The nature of viruses and their study. Nature and origin of viruses
General virology.
Exam questions
13. Ecology of viruses. Nature and origin. Principles of classification.
14. Virus as a special form of life. Principles of virion structure. The significance of viruses in human pathology. Works by D. Ivanovsky.
15. Molecular basis of viral reproduction. Reproduction of DNA viruses.
16. Molecular basis of viral reproduction. Reproduction options RNA viruses.
17. Forms of interaction between the virus and the cell. Virus persistence. Ecological significance and clinical manifestations.
18. Virus persistence: concept and molecular mechanisms.
19. The concept of oncogenes, their nature and mechanisms of action on the cell. Oncogenic viruses and mechanisms of viral carcinogenesis.
20. Bacteriophages. Phases of interaction with a bacterial cell. Temperate and virulent phages. Practical use phages.
And others…
Terms from genetics
REPLICATION (repetition) - doubling of DNA molecules (in some RNA viruses) with the participation of special enzymes.
TRANSCRIPTION (rewriting) - biosynthesis of RNA molecules on the corresponding sections of DNA.
TRANSLATION (transmission) - biosynthesis of proteins in a living cell on ribosomes.
Protein synthesis
Broadcast
(in photo 1, 2 – subunits
Ribosome transcription; 3 – polypeptide chain)
This is a special form of existence of living matter.
1892 - Ivanovsky D.I. While studying diseases of tobacco leaves, he discovered the tobacco mosaic virus. Formulated the differences between viruses and bacteria:
filterability through bacterial filters,
inability to grow on artificial media.
Fundamental differences between viruses and prokaryotes and eukaryotes
1) noncellular structure
2) smallest dimensions
3) one type of nucleic acid
4) absence of ribosomes (protein synthesizing apparatus)
5) absence of even the rudiments of one’s own energy metabolism
7) disjunctive (separated) method of reproduction
Viruses play an important role in human life, as they can cause diseases of varying severity.
According to epidemiological characteristics, viral diseases are divided into anthroponotic, then there are those that only affect humans (for example, polio) and zooanthroponotic - that are transmitted from animals to humans (for example, rabies).
The main routes of transmission of viral infection are:
1.Food route, in which the virus enters the human body with contaminated food and water (viral hepatitis A, E, etc.)
2.Parenteral ( or through blood), in which virus enters directly into the blood or internal environment of a person. This mainly occurs when manipulating contaminated surgical instruments or syringes, during unprotected sexual intercourse, and also transplacentally from mother to child. In this way, fragile viruses are transmitted that quickly break down in environment(hepatitis B virus, HIV, rabies virus, etc.).
3.Respiratory tract, which is characterized by an airborne transmission mechanism, in which the virus enters the human body along with inhaled air, which contains particles of sputum and mucus thrown out by a sick person or animal. This is the most dangerous route of transmission, since the virus can be transported through the air over significant distances and cause entire epidemics. This is how influenza, parainfluenza, mumps, chickenpox, etc. viruses are transmitted.
Most viruses have a certain affinity for one or another organ. For example, hepatitis viruses multiply primarily in liver cells. By type of target organs that are affected during a particular disease, we distinguish the following types of viral diseases: intestinal, respiratory (respiratory), affecting the central and peripheral nervous system, internal organs, skin and mucous membranes, blood vessels, immune system, etc. By type clinical development we distinguish acute and chronic viral infections. Most common acute viral diseases, which occur with severe local symptoms (damage to the mucous membrane of the respiratory tract, damage to liver tissue, damage to various areas of the brain) and general- increased body temperature, weakness, pain in joints and muscles, changes in blood composition, etc. Acute viral infection, as a rule, ends with complete recovery of the body. In some cases, the acute form of the disease becomes chronic. Chronic viral infections occur with a blurred clinical picture and sometimes may not be noticed by the patients themselves. Chronic infections difficult to treat and may progress long time, leading to significant morphological and functional changes in internal organs (for example, chronic hepatitis B can lead to cirrhosis of the liver).
Virology- the science that studies the morphology, physiology, genetics, ecology and evolution of viruses
The word "virus" meant poison. This term was also used by L. Pasteur to designate an infectious principle. Currently, a virus means tiny replicating microorganisms, found wherever there are living cells.
