The morphology and structure of viruses is studied using an electron microscope, since their size is small and comparable to the thickness of the bacterial membrane. The form of virions can be different: rod-shaped (tobacco mosaic virus), bullet-shaped (rabies virus), spherical (poliomyelitis viruses, HIV), in the form of a sperm cell (many bacteriophages).
The sizes of viruses are determined using electron microscopy, by ultrafiltration through filters with a known pore diameter, by ultracentrifugation. One of the smallest viruses is the poliomyelitis virus (about 20 nm), the largest is smallpox (about 350 nm).
Distinguish between simple (for example, polio virus) and complex (for example, influenza, measles) viruses. In viruses simply arranged, the nucleic acid is bound to a protein envelope called the capsid (from Latin capsa - sheath). The capsid consists of repeating morphological subunits - capsomeres. Nucleic acid and capsid interact with each other to form a nucleocapsid. In complex viruses, the capsid is surrounded by an additional lipoprotein envelope - the supercapsid (a derivative of the membrane structures of the host cell), which has spikes. Virions are characterized by a spiral, cubic and complex type of capsid symmetry. The spiral type of symmetry is due to the helical structure of the nucleocapsid, the cubic type of symmetry is due to the formation of an isometrically hollow body from the capsid containing viral nucleic acid.
Capsid and supercapsid protect virions from environmental influences, cause selective interaction (adsorption) with cells, and determine the antigenic and immunogenic properties of virions. The internal structures of viruses are called the core. In virology, the following taxonomic categories are used: family (name ends in viridae), subfamily (name ends in virinae), genus (name ends in virus).
However, the names of genera and especially subfamilies are not formulated for all viruses. The type of virus has not received a binomial name, as in bacteria.
The classification of viruses is based on the following categories:
§ the type of nucleic acid (DNA or RNA), its structure, the number of strands (one or two),
§ features of the reproduction of the viral genome;
§ the size and morphology of virions, the number of capsomeres and the type of symmetry;
§ the presence of a supercapsid;
§ sensitivity to ether and deoxycholate;
§ breeding site in the cage;
§ antigenic properties, etc.
Viruses infect vertebrates and invertebrates, as well as plants and bacteria. Being the main causative agents of human infectious diseases, viruses are also involved in the processes of carcinogenesis, can be transmitted in various ways, including through the placenta (rubella virus, cytomegalovirus, etc.), affecting the human fetus. They can lead to post-infectious complications - the development of myocarditis, pancreatitis, immunodeficiency, etc.
In addition to common viruses, so-called non-canonical viruses are known - prions - protein infectious particles that are agents of a protein nature, in the form of fibrils with a size of 10.20x100.200 nm. Prions, apparently, are both inducers and products of the autonomous gene of humans or animals and cause encephalopathy in them under conditions of a slow viral infection (Creutzfeldt, Jacob, Kuru, etc.). Other unusual agents close to viruses are viroids - small molecules of circular, supercoiled RNA that do not contain protein that cause disease in plants.
Chapter 3
PHYSIOLOGY OF MICROORGANISMS
Physiology of microorganisms studies the vital activity of microbial cells, the processes of their nutrition, respiration, growth, reproduction, and the laws of interaction with the environment.
The subject of medical microbiology is the physiology of pathogenic and opportunistic microorganisms that can cause human diseases. Elucidation of the physiology of these microorganisms is important for making a microbiological diagnosis, understanding pathogenesis, treating and preventing infectious diseases, regulating human relationships with the environment, etc.
The chemical composition of bacteria
The composition of microorganisms includes water, proteins, nucleic acids, carbohydrates, lipids, and minerals.
Water is the main component of a bacterial cell, accounting for about 80% of its mass. It is in a free or bound state with the structural elements of the cell. In disputes, the amount of water is reduced to 18.20%. Water is a solvent for many substances and also plays a mechanical role in providing turgor. During plasmolysis - the loss of water by the cell in a hypertonic solution - the protoplasm is detached from the cell membrane. Removal of water from the cell, drying, suspend metabolic processes. Most microorganisms tolerate drying well. With a lack of water, microorganisms do not multiply. Vacuum drying from a frozen state (lyophilization) stops reproduction and promotes long-term preservation of microbial specimens.
Proteins (40.80% dry weight) determine the most important biological properties of bacteria and usually consist of combinations of 20 amino acids. The bacteria include diaminopimelic acid (DAP), which is absent in human and animal cells. Bacteria contain more than 2000 different proteins found in structural components and involved in metabolic processes. Most of the proteins have enzymatic activity. The proteins of the bacterial cell determine the antigenicity and immunogenicity, virulence, and species of bacteria.
