Polio and smallpox, acquired human immunodeficiency syndrome and herpes are well-known diseases caused by specific pathogens. These are viruses - cell dependent, standing on the border of the living and nonliving. What is the effect of viruses on the cell, how they were discovered and what are the ways to fight and prevent this group of diseases - the topic of this article.
Birth of Virology
In 1892, in the laboratory of the Nikitsky Botanical Garden at the Academy of Sciences of Russia, a young scientist Dmitry Iosifovich Ivanovsky (1864-1920) found that tobacco mosaic disease is caused by a pathogen that passes through bacterial filters, does not grow like bacteria on artificial media and does not give a picture diseases through filtrates.
This date is considered to be the birth of the science of viruses, although their discoverer was never able to see them throughout his life. We saw them only in the 30s of the last century, when electron microscopes appeared.
Separate kingdom
The viruses are so amazing that they are isolated in a separate kingdom of Vira (from lat. Virus, poison). How are viruses arranged so that they are so different from all living organisms on our planet? First, they contain only one type of nucleic acid - either DNA or RNA. All other organisms in their cells are of both types.
Secondly, viruses lack a cellular structure and protein synthesizing systems. Thirdly, viruses are ubiquitous (ubiquitous) and very small in size. And most importantly, these are intracellular parasites at the genetic (molecular) level.
Virion and virus
The classification and structure of viruses is diverse in various sources. Today, about a thousand human viruses have been studied and described, but this is not a complete list of them. After all, we begin to study them exclusively in the context of the disease. The extracellular form of these parasites is the virion. When one enters the cell, the mechanism of virus penetration turns it into a completely different form. And just then we are talking about a viral infection.
Size and shape matter
The sizes of virions are measured in nanometers (10 -9 meters). One of the smallest - polio viruses - has a virion size of 17 nanometers. Influenza viruses are medium in size, from 80 to 120 nanometers. But there are giants - smallpox virus has a size of about 400 nanometers.
Schematically arranged viruses as astronauts. In the "suit" - a protective protein coat, sometimes containing lipids and carbohydrates called capsid - is the hereditary material of nucleic acid (RNA or DNA). Moreover, the hereditary material is presented in the form of a “minimal consumer basket” - enzymes for the actual (copying) itself and the virus genome itself.
The appearance of the “suit” can be rod-shaped, spiral, spherical, bullet-shaped, brick-shaped or even have an irregular shape. It is determined by the proteins that are necessary for the virion for the mechanism of penetration of the virus into the cell. Viruses fuse with the membrane of the host cell, and its nucleic acids enter the cytoplasm, which are ready for self-assembly in the young generation of viruses.
Alive or not?
Even virologists have no clear answer to this question. If a living system is considered capable of reproducing its own kind, then viruses are living organisms. If we add at least some vital processes, then they are not living. In addition, it should be remembered that they do not reproduce themselves outside the host cell.
Information about the origin and effect of viruses also leaves a lot of questions. The most acceptable hypothesis today is their origin from “runaway” nucleic acids, which have acquired the ability to reproduce in foreign cells.
The effect of the virus on the cell
Viruses interact with cells according to two types of scenario, the fundamental differences of which are in the degree of dependence of the parasite on the host. It completely depends on the energy and protein synthesizing economy of the cell, but behaves independently.
What is the effect of viruses on the cell? In replication of viral nucleic acids according to their own schedule. Such a scenario is called productive, of course, from the point of view of the parasite. And it can end in the death of the host cell.
The second scenario is conciliatory. In this case, the genome of the virus is incorporated into the host genome and is covalently replicated with cellular nucleic acids. The development of what the effect of viruses on a cell is manifested in can go two ways further. Or the virus behaves quietly, and only under certain conditions its genes begin to work, and young virions come out of the dying cell in search of new victims. Or the virus genes are constantly working and producing the younger generation, but the cell does not die.
Young viruses enter the external environment either as a result of destruction of the host cell, or by ejection of fragments of the cytoplasm, or by exocytosis of small groups of virions.
Everywhere and everywhere
As already mentioned, viruses are ubiquitous. But there are nuances. So, virions with a tail parasitize mainly in bacterial cells. Virions in the form of a thread or spiral are more often found in plant cells, because they have dense cellulose membranes, which, like a syringe, pierce the tail of the virions. By the way, the appearance of streaks on the flowers of tulips is often associated with infection of plants with viruses. But they sell them to us as a special sort!
In animal cells, parasites with a lipoprotein capsule predominate. Some virions have a very strict "registration". So, the virus that causes poliomyelitis can exist only in the cells of primates and humans, and even then not in all. Others, such as smallpox virus, have a wide range of possible hosts.
How do they enter the host
The most common way is an airborne infection. Billions of tiny droplets of liquid are thrown into the surrounding area when coughing, sneezing, and simply breathing. With high virulence (contagiousness) of the pathogen, it is very difficult to prevent the spread of infection.
Another way of infection is contagious, with direct physical contact. This is a small group of viral diseases, namely herpes, AIDS and sexually transmitted infections.
A rather dangerous way of infection is through carriers. It can be any living organism that has received an infection from a reservoir or carrier. In this case, the carrier becomes an intermediate host, in the cells of which the pathogenic agent can multiply or even go through certain stages of the life cycle. Such pathogens include, for example, rabies virus.
Some viruses are capable of being transported on the covers of insects, and smallpox virus can retain its virulence for a long time even in dust.
Many diseases
Given the diversity of viruses, one should not be surprised at the variety of viral infections at the cellular level, although scientists have not found a correlation between the form of the virus and the nature of the disease. For example, hepatitis A, C, and E viruses can cause inflammation in humans. Moreover, they are all different in their genetic structure and appearance and even classified into three different families.
A family of picornaviruses, whose species are similar in appearance to those like twins, cause such diseases. like foot and mouth disease, diabetes, myocarditis, conjunctivitis and various colds.
Microbiologist's dream
Viruses, and especially bacteriophages, are today a favorite subject of research by molecular biologists. Cytotechnologies actively use the unique ability of viruses to integrate their hereditary information into the genome of a host cell. Viruses are called transfer vectors of genetic material.
Modern science provides many examples of successful transgenic organisms, for example, a potato variety in the genome of which a chitinase gene is introduced - an enzyme that decomposes chitin. Chitinase accumulates in the stems and leaves of potatoes and makes it unsuitable for feeding by insects and parasitic mushrooms.
Regardless of what the effect of viruses on the cell is, their shape and content, as well as reproduction strategies, there is no panacea for fighting them as pathogens. In infectious pathology, about 75% of diseases are viral in nature. The etiology of 25% of all infections of the gastrointestinal tract is also due to viruses.
The definition is complicated by the possibility of the infection staying in the latent period. That is why it is necessary to thoroughly study each pathogen in the manifestation of its pathogenicity and look for very special ways to deal with them, which is not always the case.