Poliomyelitis, rabies, smallpox, herpes, acquired human immunodeficiency syndrome are known to everyone diseases that are caused by very specific pathogens. Organisms that stand on the border between living and non-living, obligate (obligatory) cellular parasites - viruses. Morphology, physiology and their very existence on the planet raises many questions today.
Virology: Getting Started
The scene is the laboratory of the Nikitsky Botanical Garden at the Russian Academy of Sciences, where the biologist Dmitry Iosifovich Ivanovsky (1864-1920) is studying the mysterious mosaic disease of tobacco. The causative agent of a disease in a plant passes through the smallest bacterial filters, does not grow on nutrient media and does not give symptoms when he althy plants are infected with filtrates from diseased ones.
It was then, in 1892, that the scientist concluded that it was not bacteria. And he calls the pathogen viruses (from the Latin virus,- I). Dmitry Ivanovsky tried all his life to see viruses, but we saw the morphology of viruses in the 30s of the XX century, when electron microscopes were invented.
But this very date is considered the beginning of the science of virology, and Dmitry Ivanovsky is its founder.
Amazing Kingdom
The morphology and physiology of viruses are so amazing that these organisms are isolated in an 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 distinguishing features of viruses are as follows:
- Contain only one type of nucleic acid (RNA or DNA).
- The morphology of viruses lacks protein-synthesizing and energy systems.
- Do not have a cellular structure.
- Virus parasitism is realized 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, 32% of the human genome consists of elements of a virus-likestructures.
To date, more than 6 thousand viruses have already been described, but it is estimated that there are more than one hundred million of them. This is the most numerous biological form on the planet, and it is represented in all ecosystems (ubiquitous (ubiquitous) distribution).
Their appearance on the planet today is not clear. One thing is known - when the first cellular life forms 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 a cell, its shells dissolve and the nucleic acids of the virus are incorporated into the host's genetic material. That's when we talk about a viral infection. The virus genome integrates into the natural replication mechanisms of the host cell genome and starts a chain of reactions, carrying out its parasitic existence.
Virion is essentially an inanimate part of life. And the genome of a virus in a cell is its living component, because it is there that viruses reproduce.
Morphology and ultrastructure of viruses
In this context, we are talking about a virion - an extracellular form.
The size of virions is measured in nanometers - 10-9 meters. Influenza viruses are medium in size - 80-120 nanometers, and the smallpox virus is a giant with dimensions of 400 nanometers.
The structure and morphology of viruses is similar to astronauts. Inside the capsid (protein coat, sometimescontaining fats and carbohydrates), as in a "space suit", is the most valuable part - nucleic acids, the genome of the virus. Moreover, this “cosmonaut” is also presented in a minimal amount - only directly hereditary material and a minimum of enzymes for its replication (copying).
Outwardly, the “suit” can be rod-shaped, spherical, bullet-shaped, in the form of a complex icosahedron, or not at all regular in shape. It depends on the presence in the capsid of specific proteins that are responsible for the penetration of the virus into the cell.
How the pathogen enters the host body
There are many ways to penetrate, but the most common is airborne. Myriads of tiny particles are thrown into space not only when coughing or sneezing, but simply when breathing.
Another way for virions to enter the body is contagious (direct physical contact). This method is inherent in a fairly small group of pathogens, this is how herpes, venereal infections, AIDS are transmitted.
The method of infection through a vector, which can be different groups of organisms, is quite complicated. A vector that has acquired a pathogen from a reservoir of infection becomes a site for viruses to replicate or progress through developmental stages. The rabies virus is just such a pathogen.
What happens in the host's body
With the help of external proteins of the capsid, the virus attaches to the cell membrane and penetrates through endocytosis. They areget into the lysosomes, where, under the action of enzymes, they get rid of the "space suit". And the nucleic acids of the pathogen enter the nucleus or remain in the cytoplasm.
Nucleic acids of the pathogen are built into the chains of nucleic acids of the host, and the reaction of replication (copying) of hereditary information is launched. When a sufficient number of viral particles accumulate in the cell, the virions use the energy and plastic mechanisms and resources of the host.
The last stage is the release of virions from the cell. Some viruses lead to complete destruction of cells and enter the intercellular space, others enter it through exocytosis or budding.
Pathogen strategies
Interaction between a virus and a host cell can develop according to several scenarios. The main feature of which is the degree of autonomy of the parasite.
Structure The morphology of viruses leads to the 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. Such an interaction is called productive (it is natural for a virus, but not for a cell). Having exhausted the supply of the cell, the virus leads to its death.
Another type of interaction is consensual. In this case, the virus 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 leavecell only under certain conditions. Or the pathogen genes are constantly working, producing a large number of young generation, but the cell does not die, but they leave it through exocytosis.
Difficulties in taxonomy
Classification and morphology of viruses is different in various sources. At the same time, the following features are used to classify them:
- Type of nucleic acid (RNA-containing and DNA-containing) and the method of its replication. The most common classification of viruses proposed by American virologist David B altimore in 1971.
- Morphology and structure of the virus (single-stranded, double-stranded, linear, circular, fragmented, non-fragmented).
- Dimensions, type of symmetry, number of capsomeres.
- Presence of a supercapsid (outer shell).
- Antigenic properties.
- Type of genetic interaction.
- 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 the morphology of viruses that determines the various approaches to the classification of viruses in microbiology. It's not quite easy. 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.
Picky and not so good
By host choice, these pathogens are extremely diverse in their preferences. Some attack only one biological species - they have a very strict "registration". For example, eatinfluenza viruses of cats, gulls, pigs, which are completely safe for other animals. Sometimes the specialization is surprising - the bacteriophage P-17 virus only infects males of one variety of E. coli.
Other viruses behave quite differently. For example, bullet-shaped viruses, whose morphology is similar to a bullet, cause completely different diseases and, at the same time, their range of hosts is extremely wide. Such viruses include the rabies virus, which infects all mammals, or the bovine vesicular stomatitis virus (transmitted by insects, by the way).
There are other nuances. Viruses with a tail (virions) mostly attack bacterial cells, filamentous or spiral ones are parasites of plants, and in animal cells viruses with a complex capsid and a multifaceted virion form are more likely to parasitize.