Nucleic acids of bacteria perform functions similar to those of eukaryotic cells: a DNA molecule in the form of a chromosome is responsible for heredity, ribonucleic acids (informational, or matrix, transport and ribosomal) are involved in protein biosynthesis.
Bacteria can be characterized (taxonomically) by the content of the sum of guanine and cytosine (GC) in molar percentages (M%) of the total number of DNA bases. A more accurate characteristic of microorganisms is their DNA hybridization. The basis of the hybridization method
DNA - the ability of denatured (single-stranded) DNA to be renatured, i.e. connect with a complementary strand of DNA and form a double-stranded DNA molecule.
Bacterial carbohydrates are represented by simple substances (mono- and disaccharides) and complex compounds. Polysaccharides are often found in capsules. Some intracellular polysaccharides (starch, glycogen, etc.) are reserve nutrients.
Lipids are mainly part of the cytoplasmic membrane and its derivatives, as well as the cell wall of bacteria, for example, the outer membrane, where, in addition to the biomolecular layer of lipids, there is LPS. Lipids can play the role of reserve nutrients in the cytoplasm. Bacterial lipids are represented by phospholipids, fatty acids and glycerides. Mycobacterium tuberculosis contains the largest amount of lipids (up to 40%).
Mineral substances of bacteria are found in ash after cells are burned. Phosphorus, potassium, sodium, sulfur, iron, calcium, magnesium, as well as trace elements (zinc, copper, cobalt, barium, manganese, etc.) are detected in large quantities. They are involved in the regulation of osmotic pressure, pH of the medium, redox potential , activate enzymes, are part of enzymes, vitamins and structural components of the microbial cell.
Bacteria nutrition
The nutritional features of a bacterial cell consist in the intake of nutrient substrates through its entire surface, as well as in a high rate of metabolic processes and adaptation to changing environmental conditions.
Types of food... The widespread distribution of bacteria is facilitated by a variety of types of food. Microorganisms need carbohydrates, nitrogen, sulfur, phosphorus, potassium and other elements. Depending on the carbon sources for nutrition, bacteria are divided into autotrophs (from the Greek autos - itself, trophe - food), using carbon dioxide CO2 and other inorganic compounds to build their cells, and heterotrophs (from the Greek heteros - another, trophe - food), feeding on ready-made organic compounds. Autotrophic bacteria are nitrifying bacteria found in the soil; sulfur bacteria living in water with hydrogen sulfide; iron bacteria living in water with ferrous iron, etc.
Microorganisms are divided into two groups depending on the oxidizable substrate, called an electron or hydrogen donor. Microorganisms using inorganic compounds as hydrogen donors are called lithotrophic (from the Greek lithos - stone), and microorganisms using organic compounds as hydrogen donors are called organotrophs.
Considering the energy source, phototrophs are distinguished among bacteria, i.e. photosynthetic (for example, blue-green algae that use light energy), and chemotrophs, which require chemical energy sources.
Growth factors... For microorganisms to grow on nutrient media, certain additional components are required, which are called growth factors. Growth factors are compounds necessary for microorganisms that they cannot synthesize themselves, therefore they must be added to nutrient media. Among the growth factors are distinguished: amino acids necessary for building proteins; purines and pyrimidines, which are required for the formation of nucleic acids; vitamins that are part of some enzymes. The terms "auxotrophs" and "prototrophs" are used to denote the ratio of microorganisms to growth factors. Auxotrophs require one or more growth factors; prototrophs can themselves synthesize compounds necessary for growth. They are able to synthesize components from glucose and ammonium salts.
Nutritional mechanisms. The entry of various substances into a bacterial cell depends on the size and solubility of their molecules in lipids or water, the pH of the medium, the concentration of substances, various factors of membrane permeability, etc. The cell wall allows small molecules and ions to pass through, trapping macromolecules weighing more than 600 D. The main regulator of the intake of substances the cell is the cytoplasmic membrane. Conventionally, four mechanisms can be distinguished for the penetration of nutrients into a bacterial cell: these are simple diffusion, facilitated diffusion, active transport, and group translocation. The simplest mechanism for the entry of substances into the cell is simple diffusion, in which the movement of substances occurs due to the difference in their concentration on both sides of the cytoplasmic membrane. Substances pass through the lipid part of the cytoplasmic membrane (organic molecules, drugs) and, less often, through channels filled with water in the cytoplasmic membrane. Passive diffusion takes place without energy consumption.
