. The viral DNA itself doubles (duplicates) many times with the participation of another set of enzymes, both viral and those belonging to the cell.
The synthesized protein, which is used to build the capsid, and the viral DNA, multiplied in many copies, combine and form new, “daughter” virions. The formed viral offspring leaves the used cell and infects new ones: the virus reproduction cycle repeats. Some viruses, during budding from the cell surface, capture part of the cell membrane into which viral proteins have been embedded “in advance”, and thus acquire an envelope. As for the host cell, it eventually turns out to be damaged or even completely destroyed.
In some DNA-containing viruses, the cycle of reproduction in the cell itself is not associated with immediate replication of viral DNA; instead, the viral DNA is inserted (integrated) into the DNA of the host cell. At this stage, the virus disappears as a single structural formation: its genome becomes part of the cell’s genetic apparatus and even replicates as part of cellular DNA during cell division. However, later, sometimes after many years, the virus may reappear - the mechanism of synthesis of viral proteins is launched, which, combining with viral DNA, form new virions.
In some RNA viruses, the genome (RNA) can directly act as mRNA. However, this feature is characteristic only of viruses with a “+” strand of RNA (i.e., with RNA having positive polarity). For viruses with a “-” strand of RNA, the latter must first be “rewritten” into the “+” strand; Only after this does the synthesis of viral proteins begin and virus replication occurs.
So-called retroviruses contain RNA as a genome and have an unusual way of transcribing genetic material: instead of transcribing DNA into RNA, as happens in a cell and is typical for DNA-containing viruses, their RNA is transcribed into DNA. The double-stranded DNA of the virus is then integrated into the chromosomal DNA of the cell. On the matrix of such viral DNA, a new viral RNA is synthesized, which, like others, determines the synthesis of viral proteins.
CLASSIFICATION OF VIRUSES
If viruses are truly mobile genetic elements that have received “autonomy” (independence) from the genetic apparatus of their hosts (different types of cells), then different groups of viruses (with different genomes, structures and replication) should have arisen independently of each other. Therefore, it is impossible to construct a single pedigree for all viruses, connecting them on the basis of evolutionary relationships. The principles of "natural" classification used in animal taxonomy do not apply to viruses.
Nevertheless, a virus classification system is necessary in practical work, and attempts to create it have been made repeatedly. The most productive approach was based on the structural and functional characteristics of viruses: in order to distinguish different groups of viruses from each other, they describe the type of their nucleic acid (DNA or RNA, each of which can be single-stranded or double-stranded), its size (the number of nucleotides in the nucleic acid chain acids), the number of nucleic acid molecules in one virion, the geometry of the virion and the structural features of the capsid and outer shell of the virion, the type of host (plants, bacteria, insects, mammals, etc.), features of the pathology caused by viruses (symptoms and nature of the disease), antigenic properties of viral proteins and features of the body’s immune system’s response to the introduction of the virus.
The group of microscopic pathogens called viroids (i.e., virus-like particles) does not quite fit into the classification system of viruses. Viroids cause many common plant diseases. These are the smallest infectious agents, devoid of even the simplest protein cover (found in all viruses); they consist only of single-stranded RNA closed in a ring.
VIRAL DISEASES
Evolution of viruses and viral infections.
Birds are the natural reservoir for equine encephalitis viruses, which are especially dangerous for horses and, to a slightly lesser extent, for humans. These viruses are carried by blood-sucking mosquitoes, in which the virus multiplies without significant harm to the mosquito. Sometimes viruses can be transmitted passively by insects (without reproducing in them), but most often they reproduce in vectors.
For many viruses, such as measles, herpes and partly influenza, the main natural reservoir is humans. Transmission of these viruses occurs through airborne droplets or contact.
The spread of some viral diseases, like other infections, is full of surprises. For example, in groups of people living in unsanitary conditions, almost all children at an early age contract polio, which usually occurs in mild form, and acquire immunity. If living conditions in these groups improve, young children usually do not get polio, but the disease can occur at an older age, and then it is often severe.
Many viruses cannot survive in nature for a long time at a low population density of the host species. The small populations of primitive hunters and plant gatherers created unfavorable conditions for the existence of some viruses; therefore, it is very likely that some human viruses arose later, with the advent of urban and rural settlements. It is assumed that the measles virus originally existed among dogs (as a causative agent of fever), and human smallpox could have appeared as a result of the evolution of cow or mousepox. The most recent examples of viral evolution include acquired human immunodeficiency syndrome (AIDS). There is evidence of genetic similarity between human immunodeficiency viruses and African green monkeys.
