The human body is susceptible to all kinds of diseases and infections, and animals and plants also get sick quite often. Scientists of the last century tried to identify the cause of many diseases, but even after determining the symptoms and course of the disease, they could not confidently say about its cause. It was only at the end of the nineteenth century that the term “viruses” appeared. Biology, or rather one of its sections - microbiology, began to study new microorganisms, which, as it turned out, have been neighbors for a long time and contribute to the deterioration of his health. In order to more effectively fight viruses, a new science has emerged - virology. It is she who can tell a lot of interesting things about ancient microorganisms.

Viruses (biology): what are they?

Only in the nineteenth century did scientists discover that the causative agents of measles, influenza, foot-and-mouth disease and other infectious diseases not only in humans, but also in animals and plants are microorganisms invisible to the human eye.

After viruses were discovered, biology was not immediately able to provide answers to the questions posed about their structure, occurrence and classification. Humanity has a need for a new science - virology. Currently, virologists are working to study familiar viruses, monitor their mutations and invent vaccines that can protect living organisms from infection. Quite often, for the purpose of experiment, a new strain of the virus is created, which is stored in a “dormant” state. On its basis, drugs are developed and observations are made of their effects on organisms.

In modern society, virology is one of the most important sciences, and the most sought-after researcher is a virologist. The profession of a virologist, according to sociologists, is becoming more and more popular every year, which well reflects the trends of our time. After all, according to many scientists, wars will soon be fought and ruling regimes established with the help of microorganisms. In such conditions, a state with highly qualified virologists may turn out to be the most resilient, and its population the most viable.

The emergence of viruses on Earth

Scientists attribute the emergence of viruses to the most ancient times on the planet. Although it is impossible to say with certainty how they appeared and what form they had at that time. After all, viruses have the ability to penetrate absolutely any living organisms; they have access to the simplest forms of life, plants, fungi, animals and, of course, humans. But viruses do not leave behind any visible remains in the form of fossils, for example. All these features of the life of microorganisms significantly complicate their study.

• they were part of the DNA and separated over time;
• they were built into the genome initially and, under certain circumstances, “woke up” and began to reproduce.

Scientists suggest that the genome of modern humans contains great amount viruses that infected our ancestors and are now naturally integrated into our DNA.

Viruses: when were they discovered?

The study of viruses is a fairly new branch of science, because it is believed that it appeared only at the end of the nineteenth century. In fact, it can be said that the viruses themselves and their vaccines were unknowingly discovered by an English doctor at the end of the nineteenth century. He worked on creating a cure for smallpox, which at that time killed hundreds of thousands of people during an epidemic. He managed to create an experimental vaccine directly from the sore of one of the girls who had smallpox. This vaccination turned out to be very effective and saved more than one life.

But D.I. Ivanovsky is considered the official “father” of viruses. This Russian scientist studied diseases of tobacco plants for a long time and made an assumption about small microorganisms that pass through all known filters and cannot exist on their own.

A few years later, the Frenchman Louis Pasteur, in the process of fighting rabies, identified its causative agents and introduced the term “viruses”. An interesting fact is that the microscopes of the late nineteenth century could not show viruses to scientists, so all assumptions were made about invisible microorganisms.

Development of virology

The middle of the last century gave a powerful impetus to the development of virology. For example, the invented electron microscope finally made it possible to see viruses and classify them.

In the fifties of the twentieth century, the polio vaccine was invented, which became a salvation from this terrible disease for millions of children around the world. In addition, scientists have learned to grow human cells in a special environment, which has led to the opportunity to study human viruses in the laboratory. At the moment, about one and a half thousand viruses have already been described, although fifty years ago only two hundred similar microorganisms were known.

Properties of viruses

Viruses have a number of properties that distinguish them from other microorganisms:

• Very small sizes, measured in nanometers. Large human viruses, such as smallpox, are three hundred nanometers in size (that's only 0.3 millimeters).
• Every living organism on the planet contains two types of nucleic acids, but viruses have only one.
• Microorganisms cannot grow.
• Viruses reproduce only in a living host cell.
• Existence occurs only inside the cell; outside it, the microorganism cannot show signs of vital activity.

Virus forms

To date, scientists can confidently declare two forms of this microorganism:

• extracellular - virion;
• intracellular - virus.