The discovery of viruses belongs to the Russian scientist Dmitry Iosifovich Ivanovsky, who in 1892 published a work on the study of tobacco mosaic disease. D.I. Ivanovsky showed that the causative agent of this disease is very small in size and does not linger on bacterial filters, which are an insurmountable obstacle for the smallest bacteria. In addition, the causative agent of tobacco mosaic disease is not able to be cultivated on artificial nutrient media. D.I. Ivanovsky discovered plant viruses.
In 1898, Loeffler and Frosch showed that foot-and-mouth disease, a widespread cattle disease, was caused by an agent that also passed through bacterial filters. This year is considered the year of discovery of animal viruses.
In 1901, Reed and Carroll showed that filterable agents could be isolated from the corpses of people who died of yellow fever. This year is considered the year of discovery of human viruses.
D'Herrel and Twort in 1917-1918 discovered viruses in bacteria, calling them "bacteriophages" Later, viruses were isolated from insects, fungi, and protozoa.
Viruses still remain one of the main causative agents of infectious and non-infectious human diseases. About 1000 different diseases are viral in nature. Viruses and the human diseases they cause are the object of study in medical virology.
It is generally accepted that viruses originated from isolation (autonomization) of individual genetic elements of a 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 predecessors, or progenitors, of viruses.
Prions- fundamentally new class pathogens, discovered and classified relatively recently, despite the fact that some diseases caused by these pathogens have been known for about a century. The term “prion” is formed as an anagram of the English words “proteinaceous infectious (particles)”. Prions are defined as “a small protein infectious particle that is resistant to inactivating influences that modify nucleic acids,” in other words, prions are ordinary proteins of the body, which for some reason (which are still unknown) begin to behave “incorrectly”.
The discovery of prions is closely connected with the history of the discovery and formation of the doctrine of slow infections, when in 1954 B. Sigurdsson (Sweden) presented the results of his many years of research into mass diseases among sheep brought in 1933 from Germany to the island. Iceland for the development of karakul farming. Despite the obvious clinical differences and unequal localization of damage to organs and tissues, the Swedish scientist was able to discover fundamental similarities among the diseases he studied, which modern form can be summarized as four main features that distinguish slow infections:
- unusually long (months or years) incubation period;
- slowly progressive nature of the course;
- unusualness of damage to organs and tissues;
the inevitability of death.
Viruses were discovered by the Russian botanist D.I. Ivanovsky (1864 – 1920) in 1892 while studying mosaic disease of tobacco leaves. The term “virus” was first proposed in 1898 by the Dutch scientist M. Beijerinck (1851 – 1931).
Currently, about 3,000 different types of viruses are known.
The sizes of viruses range from 15 to 350 nm (the length of some filamentous ones reaches 3,000 nm; 1 nm = 1·10–9 m), i.e. most of them are not visible in a light microscope (submicroscopic) and their study became possible only after the invention of the electron microscope.
Unlike all other organisms, viruses do not have a cellular structure!
Mature viral particle (i.e. extracellular, resting – virion) structure is very simple: it consists of one or more molecules of nucleic acids that make up core virus and protein shell (capsid)- these are the so-called simple viruses.
Complex viruses(For example, herpes or flu) in addition, capsid proteins and nucleic acid contain additional lipoprotein membrane(envelope, supercapsid formed from the plasma membrane of the host cell), various carbohydrates And enzymes(Fig. 3.1).
Enzymes facilitate the penetration of viral NK into the cell and the release of the resulting virions into the environment ( neuraminidase myxoviruses, ATPase And lysozyme some phages, etc.), and also participate in the processes of transcription and replication of viral NK (various transcriptases And replicase).
Protein shell protects nucleic acid from various physical and chemical influences, and also prevents the penetration of cellular enzymes into it, thereby preventing its breakdown (protective function). Also, the capsid contains a receptor that is complementary to the receptor of the infected cell - viruses infect a strictly defined range of hosts (defining function).
Virions many plant viruses and a number of phages have spiral a capsid in which the protein subunits (capsomeres) are arranged in a spiral around an axis. For example, VTM ( tobacco mosaic virus) has the shape of rods with a diameter of 15–17 nm and a length of up to 300 nm (Fig. 3.2.). Inside its capsid there is a hollow channel with a diameter of 4 nm. The genetic material of TMV is Rice. 3.2.
The structure of the tobacco mosaic virus.