“New” infections are usually severe, often fatal, but as the pathogen evolves, they may become milder. A good example is the story of the myxomatosis virus. In 1950, this virus, endemic to South America and quite harmless to local rabbits, it was brought to Australia along with European breeds of these animals. The disease in Australian rabbits, which had not previously encountered this virus, was fatal in 99.5% of cases. A few years later, the mortality rate from this disease decreased significantly, in some areas by up to 50%, which is explained not only by “attenuating” (weakening) mutations in the viral genome, but also by the increased genetic resistance of rabbits to the disease, and in both cases, effective natural selection occurred under powerful pressure of natural selection.
Reproduction of viruses in nature is supported different types organisms: bacteria, fungi, protozoa, plants, animals. For example, insects often suffer from viruses that accumulate in their cells in the form of large crystals. Plants are often affected by small and simple RNA viruses. These viruses do not even have special mechanisms to penetrate the cell. They are transmitted by insects (which feed on cell sap), roundworms and by contact, infecting the plant when it is mechanically damaged. Bacterial viruses (bacteriophages) have the most complex mechanism for delivering their genetic material into a sensitive bacterial cell. First, the phage “tail,” which looks like a thin tube, attaches to the wall of the bacterium. Then special “tail” enzymes dissolve a section of the bacterial wall and the phage’s genetic material (usually DNA) is injected into the resulting hole through the “tail,” like through a syringe needle.
More than ten main groups of viruses are pathogenic for humans. Among the DNA viruses, these are the poxvirus family (causing smallpox, cowpox and other smallpox infections), the herpes group of viruses (herpetic rashes on the lips, chicken pox), adenoviruses (diseases of the respiratory tract and eyes), the papovavirus family (warts and other growths skin), hepadnaviruses (hepatitis B virus). There are much more RNA-containing viruses that are pathogenic to humans. Picornaviruses (from Lat. pico – very small, English. RNA - RNA) are the smallest mammalian viruses, similar to some plant viruses; they cause polio, hepatitis A, acute colds. Myxoviruses and paramyxoviruses are the cause different forms influenza, measles and mumps (mumps). Arboviruses (from English. ar thropod bo rne - “arthropod-borne”) - the largest group of viruses (more than 300) - are carried by insects and are the causative agents of tick-borne and Japanese encephalitis, yellow fever, equine meningoencephalitis, Colorado tick fever, Scottish sheep encephalitis and other dangerous diseases. Reoviruses, rather rare causative agents of human respiratory and intestinal diseases, have become the subject of particular scientific interest due to the fact that their genetic material is represented by double-stranded fragmented RNA.
Treatment and prevention.
Reproduction of viruses is closely intertwined with the mechanisms of protein and nucleic acid synthesis of the cell in the infected organism. Therefore, creating drugs that selectively suppress the virus, but do not harm the body, is an extremely difficult task. Nevertheless, it turned out that the genomic DNA of the largest herpes and smallpox viruses encodes large number enzymes that differ in properties from similar cellular enzymes, and this served as the basis for the development of antiviral drugs. Indeed, several drugs have been created whose mechanism of action is based on suppressing the synthesis of viral DNA. Some compounds are too toxic for general use(intravenously or orally), suitable for local use, for example, when the eyes are damaged by the herpes virus.
It is known that the human body produces special proteins - interferons. They suppress the translation of viral nucleic acids and thus inhibit the replication of the virus. Thanks to genetic engineering, interferons produced by bacteria have become available and are being tested in medical practice. cm. GENETIC ENGINEERING).
The most effective elements of the body’s natural defense include specific antibodies (special proteins produced by the immune system), which interact with the corresponding virus and thereby effectively prevent the development of the disease; however, they cannot neutralize a virus that has already entered the cell. An example is a herpes infection: the herpes virus is stored in the cells of the nerve nodes (ganglia), where antibodies cannot reach it. From time to time the virus is activated and causes relapses of the disease.
Typically, specific antibodies are formed in the body as a result of the penetration of an infectious agent into it. The body can be helped by artificially enhancing the production of antibodies, including by creating immunity in advance through vaccination. It was in this way, through mass vaccination, that smallpox was practically eradicated throughout the world.
Modern methods of vaccination and immunization are divided into three main groups. Firstly, it is the use of a weakened strain of the virus, which stimulates the body to produce antibodies that are effective against a more pathogenic strain. Secondly, the introduction of a killed virus (for example, inactivated by formaldehyde), which also induces the formation of antibodies. The third option is the so-called. “passive” immunization, i.e. introduction of ready-made “foreign” antibodies. An animal, such as a horse, is immunized, then antibodies are isolated from its blood, purified, and used to inject into a patient to create immediate but short-lived immunity. Sometimes antibodies are used from the blood of a person who has had a given disease (for example, measles, tick-borne encephalitis).