Facilitated diffusion also occurs as a result of a difference in the concentration of substances on both sides of the cytoplasmic membrane. However, this process is carried out with the help of carrier molecules localized in the cytoplasmic membrane and possessing specificity. Each carrier transports the corresponding substance through the membrane or transfers to another component of the cytoplasmic membrane - the carrier itself.
Carrier proteins can be permeases, the place of synthesis of which is the cytoplasmic membrane. Facilitated diffusion proceeds without energy consumption, substances move from a higher concentration to a lower one.
Active transport occurs with the help of permeases and is aimed at transferring substances from a lower concentration to a higher one, i.e. as if against the flow, therefore, this process is accompanied by the expenditure of metabolic energy (ATP), formed as a result of redox reactions in the cell.
The transfer (translocation) of groups is similar to active transport, differing in that the transferred molecule is modified during the transfer, for example, it is phosphorylated. The release of substances from the cell is carried out by diffusion and with the participation of transport systems - enzymes of bacteria. Enzymes recognize the corresponding metabolites (substrates), interact with them and accelerate chemical reactions. Enzymes are proteins that participate in the processes of anabolism (synthesis) and catabolism (decay), i.e. metabolism. Many enzymes are interconnected with the structures of the microbial cell. For example, in the cytoplasmic membrane there are redox enzymes involved in respiration and cell division; enzymes that provide nutrition for the cell, etc. Redox enzymes of the cytoplasmic membrane and its derivatives provide energy for intensive processes of biosynthesis of various structures, including the cell wall. Enzymes associated with cell division and autolysis are found in the cell wall. The so-called endozymes catalyze the metabolism that takes place inside the cell.
Exoenzymes are secreted by the cell into the environment, breaking down the macromolecules of nutrient substrates to simple compounds that are assimilated by the cell as sources of energy, carbon, etc. Some exozymes (penicillinase, etc.) inactivate antibiotics, performing a protective function.
Distinguish between constitutive and inducible enzymes. Constitutive enzymes include enzymes that are synthesized by the cell continuously, regardless of the presence of substrates in the nutrient medium. Inducible (adaptive) enzymes are synthesized by a bacterial cell only if there is a substrate of this enzyme in the medium. For example, E. coli p-galactosidase is practically not formed on a medium with glucose, but its synthesis increases sharply when grown on a medium with lactose or other p-galactosidosis.
Some enzymes (the so-called aggression enzymes) destroy tissue and cells, causing widespread microorganisms and their toxins in the infected tissue. These enzymes include hyaluronidase, collagenase, deoxyribonuclease, neuraminidase, lecitovitellase, etc. Thus, streptococcal hyaluronidase, splitting the hyaluronic acid of the connective tissue, promotes the spread of streptococci and their toxins.
More than 2000 enzymes are known. They are grouped into six classes: oxidoreductases - redox enzymes (they include dehydrogenases, oxidases, etc.); transferases, which transfer individual radicals and atoms from one compound to another; hydrolases that accelerate hydrolysis reactions, i.e. splitting substances into simpler ones with the addition of water molecules (esterase, phosphatase, glucosidase, etc.); lyases that cleave chemical groups from substrates in a non-hydrolytic way (carboxylases, etc.); isomerases, which convert organic compounds into their isomers (phosphohexoisomerase, etc.); ligases, or synthetases, which accelerate the synthesis of complex compounds from simpler ones (asparagine synthetase, glutamine synthetase, etc.).
Differences in the enzymatic composition are used to identify microorganisms, since they determine their various biochemical properties: saccharolytic (breakdown of sugars), proteolytic (breakdown of proteins), and others, detected by the end products of breakdown (formation of alkalis, acids, hydrogen sulfide, ammonia, etc.) ...
Enzymes of microorganisms are used in genetic engineering (restriction enzymes, ligases, etc.) to obtain biologically active compounds, acetic, lactic, citric and other acids, lactic acid products, in winemaking and other industries. Enzymes are used as bioadditives in washing powders ("Oka", etc.) to destroy protein contaminants.