There is single-stranded RNA tightly packed in the groove of the helical capsid. For virions with a helical capsid is characterized by a high protein content (90 – 98%) in relation to
The capsids of virions of many viruses (for example, adenovirus, virus herpes, yellow virus turnip mosaics– VZhMT) have the shape of a symmetrical polyhedron, most often an icosahedron (a polyhedron with 12 vertices, 20 triangular faces and 30 edges). Such capsids are called isometric(Fig. 3.3.). In such virions, the protein content is about 50% relative to NK.
A virus always contains one type of nucleic acid (either DNA or RNA), therefore all viruses are divided into DNA-containing and RNA-containing. The nucleic acid molecules in the virion can be linear (RNA, DNA) or ring-shaped (DNA). Moreover, these nucleic acids can consist of one chain or two. Viral NK has from 3 to 200 genes.
The nucleic acid of the virus combines the functions of both acids (DNA and RNA) - storage and transmission of hereditary information, as well as control of protein synthesis.
Unlike viruses, all cellular organisms contain both types of nucleic acids.
Bacterial viruses have a more complex structure – bacteriophages(Fig. 3.4.). They consist of a head and tail (rod and sheath, basal plate and process filaments). A long NA molecule (RNA or DNA) is folded into a spiral inside the bacteriophage head (protein shell).
Viruses also include viroids– infectious agents that are low molecular weight (short) single-stranded circular RNAs that do not encode their own proteins (lacking a capsid). They are causative agents of a number of diseases.
TO
IN
Rice. 3.4.
The structure of a bacteriophage. Threads of the process
Scientists believe that viruses arose about 3 billion years ago from the nucleic acids of organisms (prokaryotes) as a result of the isolation of free fragments from the genome that have acquired the ability to synthesize a protein shell and divide (double, replicate) inside cells. It has been suggested that new types of viruses are still being formed from the genome of bacteria and eukaryotes(nuclei, plastids, mitochondria) .
In nature, viruses are of great importance, since they are ubiquitous and affect all groups of living organisms, often causing various diseases.
More than 1000 diseases are known plants caused by viruses (RNA-containing). The most common are various necrosis(areas of dead tissue), mosaics(spots, specks, stripes on plant organs), in which parenchyma tissue is damaged, the number of chloroplasts decreases, phloem is destroyed, etc.; wrinkled or dwarfed leaves are observed. Viruses cause stunted plant growth, which leads to reduced yields.
VZhMT – turnip yellow mosaic virus, VTM – tobacco mosaic virus, VKKT – tomato dwarf bushiness virus.
The appearance of stripes on the flowers of some varieties of tulips (variegated) is also caused by a virus, but flower growers sell these tulips, passing them off as a special variety.
U animals viruses (DNA and RNA containing) cause diseases such as: foot and mouth disease(in cattle) rabies(in dogs, foxes, wolves), myxomatosis(in rats) sarcoma, leukemia And plague(in chickens), etc. Very often people become infected with these diseases (through contact with infected animals).
U person Viruses cause diseases such as: smallpox(variola virus) piggy(paramyxovirus), flu(myxovirus), respiratory diseases(ARI; rhinoviruses RNA-), infectious hepatitis, polio(infantile paralysis; picornavirus), rabies, herpes, AIDS(human immunodeficiency virus - HIV).
Flu - the only infectious disease that manifests itself in the form of periodic global epidemics that are dangerous to human life. The infectious properties of the influenza virus (affects the mucous membranes of the respiratory tract), like other viruses, depend on specific proteins of the viral envelope, which constantly change as a result of recombinations or mutations. Therefore, new strains of the influenza virus cause new epidemics, since humans have not yet developed immunity to them.
Thus, in the winter of 1968/69, 50 million cases were registered in the USA hong kong flu while 70,000 people died. The epidemic of 1918/19 covered the entire Earth, took place in three waves and claimed 20 million human lives.
Viral diseases are difficult to treat because viruses are not sensitive to antibiotics. Fortunately, in many cases the immune system limits the infection from spreading further.
Numerous viral diseases of humans and animals can be prevented by immunization– carrying out preventive vaccinations that allow you to develop immunity against viruses.
Viruses are widely used by humans in microbiological research (biotechnology, genetic engineering). It is possible to use viruses to control crop pests.
In the USA with cotton boll effectively fight with the help of the virus. This method of control is practically harmless - the virus, as a rule, is species-specific (that is, it affects only a certain type of organism).
It was also found that, for example, rice necrotic mosaic virus inhibits the growth of rice. But other plants, for example, jute(a source of coarse fibers for bags and ropes), grow better when affected by this virus than when healthy. Scientists cannot yet explain this phenomenon.
