Representatives of the kingdom of viruses are a special group of life forms. They have not only a highly specialized structure, but also are characterized by a specific metabolism. In this article we will study the non-cellular form of life - the virus. What it consists of, how it propagates and what role it plays in nature, you will learn by reading it.
The discovery of non-cellular life forms
Russian scientist D. Ivanovsky in 1892 was studying the causative agent of tobacco disease - tobacco mosaic. He found that the pathogenic agent does not belong to bacteria, but is a special form, subsequently called the virus. At the end of the 19th century, high resolution microscopes were not yet used in biology, so the scientist could not find out what molecules the virus consists of, as well as see and describe it. After the creation of an electron microscope at the beginning of the 20th century, the world saw the first representatives of the new kingdom, who turned out to be the cause of many dangerous and difficult to cure human diseases, as well as other living organisms: animals, plants, bacteria.
The position of non-cellular forms in the taxonomy of wildlife
As mentioned earlier, these organisms are united in the fifth kingdom of wildlife - viruses. The main morphological feature characteristic of all viruses is the lack of a cellular structure. Until now, discussions in the scientific world on the question of whether non-cellular forms are living objects in the full sense of the term are ongoing. After all, all manifestations of metabolism in them are possible only after penetration into a living cell. Up to this point, viruses behave like objects of inanimate nature: they do not have metabolic reactions, they do not multiply. At the beginning of the 20th century, a whole group of questions arose before scientists: what is a virus, what does its shell consist of, what is inside a virus particle? Answers were received as a result of many years of research and experimentation, which served as the basis for a new scientific discipline. It arose at the intersection of biology and medicine and is called virology.
Structural features
The expression "all ingenious is simple" directly refers to non-cellular life forms. The virus consists of nucleic acid molecules - DNA or RNA, coated with a protein shell. He does not have his own energy and protein synthesizing apparatus. Without a host cell, viruses have no sign of living substance: no respiration, no growth, no irritability, no reproduction. For all this to appear, it takes only one thing: to find a victim - a living cell, subordinate its metabolism to its nucleic acid and finally destroy it. As mentioned earlier, the envelope of a virus consists of protein molecules having an ordered structure (simple viruses).
If the membrane also contains lipoprotein subunits, which are actually part of the cytoplasmic membrane of the host cell, such viruses are called complex (smallpox and hepatitis B pathogens). Often, the surface membrane of the virus also includes glycoproteins. They perform a signaling function. Thus, both the envelope and the virus itself are composed of molecules of the organic component - protein and nucleic acids (DNA or RNA).
How viruses enter living cells
Earlier, we examined the structural features of the intracellular parasite membrane. The virus consists of molecules of organic and biological matter, and its surface structure contains special proteins that recognize the plasmalemma of living cells. Therefore, non-cellular forms affect specific types of cells of certain biological species of organisms. For example, canine distemper viruses are not harmful to human health. The parasite enters the cell in several ways:
- By fusion of its membrane with the cell membrane (influenza virus).
- By pinocytosis (causative agent of animal poliomyelitis).
- Through damage to the cell wall (plant viruses).
Virus propagation
As soon as the parasite enters the cell, its nucleic acid molecules, wedging into the genome of the nucleus, transmit information about the structure of protein particles and start the process of biosynthesis of their own proteins. In this case, ribosomes, ATP molecules, and t-RNA of the host cell are used. In parallel, hereditary information is reduced in an infected cell. Recall that viruses, called simple viruses, are made up of protein and nucleic acid. Their particles contain RNA, which immediately binds to the ribosome subunits of the host cell and induces the biosynthesis of virus protein molecules.
The result of the attack of the pathogen on the cell is the connection of the DNA or RNA of the virus with its own protein particles. Thus, a newly formed virus consists of nucleic acid molecules coated with ordered particles of proteids. The membrane of the host cell is destroyed, the cell dies, and the viruses emerging from it are introduced into healthy cells of the body.
The phenomenon of reverse reduction
At the beginning of the study, representatives of this kingdom believed that viruses consist of cells, but D. Ivanovsky’s experiments already proved that pathogens cannot be isolated using microbiological filters: pathogens passed through their pores and ended up in a filtrate that retained virulent properties.
Further research established the fact that the virus consists of molecules of organic matter and shows signs of living substance only after its direct penetration into the cell. In it, he begins to multiply. Most RNA-containing viruses multiply as described above, but some of them, such as the AIDS virus, cause DNA synthesis in the nucleus of the host cell. This phenomenon is called reverse replication. Then, the viral i-RNA is synthesized on the DNA molecule, and already on it begins the assembly of the viral protein subunits that form its envelope.
Features of bacteriophages
What is a bacteriophage - a cell or a virus? What does this non-cellular life form consist of? The answers to these questions are as follows: it is a virus that infects exclusively prokaryotic organisms - bacteria. Its structure is quite peculiar. The virus consists of molecules of organic matter and is divided into three parts: the head, rod (cover) and tail threads. In the front part - the head - is a DNA molecule. This is followed by a cover having a hollow shaft inside. Tail filaments attached to it provide the connection of the virus with the receptor loci of the plasma membrane of the bacterium. The principle of action of a bacteriophage resembles a syringe. After contraction of the cover proteins, the DNA molecule enters the hollow rod and is then injected into the cytoplasm of the target cell. Now the infected bacterium will synthesize the DNA of the virus and its proteins, which will inevitably lead to its death.
How the body protects itself from viral infections
Nature has created special defenses against the viral diseases of plants, animals, and humans. Pathogens themselves are perceived by their cells as antigens. In response to the presence of viruses in the body, immunoglobulins are produced - protective antibodies. The organs of the immune system - the thymus, lymph nodes - respond to viral invasion and contribute to the production of protective proteins - interferons. These substances inhibit the development of viral particles and inhibit their reproduction. Both types of defense reactions discussed above are related to humoral immunity. Another form of protection is cellular. White blood cells, macrophages, neutrophils absorb viral particles and break them down.
The importance of viruses
It's no secret that it is mostly negative. These ultrafine pathogenic particles (from 15 to 450 nm), visible only through an electron microscope, cause a whole bunch of dangerous and difficult to cure diseases of all organisms existing on Earth. So, in humans, viruses infect vital organs and systems, for example, the nervous (rabies, encephalitis, polio) immune (AIDS), digestive (hepatitis), respiratory (flu, adenoinfection). Animals suffer from a lizard, plague, and plants - various necrosis, spotting, mosaic.
The diversity of the kingdom is not fully understood. The proof is that they still discover new types of viruses and diagnose previously unexplored diseases. For example, in the mid-20th century, Zika virus was discovered in Africa. It is found in mosquitoes, which, when bitten, infect humans and other mammals. Symptoms of the disease indicate that the pathogen primarily affects parts of the central nervous system and causes microcephaly in newborns. People who are carriers of this virus should remember that they pose a potential danger to their partners, as cases of sexual transmission of the disease have been recorded in medical practice.
The positive role of viruses can be attributed to their use in the fight against pest species, in genetic engineering.
In this work, we told what a virus is, what its particle consists of, how organisms protect themselves from pathogenic agents. We also determined what role non-cellular life forms in nature play.