Outside the cell, the virion is in a “sleeping” state; it shows no signs of life. Once in the human body, it finds a suitable cell and, only having penetrated it, begins to actively multiply, turning into a virus.

Virus structure

Almost all viruses, despite the fact that they are quite diverse, have the same structure:

• nucleic acids that form the genome;
• protein shell (capsid);
• Some microorganisms also have a membrane coating on top of the shell.

Scientists believe that this simplicity of structure allows viruses to survive and adapt to changing conditions.

Currently, virologists distinguish seven classes of microorganisms:

• 1 - consist of double-stranded DNA;
• 2 - contain single-stranded DNA;
• 3 - viruses that copy their RNA;
• 4 and 5 - contain single-stranded RNA;
• 6 - transform RNA into DNA;
• 7 - transform double-stranded DNA through RNA.

Despite the fact that the classification of viruses and their study have made great progress, scientists admit the possibility of the emergence of new types of microorganisms that differ from all those already listed above.

Types of Viral Infection

The interaction of viruses with a living cell and the method of exit from it determines the type of infection:

• Lytic

During the infection process, all viruses simultaneously exit the cell, and as a result, the cell dies. Subsequently, the viruses “settle” in new cells and continue to destroy them.

• Persistent

Viruses leave the host cell gradually and begin to infect new cells. But the old one continues its life activity and “gives birth” to new viruses.

• Latent

The virus is embedded in the cell itself, during its division it is transmitted to other cells and spreads throughout the body. Viruses can remain in this state for quite a long time. Under the necessary circumstances, they begin to actively multiply and the infection proceeds according to the types already listed above.

Russia: where are viruses studied?

In our country, viruses have been studied for quite a long time, and it is Russian specialists who are leaders in this field. The D.I. Ivanovsky Research Institute of Virology is located in Moscow, whose specialists make a significant contribution to the development of science. On the basis of the research institute, I operate research laboratories, maintain an advisory center and a department of virology.

At the same time, Russian virologists are working with WHO and expanding their collection of virus strains. Research institute specialists work in all areas of virology:

• general:
• private;
• molecular.

It is worth noting that in recent years there has been a tendency to unite the efforts of virologists around the world. Such collaboration is more effective and allows serious progress in studying the issue.

Viruses (biology as a science has confirmed this) are microorganisms that accompany all living things on the planet throughout their entire existence. Therefore, their study is so important for the survival of many species on the planet, including humans, who have more than once in history fallen victim to various epidemics caused by viruses.

Viruses were discovered by D.I. Ivanovsky (1892, tobacco mosaic virus).

If viruses are isolated in their pure form, then they exist in the form of crystals (they do not have their own metabolism, reproduction and other properties of living things). Because of this, many scientists consider viruses to be an intermediate stage between living and nonliving objects.

Viruses are non-cellular form life. Viral particles (virions) are not cells:

• viruses are much smaller than cells;
• viruses are much simpler in structure than cells - they consist only of nucleic acid and a protein shell, consisting of many identical protein molecules.
• viruses contain either DNA or RNA.

Synthesis of virus components:

• The nucleic acid of the virus contains information about viral proteins. The cell makes these proteins itself, on its ribosomes.
• The cell reproduces the nucleic acid of the virus itself, using its enzymes.
• Then the self-assembly of viral particles occurs.

Virus meaning:

• cause infectious diseases (flu, herpes, AIDS, etc.)
• Some viruses can insert their DNA into the chromosomes of the host cell, causing mutations.

AIDS

The AIDS virus is very unstable and is easily destroyed in air. You can become infected with it only through sexual intercourse without a condom and through a transfusion of contaminated blood.

Answer

Establish a correspondence between the characteristics of a biological object and the object to which this characteristic belongs: 1) bacteriophage, 2) E. coli. Write numbers 1 and 2 in the correct order.
A) consists of nucleic acid and capsid
B) cell wall made of murein
C) outside the body is in the form of crystals
D) can be in symbiosis with humans
D) has ribosomes
E) has a tail canal

Answer

Choose the one that suits you best correct option. Science studies precellular life forms
1) virology
2) mycology
3) bacteriology
4) histology

Answer

Choose one, the most correct option. The AIDS virus infects human blood
1) red blood cells
2) platelets
3) lymphocytes
4) blood platelets

