Botany - B.Ye. Yakubenko 2017

Part Two. System of the Organic World
Chapter V. System of the Organic World
5.1. KINGDOM VIRUSES (VIRA)

This kingdom includes only one division—Viruses (Virophyta). It comprises the tiniest acellular organisms that lack both a cellular Structure and their own METABOLISM. Their sole indication of being alive is The ability to reproduce, transmit hereditary traits from generation to generation, and adapt under The Influence of various environmental factors.

Plant viruses were discovered in 1892 by Dmitry Ivanovsky while studying tobacco mosaic disease. However, The structure of viruses was revealed later, thanks to Electron Cell/15.html">Microscopy and the ability to examine them within living Cells, as viruses do not reproduce on artificial nutrient media—they are intracellular parasites consisting of a protein coat, the capsid, enclosing Nucleic Acids. Notably, a virus contains only one type of nucleic acid: either RNA or DNA, whereas other organisms have both in their cells. The Chemical Nature of viruses was determined in the 1930s. In 1935, the American scientist W. Stanley proved that the tobacco mosaic virus contains 94% protein, and in 1936, English scientists F. Bawden and N. Pirie demonstrated that this virus contains 6% nucleic acid.

In terms of shape, viruses can be spherical (Influenza Viruses), rod-shaped (tobacco mosaic virus), or cubic (smallpox). Viral Particles, known as virions, consist of nucleic acid (RNA or DNA), which carries the hereditary Properties of viruses. Accordingly, viruses are classified into DNA and RNA viruses. Plant-infecting viruses contain RNA, insect viruses contain DNA, while those parasitizing animal and human cells contain either RNA or DNA. The outer coat Proteins perform a protective function.

In addition to viruses, Viroids have been discovered and described; these are nucleic acids (RNA) that cause plant diseases and lack a protein coat. Prions—protein molecules capable of causing infections in animals—have also been identified.

Over 3,000 viruses have been described. Among them are monophages, which parasitize only a single plant species, and polyphages, which infect various plant species.

The distinguishing features of viruses include: a) the ability to crystallize, turning into a chemical substance and then returning to their active state without losing their traits; b) upon entering a living cell, the viral particle (virion) sheds its protein coat, after which the biosynthetic processes of the host cell are hijacked by the Genetic information encoded in the viral nucleic acid. As a result, The Cell synthesizes viral Nucleic Acids and Proteins instead of its own. The viral nucleic acid becomes enveloped in a protein coat, forming new viral particles (virions) that invade neighboring healthy cells, generalizing the infection and causing plant or animal disease. Virus-infected cells do not recover.

Viruses that settle in bacterial cells are called Bacteriophages. They possess a different structure: most commonly, a bacteriophage (or phage) consists of a prismatic HEAD, a tail, a collar, a sheath, a base plate, and filamentous structures. The nucleic acid is concentrated in the head. Some phages consist solely of a head or a head with a short tail. Upon penetrating a bacterial cell, a phage induces its destruction, or lysis, by producing new phage particles.

Viruses cannot spread independently; intermediary organisms assist them in this process. These can include soil nematodes, Fungi, aphids, thrips, and mites. About 50 species of viruses are transmitted by parasitic weeds from the genus Cuscuta, spreading from diseased plants to healthy ones. Humans also contribute to the spread of plant viruses during tree grafting and pruning, as well as seedling picking.

Viruses that cause diseases in animals and humans are most frequently transmitted via airborne droplets, insect bites, through Blood from an infected person to a healthy one (such as the HIV virus), or when Sanitary and hygienic standards are violated.

Significance of viruses and bacteriophages. In human life, viruses play a predominantly negative role, causing a wide range of illnesses. In plants, they induce leaf mosaicism and spotting, various tumors, and rots. The most common viral diseases in plants include potato aucuba mosaic, wheat streak mosaic, and barley stripe mosaic. During mass proliferation, viruses can destroy up to 50% of the crop.

In humans, viruses cause such dangerous diseases as influenza, poliomyelitis, encephalitis, rabies, herpes, measles, chickenpox, and AIDS. In animals, they trigger FOOT-and-Mouth disease, rabies, enteritis, canine distemper, avian plague, and others.

Certain bacteriophages harm various microbiological industries that specialize in The production of fermented dairy products, Antibiotics, Vitamins, fertilizers, and more.

However, viruses also have a positive significance: some are utilized in biological Methods for controlling forest pests, particularly the pine moth, the spongy moth, and the European pine sawfly. Bacteriophages are also employed to combat bacterial infections.

In Introduction/32.html">Genetic Engineering, bacteriophages are used to facilitate the synthesis of Hormones, Enzymes, and interferons, and they serve as models for experimental research in molecular biology.

Regarding THE ORIGIN OF viruses, several hypotheses exist: the acellular structure of viruses indicates their relict nature, suggesting they represent a stage in the evolution of chemical structures transitioning into living cells. According to other hypotheses, the structural simplicity of viruses is secondary, representing a genomic simplification of parasitic Bacteria that have lost a significant portion of their genome. Some scientists link the origin of viruses to viroids, believing that the acquisition of RNA Replication ability by viroids led to The Emergence of the first RNA viruses.

Class="center">Self-Control Questions

1. Which scientist first described viruses?

2. What is the structure of a virus?

3. How do viruses reproduce?

4. How do viruses spread?

5. Name viral plant diseases.

6. What is The Role of viruses?