Bacteria breathing
Breathing, or biological oxidation, is based on redox reactions that lead to the formation of ATP, a universal accumulator of chemical energy. The microbial cell needs energy for its vital activity. During respiration, the processes of oxidation and reduction take place: oxidation is the release of hydrogen or electrons by donors (molecules or atoms); reduction - the addition of hydrogen or electrons to an acceptor. The acceptor of hydrogen or electrons can be molecular oxygen (such respiration is called aerobic) or nitrate, sulfate, fumarate (such respiration is called anaerobic - nitrate, sulfate, fumarate). Anaerobiosis (from the Greek aeg - air + bios - life) is a vital activity that occurs in the absence of free oxygen. If organic compounds are donors and acceptors of hydrogen, then this process is called fermentation. During fermentation, enzymatic breakdown of organic compounds, mainly carbohydrates, occurs under anaerobic conditions. Taking into account the final product of the breakdown of carbohydrates, alcohol, lactic acid, acetic acid and other types of fermentation are distinguished.
In relation to molecular oxygen, bacteria can be divided into three main groups: obligate, i.e. obligatory, aerobes, obligate anaerobes and facultative anaerobes.
Obligate aerobes can only grow in the presence of oxygen. Obligate anaerobes (clostridia botulism, gas gangrene, tetanus, bacteroids, etc.) grow only in an environment without oxygen, which is toxic to them. In the presence of oxygen, bacteria form oxygen peroxide radicals, including hydrogen peroxide and oxygen superoxide anion, which are toxic to obligate anarobic bacteria, since they do not form the corresponding inactivating enzymes. Aerobic bacteria inactivate hydrogen peroxide and superoxide anion with the corresponding enzymes (catalase, peroxidase and superoxide dismutase). Facultative anaerobes can grow in the presence or absence of oxygen, since they are able to switch from respiration in the presence of molecular oxygen to fermentation in the absence of it. Facultative anaerobes are able to carry out anaerobic respiration, called nitrate: nitrate, which is a hydrogen acceptor, is reduced to molecular nitrogen and ammonia. Among obligate anaerobes, aerotolerant bacteria are distinguished, which persist in the presence of molecular oxygen, but do not use it.
For the cultivation of anaerobes in bacteriological laboratories, anaerostats are used - special containers in which the air is replaced by a mixture of gases that do not contain oxygen. Air can be removed from culture media by boiling, using chemical oxygen adsorbents placed in anaerostats or other containers with crops.
Growth and reproduction of bacteria
The vital activity of bacteria is characterized by growth - the formation of structural and functional components of the cell and an increase in the bacterial cell itself, as well as reproduction - self-reproduction, leading to an increase in the number of bacterial cells in the population.
Bacteria multiply by binary dividing in half, less commonly by budding.
Actinomycetes, like fungi, can multiply by spores. Actinomycetes, being branching bacteria, multiply by fragmentation of filamentous cells. Gram-positive bacteria divide by ingrowing synthesized division partitions into the cell, and gram-negative bacteria divide by constriction, as a result of the formation of dumbbell-shaped figures, from which two identical cells are formed.
Cell division is preceded by the replication of the bacterial chromosome in a semi-conservative manner (the double-stranded DNA strand opens and each strand is completed with a complementary strand), which leads to a doubling of the DNA molecules of the bacterial nucleus - the nucleoid. Replication of chromosomal DNA is carried out from the starting point ogi (from the English, origin - beginning).
The chromosome of a bacterial cell is connected in the region with the cytoplasmic membrane. DNA replication is catalyzed by DNA polymerases. First, unwinding (despiralization) of the double DNA strand occurs, resulting in the formation of a replicative fork (branched chains); one of the chains, completing construction, binds nucleotides from 5 "- to the Z" -end, the other is completed segment by segment.
DNA replication occurs in three stages: initiation, elongation, or chain growth, and termination. The two chromosomes formed as a result of replication diverge, which is facilitated by an increase in the size of the growing cell: the chromosomes attached to the cytoplasmic membrane or its derivatives (for example, mesosomes) move away from each other as the cell volume increases. Their final isolation ends with the formation of a constriction or division partition. Cells with a division septum diverge as a result of the action of autolytic enzymes that destroy the core of the division septum. In this case, autolysis can proceed unevenly: dividing cells in one area remain connected by a part of the cell wall in the area of the division septum. Such cells are located at an angle to each other, which is characteristic of diphtheria corynebacteria.
Reproduction of bacteria in a liquid nutrient medium. Bacteria inoculated in a certain, unchanging volume of the nutrient medium, multiplying, consume nutrients, which further leads to depletion of the nutrient medium and the cessation of bacterial growth. Cultivating bacteria in such a system is called batch culture, and culture is called batch culture. If the cultivation conditions are maintained by continuous supply of fresh nutrient medium and the outflow of the same volume of culture liquid, then such cultivation is called continuous, and the culture is called continuous.