Accumulation of viruses.
To prepare vaccine preparations, it is necessary to accumulate the virus. For this purpose, developing chicken embryos are often used, which are infected with this virus. After incubating infected embryos for a certain time, the virus that has accumulated in them due to reproduction is collected, purified (by centrifugation or other means) and, if necessary, inactivated. It is very important to remove all ballast impurities from the virus preparations, which can cause serious complications during vaccination. Of course, it is equally important to ensure that no uninactivated pathogenic virus remains in the preparations. IN recent years widely used for virus accumulation various types cell cultures.
METHODS FOR STUDYING VIRUSES
Bacterial viruses were the first to become the object of detailed research as the most convenient model, which has a number of advantages over other viruses. The complete cycle of phage replication, i.e. The time from infection of a bacterial cell to the release of multiplied viral particles occurs within one hour. Other viruses usually accumulate over several days or even longer. Just before World War II and shortly after its end, methods were developed to study individual viral particles. Plates with nutrient agar on which a monolayer (solid layer) of bacterial cells is grown are infected with phage particles using serial dilutions. As the virus multiplies, it kills the cell that “shelters” it and penetrates into neighboring ones, which also die after the accumulation of phage progeny. The area of dead cells is visible to the naked eye as a bright spot. Such spots are called “negative colonies”, or plaques. The developed method made it possible to study the progeny of individual viral particles, detect genetic recombination of viruses, and determine the genetic structure and methods of replication of phages in detail that previously seemed incredible.
Work with bacteriophages contributed to the expansion of the methodological arsenal in the study of animal viruses. Until now, research on vertebrate viruses had been performed primarily in laboratory animals; such experiments were very labor-intensive, expensive and not very informative. Subsequently, new methods based on the use of tissue cultures emerged; the bacterial cells used in the phage experiments were replaced with vertebrate cells. However, to study the mechanisms of development of viral diseases, experiments on laboratory animals are very important and continue to be carried out at the present time.
Viruses. Classification of viruses. Types of interaction between cells and viruses
Sizes – from 15 to 2000 nm (some plant viruses). The largest among animal and human viruses is the causative agent of smallpox - up to 450 nm.
Simple viruses have an envelope - capsid, which consists only of protein subunits ( capsomeres). The capsomeres of most viruses have helical or cubic symmetry. Virions with helical symmetry are rod-shaped. Most viruses that infect plants are built according to the spiral type of symmetry. Most viruses that infect human and animal cells have a cubic type of symmetry.
Complex
Complex viruses can be additionally covered with a lipoprotein surface membrane with glycoproteins that are part of the plasma membrane of the host cell (for example, smallpox viruses, hepatitis B), that is, they have supercapsid. With the help of glycoproteins, specific receptors are recognized on the surface of the host cell membrane and the viral particle attaches to it. The carbohydrate regions of the glycoproteins protrude above the surface of the virus in the form of pointed rods. The additional envelope can merge with the plasma membrane of the host cell and facilitate the penetration of the contents of the viral particle deep into the cell. Additional shells may include enzymes that ensure the synthesis of viral nucleic acids in the host cell and some other reactions.
Bacteriophages have a rather complex structure. They are classified as complex viruses. For example, bacteriophage T4 consists of an expanded part - a head, a process and tail filaments. The head consists of a capsid that contains nucleic acid. The process includes a collar, a hollow shaft surrounded by a contractile sheath resembling an extended spring, and a basal plate with caudal spines and filaments.
Classification of viruses
The classification of viruses is based on the symmetry of the viruses and the presence or absence of an outer shell.
| Deoxyviruses | Riboviruses | ||
| DNA double-stranded | DNA single-stranded | RNA double-stranded | RNA single-stranded |
| Cubic symmetry type: – without outer shells (adenoviruses); – with external membranes (herpes) | Cubic symmetry type: – without outer membranes (some phages) | Cubic symmetry type: – without outer shells (retroviruses, plant wound tumor viruses) | Cubic symmetry type: – without outer shells (enteroviruses, poliovirus) Spiral symmetry type: – without outer shells (tobacco mosaic virus); – with outer membranes (influenza, rabies, oncogenic RNA-containing viruses) |
| Mixed type of symmetry (T-paired bacteriophages) | |||
| Without a certain type of symmetry (pox) | |||
Viruses exhibit vital activity only in the cells of living organisms. Their nucleic acid is capable of causing the synthesis of viral particles in the host cell. Outside the cell, viruses do not show signs of life and are called virions .