Answer

Answer

Choose one, the most correct option. The cells of which organisms are affected by the bacteriophage?
1) lichens
2) mushrooms
3) prokaryote
4) protozoa

Answer

Choose one, the most correct option. The immunodeficiency virus primarily affects
1) red blood cells
2) platelets
3) phagocytes
4) lymphocytes

Answer

Choose one, the most correct option. In what environment does the AIDS virus usually die?
1) in the lymph
2) in breast milk
3) in saliva
4) in the air

Answer

Choose one, the most correct option. Viruses have such signs of living things as
1) food
2) growth
3) metabolism
4) heredity

Answer

Answer

1. Establish the correct sequence of stages of reproduction of DNA viruses. Write down the corresponding sequence of numbers in the table.
1) release of the virus into the environment
2) virus protein synthesis in the cell
3) introduction of DNA into the cell
4) synthesis of viral DNA in the cell
5) attachment of the virus to the cell

Answer

2. Establish the sequence of stages of the bacteriophage life cycle. Write down the corresponding sequence of numbers.
1) biosynthesis of DNA and bacteriophage proteins by a bacterial cell
2) rupture of the bacterial membrane, release of bacteriophages and infection of new bacterial cells
3) penetration of bacteriophage DNA into the cell and its integration into the circular DNA of the bacterium
4) attachment of the bacteriophage to the bacterial cell membrane
5) assembly of new bacteriophages

Answer

Answer

1) have an unformed core
2) reproduce only in other cells
3) do not have membrane organelles
4) carry out chemosynthesis
5) capable of crystallizing
6) formed by a protein shell and nucleic acid

Answer

Answer

Choose three correct answers out of six and write down the numbers under which they are indicated. Viruses as opposed to bacteria
1) have a cellular structure
2) have an unformed core
3) formed by a protein shell and nucleic acid
4) belong to free-living forms
5) reproduce only in other cells
6) are a non-cellular form of life

Answer

1. Establish a correspondence between the characteristic of an organism and the group for which it is characteristic: 1) prokaryotes, 2) viruses.
A) cellular structure of the body
B) the presence of its own metabolism
C) integration of one’s own DNA into the DNA of the host cell
D) consists of a nucleic acid and a protein shell
D) reproduction by division in two
E) the ability to reverse transcription

Answer

Answer

Answer

Answer

Answer

Answer

Choose two correct answers out of five and write down the numbers under which they are indicated. Metabolism as a property of living things is characteristic of
1) plant viruses
2) protozoa
3) soil bacteria
4) animal viruses
5) bacteriophages

Answer

© D.V. Pozdnyakov, 2009-2019

Structure

Examples of icosahedral virion structures.
A. A virus that does not have a lipid envelope (for example, picornavirus).
B. Enveloped virus (eg, herpesvirus).
The numbers indicate: (1) capsid, (2) genomic nucleic acid, (3) capsomere, (4) nucleocapsid, (5) virion, (6) lipid envelope, (7) membrane envelope proteins.

Classification

Squad ( -virales) Family ( -viridae) Subfamily ( -virinae) Genus ( -virus) View ( -virus)

Baltimore classification

Nobel laureate, biologist David Baltimore, proposed his own classification scheme for viruses based on differences in the mechanism of mRNA production. This system includes seven main groups:

• (I) Viruses that contain double-stranded DNA and do not have an RNA stage (for example, herpesviruses, poxviruses, papovaviruses, mimivirus).

• (II) Double-stranded RNA viruses (eg rotaviruses).

• (III) Viruses containing a single-stranded DNA molecule (eg, parvoviruses).

• (IV) Viruses containing a single-stranded RNA molecule of positive polarity (for example, picornaviruses, flaviviruses).

• (V) Viruses containing a single-stranded RNA molecule of negative or double polarity (for example, orthomyxoviruses, filoviruses).

• (VI) Viruses containing a single-stranded RNA molecule and having in their life cycle the stage of DNA synthesis on an RNA template, retroviruses (for example, HIV).

• (VII) Viruses containing double-stranded DNA and having in their life cycle the stage of DNA synthesis on an RNA template, retroid viruses (for example, hepatitis B virus).

Currently, both systems are used simultaneously to classify viruses, as complementary to each other.