When bacteria are grown on a liquid nutrient medium, a bottom, diffuse, or superficial (in the form of a film) culture growth is observed. The growth of a batch culture of bacteria grown on a liquid nutrient medium is subdivided into several phases, or periods:
§ lag phase;
§ phase of logarithmic growth;
§ phase of stationary growth, or maximum concentration
§ bacteria;
§ phase of death of bacteria.
These phases can be depicted graphically in the form of segments of the bacterial growth curve, reflecting the dependence of the logarithm of the number of living cells on the time of their cultivation. Lag phase (from English, lag - lag) - the period between the sowing of bacteria and the beginning of reproduction. The duration of the lag-Phase is on average 4.5 hours. The bacteria increase in size and prepare for division; the amount of nucleic acids, protein and other components increases. The phase of logarithmic (exponential) growth is a period of intensive division of bacteria.
Its duration is about 5.6 hours. Under optimal growth conditions, bacteria can divide every 20-40 minutes. During this phase, bacteria are most vulnerable, which is explained by the high sensitivity of the metabolic components of an intensively growing cell to inhibitors of protein synthesis, nucleic acids, etc. ... Its duration is expressed in hours and varies depending on the type of bacteria, their characteristics and cultivation. The process of bacterial growth is completed by the phase of death, characterized by the death of bacteria under conditions of depletion of the sources of the nutrient medium and the accumulation of products of bacterial metabolism in it. Its duration ranges from 10 hours to several weeks. The intensity of growth and reproduction of bacteria depends on many factors, including the optimal composition of the nutrient medium, redox potential, pH, temperature, etc.
Reproduction of bacteria on a dense nutrient medium. Bacteria growing on solid nutrient media form isolated colonies of a rounded shape with smooth or uneven edges (S- and R-shaped; see Chapter 5), of varying consistency and color, depending on the bacterial pigment.
Water-soluble pigments diffuse into the culture medium and color it, for example Pseudomonas aeruginosa (Pseudomonas aeruginosa) colors the medium blue. Another group of pigments is insoluble in water, but soluble in organic solvents. So, the colonies of the "miracle stick" have a blood-red pigment, soluble in alcohol. And finally, there are pigments that are not soluble in water or in organic compounds.
The most common among microorganisms are pigments such as carotenes, xanthophylls and melanins. Melanins are insoluble black, brown or red pigments synthesized from phenolic compounds. Melanins, along with catalase, superoxide cismutase and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.
The type, shape, color and other features of colonies on a solid nutrient medium can be taken into account when identifying bacteria, as well as selecting colonies to obtain pure cultures.
Under industrial conditions, when biomass of microorganisms is obtained for the preparation of antibiotics, vaccines, diagnostic preparations, eubiotics, the cultivation of bacteria and fungi is carried out in fermenters under strict observance of the optimal parameters for the growth and reproduction of cultures (see Chapter 6).
Microbiology: lecture notes Tkachenko Ksenia Viktorovna
1. Morphology and structure of viruses
Viruses are the microorganisms that make up the Vira kingdom.
Features:
2) do not have their own protein-synthesizing and energy systems;
3) do not have a cellular organization;
4) have a disjunctive (disconnected) way of reproduction (the synthesis of proteins and nucleic acids occurs in different places and at different times);
6) viruses pass through bacterial filters.
Viruses can exist in two forms: extracellular (virion) and intracellular (virus).
In shape, virions can be:
1) rounded;
2) rod-shaped;
3) in the form of regular polygons;
4) threadlike, etc.
Their sizes range from 15-18 to 300-400 nm.
In the center of the virion there is a viral nucleic acid covered with a protein envelope - a capsid, which has a strictly ordered structure. The capsid membrane is built of capsomeres. Nucleic acid and capsid envelope make up the nucleocapsid.
The nucleocapsid of complex virions is covered with an outer shell - a supercapsid, which can include many functionally different lipid, protein, and carbohydrate structures.
The structure of DNA and RNA viruses does not fundamentally differ from the NK of other microorganisms. Some viruses have uracil in their DNA.
DNA can be:
1) double-stranded;
2) single-stranded;
3) circular;
4) double-stranded, but with one shorter chain;
5) double-stranded, but with one continuous, and with the other fragmented chains.