Bacteriophages infect bacteria (penetrate and actively destroy them), including pathogenic ones. Therefore, it is possible to use them to prevent and treat many infectious diseases and to combat pathogenic bacteria: plague, typhoid fever, cholera and etc.
Nature of viruses
The existence of viruses was first established by studying tobacco mosaic disease. It turned out that the causative agent of this disease can pass through a porcelain filter, usually used to trap bacteria. The size of viruses ranges from 17 to 300 nm in diameter. Thus, they are comparable in size to molecules; for example, a hydrogen atom has a diameter of about 0.1 nm, and the size of a protein molecule is on average tens of nanometers.
Viruses reproduce only in living cells. Many of them are highly specific to the type of cells being infected. They radically alter the biosynthetic processes of the host cell. In this case, the nucleic acid of the virus switches the cell to the synthesis of virus-specific structures, thus competing with its genetic apparatus. For example, respiratory viruses multiply in the cells of the mucous membrane of the respiratory tract, causing characteristic symptoms colds. Most often, viruses have a narrow range of hosts. One of quick methods identification of unknown bacteria can be achieved by using specific bacteriophages that destroy certain bacterial cells. Conversely, the response of some plant species to an unknown virus can be used (in conjunction with other methods) to identify the virus.
Until the 1930s viruses were considered to be the smallest bacteria. In 1933, this point of view was refuted. Wendell Stanley, who worked at the Rockefeller Institute, obtained an extract of the tobacco mosaic virus from infected plants and purified it. The purified virus precipitated in the form of crystals. Crystallization is one of the main tests for the presence of a chemically pure compound free of impurities. Thus, it became clear that from a chemical point of view, the virus is much simpler than a living organism. When Stanley dissolved the needle-shaped crystals and applied them to a tobacco leaf, the characteristic symptoms of mosaic disease reappeared. Thus, it was shown that the virus remains infective after crystallization and resuspension.
Most plant viruses, like tobacco mosaic virus, contain only RNA, while other viruses contain only DNA. Unlike viruses, all cellular organisms contain both types of nucleic acids. Viruses lack ribosomes as well as enzymes necessary for protein synthesis and energy generation. In this respect, viruses are fundamentally different from organisms that have a cellular organization.
Viroids and other infectious particles
Several molecular pathogens are known, similar to viruses and. apparently derived from the genome of bacteria and eukaryotes. Of particular importance among them are viroids, which, despite their name, are sharply different from viruses.
Viroids are the smallest known pathogens. They are much smaller than the smallest viral genomes and lack a protein coat. Only plant viroids are known. They consist of a single-stranded RNA molecule that replicates autonomously in infected cells. Viroids have been identified as causative agents of dangerous diseases. One of them has caused the death of millions of coconut trees in the Philippines over the past fifty years, the other crippled the commercial cultivation of chrysanthemums in the United States in the early 1950s.
The first viroid, potato spindle tuber tuber viroid, or PSTV, was identified by Theodore Diner of the US Department of Agriculture in 1971. Potato tubers infected with PSTV are elongated and twisted in shape. Sometimes deep cracks appear on them. PSTV is the largest virion known. Its RNA consists of 359 bases and has either a closed ring shape or a hairpin structure. In both cases, complementary base pairs are joined by hydrogen bonds, forming double-stranded RNA, similar to DNA. Under an electron microscope, both forms of PSTV appear rod-shaped; their length is 50 nm. Although it is the largest virion, it is only one-tenth the size of the genome of the smallest virus. Viroids are found only in the nuclei of infected cells. They replicate like viruses, that is, by synthesizing a complementary chain that functions as a template. In this case, viroids use the enzyme systems of the host cell.
Because viroids are localized in the nucleus and probably cannot function as mRNA, they are thought to cause disease by interfering with host cell gene regulation. Some proteins are present in larger quantities in infected plant cells than in healthy ones. Although nucleotide sequences complementary to PSTV have not been found in healthy plants, it is suggested that PSTV may have arisen as a result of a change in the genome of some potato species, its main host.
Molecular pathogens that are not related to viroids occur in living organisms. The existence of structures similar to viroids, but made from DNA, is also assumed in animals. These are called "subviral particles". Surprisingly, some protein fragments are able to control their reproduction in animal cells without the participation of nucleic acids; such particles are called “prions.”