Further division is made on the basis of such characteristics as genome structure (presence of segments, circular or linear molecule), genetic similarity with other viruses, the presence of a lipid membrane, taxonomic affiliation of the host organism, and so on.

Story

Application of viruses

Links

• “The Nobel Committee was hit by viruses” Article. Newspaper "Kommersant" No. 181 (3998) dated 10/07/2008.

Literature

• Mayo M.A., Pringle C.R. Virus taxonomy - 1997 // Journal of General Virology. - 1998. - No. 79. - P. 649-657.

Biology. General biology. Grade 10. Basic level Sivoglazov Vladislav Ivanovich

14. Non-cellular life form: viruses

Remember!

How are viruses different from all other living things?

Why does the existence of viruses not contradict the basic principles of cellular theory?

Which ones do you know viral diseases?

In 1892, Russian botanist Dmitry Iosifovich Ivanovsky, studying mosaic disease of tobacco plants, discovered that when the juice isolated from a diseased plant was passed through filters that retained bacteria, the liquid retained the ability to cause diseases in healthy plants. The causative agent of the disease was so small that it and similar structures, which later received the name viruses(from lat. virus- poison), it became possible to study only after the invention of the electron microscope.

The structure of viruses. Viruses have a very simple structure (Fig. 46). Every virus consists of a nucleic acid (or DNA or RNA) and a protein. Nucleic acid is genetic material virus. It is surrounded by a protective protein shell - capsid. The capsid may also contain its own viral enzymes. Some viruses, such as influenza and HIV, have additional shell, which is formed from the cell membrane of the host cell. The virus capsid, consisting of many protein molecules, has a high degree of symmetry, usually having a spiral or polyhedral shape. This structural feature allows individual viral proteins to combine into a complete viral particle through self-assembly.

Rice. 46. ​​Viruses: structure and diversity

Rice. 47. Life cycle of viruses (A) and electronic photography bacteriophage (B)

Rice. 48. Bacteriophages on the surface of the host cell (electronic photography)

Viruses as pathogens. Viruses can infect both eukaryotic and prokaryotic cells. Viruses that infect bacteria are called bacteriophages. Viruses cause many different diseases in animals, plants and fungi, each with its own specific host. The tobacco mosaic virus, for example, infects tobacco plants, causing the formation of characteristic spots on the leaves - these are places where tissues die. The smallpox virus infects only epithelial cells, and the polio virus infects nervous tissue cells. Human viral diseases also include influenza, measles, rubella, hepatitis, chicken pox, rabies, herpes, AIDS and many others.

AIDS. The human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), was first isolated in the United States in 1981. By 2000, the number of people infected with this virus had already exceeded 30 million people. The disease is currently spreading very quickly in Asia, Africa, and Central and Eastern Europe.

HIV belongs to the group retroviruses, the genetic material of which is RNA (Fig. 49). Typically, the transfer of genetic information in a cell occurs in the direction from DNA to RNA (transcription). In retroviruses, when they enter a host cell, the opposite process occurs, the so-called reverse transcription, in which DNA is synthesized based on viral RNA, which is then integrated into the host DNA.

Rice. 49. Human immunodeficiency virus (HIV): A – model of the virus; B – structure diagram; B – electronic photograph

Rice. 50. Life cycle of the human immunodeficiency virus (HIV)

Let's consider the life cycle of the immunodeficiency virus (Fig. 50). HIV infects and destroys white blood cells, including so-called helper lymphocytes. help- help), which ensure the formation of human immunity. After HIV enters the cell through endocytosis (Fig. 50, 1–3 ) viral RNA enters the cytoplasm (Fig. 50, 4 ), where on its basis, viral DNA is synthesized using a special enzyme (Fig. 50, 5 ). The latter penetrates through the pores into the cell nucleus and is integrated into the host DNA (Fig. 50, 6 ). Subsequently, when the cell divides, simultaneously with the copying of cellular DNA, the built-in viral DNA is also copied, as a result of which the number of infected lymphocytes grows rapidly. This process can continue for many years. After some time, the virus becomes active again (Fig. 50, 7 ) and “forces” the cell to work for itself, synthesizing viral RNA and proteins (Fig. 50, 8 ), from which new viral particles are assembled leaving the host cell (Fig. 50, 9 ). The reasons why the virus becomes active after 5–6 years of latent existence are unknown. New viral particles infect still healthy lymphocytes. As a result, the immune system is destroyed, lymphocytes cease to recognize foreign proteins and pathogenic bacteria entering the body, and the person becomes vulnerable to any infectious diseases. Every year, 1–2% of HIV-infected people develop AIDS. AIDS patients are susceptible to various bacterial, viral and fungal infections, which cause their death. More than 60% of people with AIDS die from pneumonia, which is usually successfully treated by the immune system healthy person. Many HIV carriers develop malignant tumors, and when infected with toxoplasmosis, the large hemispheres of the brain are affected, which can subsequently lead to paralysis and coma.