The life cycle of the virus consists of two phases: extracellular(virion), in which it does not show signs of vital activity, and intracellular . Viral particles outside the host’s body do not lose their ability to infect for some time. For example, the polio virus can remain infectious for several days, and smallpox for months. The hepatitis B virus retains it even after short-term boiling.
The active processes of some viruses occur in the nucleus, others in the cytoplasm, and in some, both in the nucleus and in the cytoplasm.
Types of interaction between cells and viruses
There are several types of interactions between cells and viruses:
- Productive – the nucleic acid of the virus induces the synthesis of its own substances in the host cell with the formation of a new generation.
- Abortive – reproduction is interrupted at some stage, and a new generation is not formed.
- Virogenic – the nucleic acid of the virus is integrated into the genome of the host cell and is not capable of reproduction.
In the centuries-old history of our planet, invisible invaders constantly interfered with 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 susceptible to the influence of 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 tiny pathogens, about which a great many theories had already been expressed up to that point. Certain human viruses differed in shape and size. Depending on the type of disease, the symptoms of different diseases manifest themselves differently: the skin becomes inflamed, internal organs or joints.
Viral infection
In 1852, Dmitry Iosifovich Ivanovsky (Russian botanist) managed to obtain an infectious extract from tobacco plants that were infected with mosaic disease. This structure is called the tobacco mosaic virus.
Virus structure
At the very center of the viral particle is the genome (hereditary information that is represented by DNA or RNA structure - position 1). Around the genome there is a capsid (position 2), which is represented by a protein shell. On the surface of the protein shell of the capsid there is a lipoprotein shell (position 3). Capsomeres are located inside the shell (position 4). Each capsomere consists of one or two protein strands. The number of capsomeres for each virus is strictly constant. Each virus contains a certain number of capsomeres, so their number is different types virus 
significantly different. Some viruses do not have a protein shell (capsid) in their structure. Such viruses are called simple viruses. Conversely, viruses that have another outer (additional lipoprotein) shell in their structure are called complex. Viruses have two life forms. The extracellular life form of the 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) .
Classification of viruses
How do body cells absorb substances?
Unlike other living organisms, a virus requires living cells to reproduce. By itself, it cannot reproduce. For example, the cells of the human body consist of a nucleus (DNA is concentrated in it - the genetic map, the cell’s action plan for maintaining its vital functions). The cell nucleus is surrounded by cytoplasm, in which mitochondria are located (they produce energy for chemical reactions, lysosomes (they break down materials received from outside), polysomes and ribosomes (they produce proteins and enzymes for carrying out chemical reactions that occur in the cell). The whole The cytoplasm of the cell, or rather its space, is penetrated by a network of tubules through which the necessary substances are absorbed and unnecessary substances 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. When there is a protein corpuscle on the surface of the membrane, it bends and bends. encloses it in a digestive vesicle, which draws it into the cell. Next, the brain center of the cell (nucleus) recognizes the substance coming 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 and use some of the useful compounds to maintain life. perform programmed functions, and unnecessary connections are removed outside the cell. This is how the process of absorption, digestion, assimilation of substances in the cell and removal of unnecessary substances is carried out.
Reproduction of viruses
As noted above, a virus needs living cells to reproduce its own kind, because it cannot reproduce on its own. The process of virus penetration into a cell consists of several stages.
The first stage of virus penetration into a cell is its deposition (adsorption through electrical interaction) on the surface of the target cell. The target cell, in turn, must have the appropriate surface receptors. Without the presence of appropriate surface receptors, the virus cannot attach to the cell. Therefore, a virus that has attached itself to a cell as a result of electrical interaction can be removed by shaking. The second stage of virus penetration into the cell is called irreversible. If the appropriate receptors are present, the virus attaches to the cell and protein spikes or threads begin to interact with the cell's receptors. The cell receptors are a protein or glycoprotein, which is usually specific to each virus.
During the third stage, the virus is absorbed (moved) in the cell membrane using intracellular membrane vesicles.
In the fourth stage, cell enzymes break down viral proteins, and thus the virus genome, which contains hereditary information, which is represented by DNA or RNA structure, is released from “imprisonment.” Then the RNA helix quickly unwinds and rushes into the cell nucleus. In the cell nucleus, the virus genome changes the genetic information of the cell 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 variants is called the latent or latent period. It can vary from several hours (smallpox, influenza) to several days (measles, adenovirus).
