Poliomyelitis, rabies, smallpox, herpes, human acquired immunodeficiency syndrome are diseases known to everyone that are caused by very specific pathogens. Organisms that stand on the border between living and nonliving, obligate (obligatory) cell parasites are viruses. Morphology, physiology and their very existence on the planet today raises many questions.
Virology: the beginning
The scene is the laboratory of the Nikitsky Botanical Garden at the Russian Academy of Sciences, where the biologist Dmitry Iosifovich Ivanovsky (1864-1920) studies the mysterious mosaic disease of tobacco. The causative agent of the disease in the plant passes through the smallest bacterial filters, does not grow on nutrient media and does not give symptoms when healthy plants are infected with filtrates from patients.
It was then, in 1892, the scientist concludes that these are not bacteria. And calls the causative agent viruses (from the Latin virus, - poison). Dmitry Ivanovsky tried to see viruses all his life, but we saw the morphology of viruses in the 30s of the XX century, when electron microscopes were invented.
But this date is considered the beginning of the science of virology, and Dmitry Ivanovsky its founder.
Amazing kingdom
The morphology and physiology of viruses is so amazing that these organisms are isolated in the independent kingdom of Vira. This simplest form of life has microscopic dimensions (from 25 to 250 nanometers) and is a nucleic acid with a set of genes enclosed in a shell. These are parasites that can reproduce only in the cells of other living organisms - plants, fungi, animals, bacteria, and even other viruses (satellite viruses).
The hallmarks of viruses are as follows:
- They contain only one type of nucleic acid (RNA or DNA).
- In the morphology of viruses, there are no protein synthesizing and energy systems.
- They do not have a cellular structure.
- Virus parasitism is implemented at the genetic level.
- Pass through bacterial filters and are not cultured on artificial media.
Part of the organic world of the planet
Viruses, as obligate parasites, have a clear genetic connection with representatives of both the flora and fauna of the Earth. Moreover, according to recent studies, the human genome consists of 32% of the elements of the virus-like structure.
To date, more than 6 thousand viruses have already been described, but it is estimated that more than one hundred million of them exist. This is the largest biological form on the planet, and it is represented in all ecosystems (ubiquitous) distribution.
Their appearance on the planet today is not clear. One thing is known - when the first cellular forms of life appeared, viruses already existed.
Alive and not Alive
These amazing organisms have two forms of their existence, which are significantly different from each other.
Outside the cell, the form of their existence is the virion. When it enters the cell, its membranes dissolve, and the nucleic acids of the virus integrate into the host gene material. And then we’re talking about a viral infection. The genome of the virus integrates into the natural mechanisms of replication of the genome of the host cell and triggers a chain of reactions, realizing its parasitic existence.
Virion is essentially an inanimate part of life. And the genome of the virus in the cell is its living component, because it is there that the reproduction of viruses takes place.
Morphology and ultrastructure of viruses
In this context, we are talking about the virion - extracellular form.
The size of virions is measured in nanometers - 10 -9 meters. Influenza viruses have average sizes of 80-120 nanometers, and smallpox virus is a giant with sizes of 400 nanometers.
The structure and morphology of viruses is similar to astronauts. Inside the capsid (the protein coat sometimes containing fats and carbohydrates), as in the “suit”, is the most valuable part - the nucleic acids, the genome of the virus. Moreover, this "astronaut" is represented in a minimal amount - only directly hereditary material and a minimum of enzymes for its replication (copying).
Externally, the “spacesuit” can be rod-shaped, spherical, bullet-shaped, in the form of a complex icosahedron or not at all in the correct form. It depends on the presence of specific proteins in the capsid, which are responsible for the penetration of the virus into the cell.
How the pathogen enters the host
There are many ways to penetrate, but the most common is airborne. A myriad of tiny particles are thrown into space not only when coughing or sneezing, but simply when breathing.
Another way for the virions to enter the body is contagious (direct physical contact). This method is inherent in a fairly small group of pathogens, that is how herpes, sexually transmitted infections, and AIDS are transmitted.
The method of infection through the carrier, which may be various groups of organisms, is quite complicated. A carrier that receives a pathogen from an infection reservoir becomes a place where viruses can multiply or undergo developmental stages. Rabies virus is just such a pathogen.
What happens in the host
Using external capsid proteins, the virus attaches to the cell membrane and penetrates through endocytosis. They enter the lysosomes, where under the action of enzymes get rid of the "spacesuit". And pathogen nucleic acids enter the nucleus or remain in the cytoplasm.
The pathogen nucleic acids are inserted into the host nucleic acid chains, and the reaction of replication (copying) of hereditary information is triggered. When a sufficient number of viral particles accumulate in the cell, while the virions use the energy and plastic mechanisms and resources of the host.
The last stage is the exit of virions from the cell. Some viruses lead to complete destruction of cells and enter the intercellular space, others exit into it through exocytosis or budding.
Pathogen strategies
The interaction of the virus and the host cell can develop in several ways. The main feature of which is the degree of autonomy of the parasite.
The structure of the morphology of viruses leads to a complete dependence of the pathogen on the energy and protein synthesizing potential of the cell, the only condition is that it replicates its nucleic acids according to its own schedule. This interaction is called productive (natural for the virus, but not for the cell). Having exhausted the supply of the cell, the virus leads to its death.
Another type of interaction is conciliatory. In this case, the viral genome integrated into the host genome replicates covalently with the cell’s own nucleic acids. And then the development of the scenario can go in two directions. The virus behaves quietly and does not manifest itself. Young virions leave the cell only under certain conditions. Or the pathogen genes are constantly working, producing a large number of the younger generation, but the cell does not die, and they leave it through exocytosis.
The complexities of taxonomy
The classification and morphology of viruses is different in a variety of sources. At the same time, the following features are used to classify them:
- The type of nucleic acid (RNA-containing and DNA-containing) and the method of its replication. The most common virus classification proposed by American virologist David Baltimore in 1971.
- Morphology and structure of the virus (single-stranded, double-stranded, linear, circular, fragmented, not fragmented).
- Dimensions, type of symmetry, number of capsomers.
- The presence of supercapsid (outer shell).
- Antigenic properties.
- Type of genetic interaction.
- The circle of potential hosts.
- Localization in the host cell - in the nucleus or in the cytoplasm.
It is the choice of the main criterion and morphology of viruses in microbiology that defines various approaches to the classification of viruses. It is rather difficult. The difficulty lies in the fact that we begin to study the morphology and structure of the virus only when they lead to pathological processes.
Finicky and not so
At the choice of the host, these pathogens are extremely diverse in their preferences. Some attack only one biological species - they have a very strict "registration". For example, it eats flu viruses of cats, gulls, pigs, which are completely safe for other animals. Sometimes the specialization is surprising - the bacteriophage R-17 virus infects only males of one species of E. coli.
Other viruses behave completely differently. For example, bullet-like viruses, the morphology of which is similar to a bullet, cause completely different diseases and at the same time the host circle is extremely wide. Such viruses include rabies virus, which infects all mammals, or the virus of cattle vesicular stomatitis (transmitted, by the way, through insects).
There are other nuances. Viruses with a tail (virions) mostly attack bacterial cells, filamentous or spiral - plant parasites, and in animal cells viruses with a complex capsid and multifaceted virion form are more often parasitic.