HIV is usually transmitted through blood or semen. In 90% of cases, infection occurs through sexual contact, and the risk of infection increases in proportion to the increase in the number of sexual partners. Repeated use of the same syringe leads to rapid spread virus among drug addicts. HIV can enter the human body through contact with the blood of a patient, for example when treating wounds. There is a risk of infection from blood transfusions that have not been tested for HIV. From an HIV-infected mother, the virus can enter the blood of the fetus through the placenta or be transmitted to the newborn through breastfeeding. But this virus does not spread through airborne droplets or a handshake.

HIV is a virus, so antibiotics used to treat bacterial infections in this case powerless. Modern medicine is developing drugs that suppress HIV replication, but their use has many side effects and the prospects for their use are still unclear. Developing a vaccine against HIV also has its challenges; this is due to the structural features of this virus and the severity of the disease it causes. Today, an important direction in the treatment of AIDS is the restoration of the immune system of those infected.

Doesn't exist yet effective ways treatment of this disease, the best way protection against AIDS is compliance with the following precautions:

– you should avoid casual sexual intercourse, and during sexual intercourse, isolate yourself from the sperm and blood of your partner using a condom;

- in hospitals, dental clinics, clinics and beauty salons, it is necessary to use disposable syringes, and reusable instruments must be thoroughly sterilized, observing all the necessary conditions;

– Donated blood should be tested for the presence of HIV antibodies.

Viruses as carriers of genetic information. There is a hypothesis that viruses are genetic material that once left the cell, but retained the ability to reproduce itself when returned to it. Consequently, in the process of evolution, viruses arose later than the appearance of the cellular form, and any viral infection should be considered as the cell receiving some foreign genetic information.

Many viruses are capable of not only introducing their hereditary information into the host’s body, but also, by integrating into the host’s DNA, changing the functioning of cellular genes. In the process of copying viral DNA, partial copying of the host's genetic material sometimes occurs. In this case, the newly assembled viral particles leaving the cell will take with them a copy of some hereditary information of the host. This is how viruses can transfer genes between organisms. different types, units and even classes, the crossing of which is in principle impossible. Currently, viruses are considered not only as causative agents of infectious diseases, but also as carriers of genes between organisms.

Review questions and assignments

1. How do viruses work?

2. What is the principle of interaction between a virus and a cell?

3. Describe the process of virus penetration into a cell.

4. What is the effect of viruses on the cell?

5. Using knowledge about the ways of spreading viral and bacterial infections, suggest ways to prevent infectious diseases.

6. Suggest several different classifications of viruses. What criteria did you base these classifications on? Compare your classifications and the classifications created by your classmates.

Think! Do it!

1. Explain why a virus can exhibit the properties of a living organism only by invading a living cell.

2. Why do viral diseases become epidemics? Describe measures to combat viral infections.

3. Express your opinion about the time of appearance of viruses on Earth in the historical past, taking into account that viruses can only reproduce in living cells.

4. Explain why in the middle of the 20th century. viruses have become one of the main objects of experimental genetic research.

5. What difficulties arise when trying to create a vaccine against HIV infection?

6. Explain why the transfer of genetic material from one organism to another by viruses is called horizontal transfer. What then, in your opinion, is the transfer of genes from parents to children called?

7. Over the years, at least seven Nobel Prizes in physiology or medicine and three Nobel Prizes in chemistry have been awarded for research directly related to the study of viruses. Using additional literature and Internet resources, prepare a report or presentation on current advances in virus research.

8. Create a portfolio on the topic “The role of viruses in the life of organisms and the evolution of the organic world on Earth.”