RNA can be:
1) single-stranded;
2) linear double-strand;
3) linear fragmented;
4) circular;
Viral proteins are classified into:
1) genomic - nucleoproteins. Provide viral nucleic acid replication and viral reproduction processes. These are enzymes, due to which there is an increase in the number of copies of the parent molecule, or proteins, with the help of which molecules are synthesized on the nucleic acid matrix that ensure the implementation of genetic information;
2) proteins of the capsid membrane - simple proteins with the ability to self-assemble. They add up to geometrically regular structures, in which several types of symmetry are distinguished: spiral, cubic (form regular polygons, the number of faces is strictly constant) or mixed;
3) proteins of the supercapsid membrane are complex proteins that are diverse in function. Due to them, viruses interact with a sensitive cell. They perform protective and receptor functions.
Among the proteins of the supercapsid membrane are:
a) anchor proteins (at one end they are located on the surface, and at the other end they go into the depth; provide contact of the virion with the cell);
b) enzymes (can destroy membranes);
c) hemagglutinins (cause hemagglutination);
d) elements of the host cell.
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Table of contents of the subject "Types of microorganisms. Viruses. Virion.":1. Microorganisms. Types of microorganisms. Classification of microorganisms. Prions.
2. Viruses. Virion. Morphology of viruses. The size of the viruses. Nucleic acids of viruses.
3. Capsid of the virus. Capsid functions of viruses. Capsomers. Nucleocapsid of viruses. Spiral symmetry of the nucleocapsid. Cubic symmetry of the capsid.
4. Supercapsid of the virus. Dressed up viruses. Naked viruses. Matrix proteins (M-proteins) of viruses. Reproduction of viruses.
5. Interaction of the virus with the cell. The nature of the virus-cell interaction. Productive interaction. Virogeny. Virus interference.
6. Types of cell infection with viruses. The reproductive cycle of viruses. The main stages of the reproduction of viruses. Adsorption of the virion to the cell.
7. Penetration of the virus into the cell. Viropexis. Undressing the virus. Shadow phase (eclipse phase) of viral reproduction. Formation of viral particles.
8. Transcription of the virus in the cell. Broadcasting viruses.
9. Replication of the virus in the cell. Build viruses. Release of daughter virions from the cell.
Viruses. Virion. Morphology of viruses. The size of the viruses. Nucleic acids of viruses.
Extracellular form - virion- includes all the constituent elements (capsid, nucleic acid, structural proteins, enzymes, etc.). Intracellular form - virus- can be represented by only one nucleic acid molecule, since, entering the cell, the virion breaks down into its constituent elements.
Morphology of viruses. The size of the viruses.
Nucleic acids of viruses
Viruses contain only one type of nucleic acid, DIC or RNA, but not both types at the same time. For example, viruses of smallpox, herpes simplex, Epstein-Barr - DNA-containing, and togaviruses, picornaviruses - RNA-containing. The genome of the viral particle is haploid. The simplest viral genome encodes 3-4 proteins, the most complex - more than 50 polypeptides. Nucleic acids are represented by single-stranded RNA molecules (excluding reoviruses, in which the genome is formed by two RNA strands) or by double-stranded DNA molecules (excluding parvoviruses, in which the genome is formed by one DNA strand). In the hepatitis B virus, the strands of the double-stranded DNA molecule are not the same in length.
Viral DNA form circular, covalently linked supercoiled (for example, in papovaviruses) or linear double-stranded structures (for example, in herpes and adenoviruses). Their molecular weight is 10-100 times less than that of bacterial DNA. Transcription of viral DNA (mRNA synthesis) is carried out in the nucleus of a virus-infected cell. In the viral DNA, at the ends of the molecule there are straight or inverted (unfolded 180 ") repetitive nucleotide sequences. Their presence provides the ability of the DNA molecule to close into a ring. These sequences, present in single- and double-stranded DNA molecules, are peculiar markers of viral DNA.
Rice. 2-1. Sizes and morphology of the main causative agents of human viral infections.
Viral RNA are represented by single- or double-stranded molecules. Single-stranded molecules can be segmented - from 2 segments in arenoviruses to 11 in rotaviruses. The presence of segments leads to an increase in the coding capacity of the genome. Viral RNA subdivided into the following groups: plus-strand RNA (+ RNA), minus-strand RNA (-RNA). In various viruses, the genome can form strands + RNA or -RNA, as well as double strands, one of which is -RNA, the other (complementary to it) - + RNA.
Plus strand RNA are represented by single chains with characteristic endings ("caps") for recognition of ribosomes. This group includes RNAs that can directly translate genetic information on the ribosomes of a virus-infected cell, that is, perform the functions of mRNA. Plus strands perform the following functions: they serve as mRNA for the synthesis of structural proteins, a matrix for RNA replication, and are packed into a capsid to form a daughter population. RNA minus strands are not capable of translating genetic information directly on ribosomes, that is, they cannot function as mRNA. However, such RNAs serve as a template for mRNA synthesis.
Infectivity of Viral Nucleic Acids
Many viral nucleic acids infectious in themselves, as they contain all the genetic information necessary for the synthesis of new viral particles. This information is realized after the penetration of the virion into the sensitive cell. Infectious properties are shown by nucleic acids of most + RNA- and DNA-containing viruses. Double-stranded RNAs and most -RNAs are not infectious.
The rapid pace of development of virology in the second half of the XX century. made it possible to obtain the most important information about the structure and chemical composition of various viruses, including their genome, as well as about the nature of interaction with host cells. The materials obtained indicate that viruses exist in two qualitatively different forms: extracellular - virion and intracellular - virus. The virion of the simplest virus is a nucleoprotein, which includes a viral genome protected by a protein envelope - a capsid. At the same time, the intracellular virus is a self-replicating form that is not capable of binary division.
Thus, in the definition of a virus, a fundamental difference is laid between the cellular form of microorganisms that reproduce by binary fission, and the replicating form, which is reproduced only from viral nucleic acid. However, the qualitative difference between viruses from pro- and eukaryotes is not limited to this one side only, but includes a number of others:
the presence of one type of nucleic acid (DNA or RNA);
lack of cellular structure and protein synthesizing systems;
the ability to integrate into the cellular genome and replicate synchronously with it.
At the same time, viruses differ from ordinary replicons, which are the DNA molecules of all microorganisms and any other cells, as well as plasmids and transposons, since these replicons are biomolecules that cannot be classified as living matter.
Classification and taxonomy of viruses. Viruses make up the Vira kingdom, which is subdivided by the type of nucleic acid into two sub-kingdoms - riboviruses and deoxyriboviruses. Subkingdoms are divided into families, which in turn are subdivided into genera. The concept of the type of viruses has not yet been clearly formulated, as well as the designation of different types.
As taxonomic characteristics, primary importance is attached to the type of nucleic acid and its molecular biological characteristics: double-stranded, single-stranded, segmented, non-segmented, with repetitive and inverted sequences, etc. However, in practical work, the characteristics of viruses obtained as a result of electron microscopy and immunological studies: the morphology, structure and size of the virion, the presence or absence of the outer shell (supercapsid), antigens, intranuclear or cytoplasmic localization, etc. Along with the mentioned signs, resistance to temperature, pH, detergents, etc. are taken into account.
Currently, human and animal viruses are included in 18 families. The belonging of viruses to certain families is determined by the type of nucleic acid, its structure, as well as the presence or absence of an outer envelope. When determining belonging to the family of retroviruses, the presence of reverse transcriptase must be taken into account.
Morphology and structure of virions
The sizes of virions of various viruses vary widely: from 15-18 to 300-400 nm. They have a variety of shapes: rod-shaped, filamentous, spherical parallelepiped, spermatozoid (Fig. 5.1). The structure of a simple virion - a nucleocapsid - indicates that the viral nucleic acid - DNA or RNA - is reliably protected by a protein envelope - a capsid. The latter has a strictly ordered structure, which is based on the principles of spiral or cubic symmetry. Capsids of rod-shaped and filamentous virions consist of structural subunits arranged in a spiral around an axis. With this arrangement of the subunits, a hollow channel is formed, inside which the viral nucleic acid molecule is compactly packed. Its length can be many times the length of the rod-shaped virion. For example, the length of the tobacco mosaic virus (TMV) is 300 nm, and its RNA reaches 4000 nm, or 4 μm. In this case, the RNA is so bound to the capsid that it cannot be released without damaging the latter. Similar capsids are found in some bacterial viruses and in human viruses (for example, the influenza virus).
The spherical structure of virions is determined by the capsid, built according to the principles of cubic symmetry, which is based on the figure of the icosahedron - a twenty-sided one. The capsid consists of asymmetric subunits (polypeptide molecules), which are combined into morphological subunits - capsomeres. One capsomere contains 2, 3, or 5 subunits located along the corresponding symmetry axes of the icosahedron. Depending on the type of rearrangement and the number of subunits, the number of capsomeres will be 30, 20, or 12. In Fig. 5.1 shows the possible types of simple virions, consisting of a certain number of capsomeres, depicted in the form of balls, as well as capsomeres of increasing volume.