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

Find out more

Viroids. Infectious agents found in nature are much smaller than viruses - viroids. They consist only of a circular RNA molecule and are devoid of any shells. The smallest viroids are only 220 nucleotides long. Viroids are found in the cells of many plants. They are believed to be excised sections of mRNA that have acquired the ability to replicate. However, they do not work like mRNA and do not code for proteins.

Once in plant cells, viroids interfere with the genome of the host cell and cause serious plant diseases. This is how millions of coconut trees died in the Philippines in the second half of the 20th century. From time to time, plantings of potatoes, citrus fruits, cucumbers, ornamental flowers and other wild and agricultural plants are seriously affected by viroids. Viroids have not yet been found in animal cells or humans.

Viruses and cancer. Many viruses are capable of penetrating into the cells of the body and integrating their genome into the genome of the cell, thereby causing serious violations in the functioning of the genetic apparatus of normal cells. As a result, a normal cell can transform into a cancerous one.

Dozens of viruses that cause cancer have been found in many animals (fish, amphibians, birds, mammals). Entire groups of oncoviruses have been discovered in humans. It is believed that about 15% of human tumors are caused by a viral infection.

Repeat and remember!

Human

Immunity. Proteins or polysaccharides of viruses that enter the body are antigens. Antigens– these are any foreign substances that, upon penetration into the body, are perceived as genetically foreign and cause an immune reaction. Immunity is the ability of organisms to protect themselves from pathogens, viruses and other foreign bodies and substances, thereby maintaining the constancy of their composition and properties.

There are several types of immunity. If immunity exists or occurs in a person without any special influence, it is called natural. Immunity obtained through the use of medications is called artificial.

Natural innate immunity is the same in all individuals of the species and is inherited, i.e., genetically fixed. Thus, humans do not suffer from many diseases that occur in animals. For example, a person will never get canine distemper, just as a dog will never get the flu.

Natural acquired immunity differs from person to person and is not inherited, which is why it is also called individual immunity. Passive natural immunity provide antibodies received by the child from the mother along with breast milk. Active natural immunity formed after an illness. This type of immunity is also called post-infectious immunity. It remains in the body for a long time. After some diseases, immunity lasts for life, for example after measles, rubella, scarlet fever and other “childhood diseases”.

Artificial immunity can only be acquired. Artificial active immunity is formed in response to the introduction of a vaccine into the body. Vaccine is a preparation made from weakened or killed pathogens, their fragments or toxins. When a vaccine is administered (vaccination), a weak immune response develops in the body, as a result of which special cells are formed in the blood that are capable of synthesizing antibodies to this pathogen. Antibodies- These are complex proteins (immunoglobulins). They are able to bind to antigens and neutralize them. When antigen binds, an inactive antigen-antibody complex is formed, which can be destroyed by leukocytes.

Artificial active immunity is persistent and lasts for years. For the first time, systematic vaccinations against smallpox began to be used at the beginning of the 19th century. after the works of the English physician Edward Jenner (1749–1823). His work was continued by the French microbiologist Louis Pasteur (1822–1895). He coined the term “vaccine” and used vaccination in medical practice.

Artificial passive immunity occurs when a person is given a medicinal serum, which already contains ready-made antibodies against the pathogen. This is especially important if infection has already occurred. Passive immunity is unstable and lasts for 4–6 weeks, during which the antibodies are gradually destroyed.

Your future profession

1. Prove that basic knowledge about the processes occurring at the molecular and cellular levels of the organization of living things are necessary not only for biologists, but also for specialists in other fields of natural sciences.

2. What professions in modern society require knowledge of the structure and vital functions of prokaryotic organisms? Prepare a short (no more than 7-10 sentences) message about the profession that most impressed you. Explain your choice.

3. “These specialists are needed in veterinary and medical scientific institutes, academic institutes, and biotechnology-related enterprises. They will not be left without work in laboratories of clinics and hospitals, at agronomic breeding stations, in veterinary laboratories and hospitals. Sometimes they are the ones who can make the most reliable and accurate diagnosis. Their research is indispensable for the early diagnosis of cancer.” Guess which professions we are talking about in these sentences. Prove your point.

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From the book Microbiology author Tkachenko Ksenia Viktorovna

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From the book Genes and the development of the body author Neyfakh Alexander Alexandrovich

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From the author's book

Chapter II. Viruses in nature and human life

From the author's book

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