Virions with a complex capsid built of more than 60 structural subunits contain groups of 5 subunits - pentamers, or of 6 subunits - hexamers. The nucleocapsid of complex virions, called the "core", is covered with an outer shell - a supercapsid.
The chemical composition of virions
Simple virions contain one type of nucleic acid - RNA or DNA - and proteins. In complex virions, the outer shell contains lipids and polysaccharides, the former are obtained from the host cells, the latter are encoded in the genome of the virus in the form of glycoproteins.
Viral DNA. The molecular weight of the DNA of different viruses varies widely (1 * 106-1 * 108). It is about 10-100 times less than the molecular weight of bacterial DNA. The genome of viruses contains up to several hundred genes. In terms of their structure, viral DNAs are characterized by a number of features, which makes it possible to subdivide them into several types. These include double-stranded and single-stranded DNA, which can be linear or circular.
Although nucleotide sequences occur only once in each DNA strand, there are straight or inverted (180 ° rotated) repeats at its ends. They are represented by the same nucleotides that are located in the initial region of DNA. Nucleotide repeats, inherent in both single-stranded and double-stranded viral DNA, are a kind of markers that distinguish viral from cellular DNA. The functional significance of these repetitions is the ability to close into a ring. In this form, it is replicated, transcribed, becomes resistant to endonucleases, and can be incorporated into the cellular genome.
Viral RNA. In RNA viruses, genetic information is encoded in RNA by the same code as in the DNA of all other viruses and cellular organisms. In terms of their chemical composition, viral RNAs do not differ from RNAs of cellular origin, but they are characterized by a different structure. In addition to the typical single-stranded RNA, a number of viruses have double-stranded RNA. Moreover, it can be linear and circular. As part of single-stranded RNAs, there are helical regions such as a double helix of DNA, formed as a result of the pairing of complementary nitrogenous bases. Single-stranded RNAs, depending on the functions they perform, are divided into two groups.
The morphology and structure of viruses is studied using an electron microscope, since their size is small and comparable to the thickness of the bacterial membrane.
The form of virions can be different: rod-shaped (tobacco mosaic virus), bullet-shaped (rabies virus), spherical (poliomyelitis viruses, HIV), in the form of a spermatozoon (many bacteriophages) (Fig. 8).
Rice. 8. Forms of virions:
1 smallpox virus; 2 herpes virus; 3 adenovirus; 4 papovavirus; 5 hepadnavirus; 6 paramyxovirus; 7 influenza virus; 8 coronavirus; 9 arenavirus; 10 retrovirus;
The sizes of viruses are determined using electron microscopy, by ultrafiltration through filters with a known pore diameter, by ultracentrifugation. Some of the smallest viruses are polio and foot and mouth disease (about 20 nm), circoviruses (16 nm), the largest variola virus (about 350 nm). Viruses have a unique genome as they contain either DNA or RNA. Therefore, a distinction is made between DNA-containing and RNA-containing viruses. They are usually haploid, meaning they have one set of genes. The genome of viruses is represented by various types of nucleic acids: double-stranded, single-stranded, linear, circular, fragmented.
Distinguish between simple (for example, polio virus) and complex (for example, influenza, measles) viruses. In viruses simply arranged, the nucleic acid is bound to a protein membrane called the capsid (from the Latin capsa sheath). The capsid consists of repeating morphological subunits of capsomeres. Nucleic acid and capsid interact with each other to form a nucleocapsid. In complex viruses, the capsid is surrounded by an additional lipoprotein envelope, a supercapsid (a derivative of the membrane structures of the host cell), which has spikes. Capsid and supercapsid protect virions from environmental influences, cause selective interaction (adsorption) with cells, and determine the antigenic and immunogenic properties of virions. The internal structures of viruses are called the core.
Virions are characterized by spiral, cubic and complex types of capsid symmetry. The spiral type of symmetry is due to the helical structure of the nucleocapsid, cubic formation of an isometric hollow body from a capsid containing viral nucleic acid.
In addition to common viruses, so called non-canonical viruses, prions, are protein infectious particles in the form of fibrils 10–20 x 100–200 nm in size. Prions, apparently, are both inducers and products of the autonomous gene of humans or animals and cause encephalopathy in them under conditions of a slow viral infection (Creutzfeldt Jakob disease, kuru, etc.). Other unusual agents close to viruses are viroids, small molecules of circular, supercoiled RNA that do not contain protein, which cause disease in plants.
