The Discovery, Characteristics, and Structure of Virus

History of Virus Discovery

• The word "virus" comes from the Latin word meaning poison.
• The discovery of the virus began when a German scientist, Adolf Mayer, in 1883 did research on mosaic disease in tobacco.
• Mayer sprayed mosaic leaf tobacco extracts into healthy tobacco plantings. The tobacco plant then becomes dwarfed and appears the color of the leaves to be dappled.
• Initially, Mayer thought that the disease is caused by bacteria that are very small.
• In 1892, Russian scientist Dimitri Ivanowsky re-examined Mayer's research. Ivanowsky filtered out a mosaic leaf tobacco extract using a bacterial filter. Then the filtrate is sprayed back into healthy tobacco plants. Supposedly, if the mosaic disease is caused by bacteria, the tobacco plant sprayed with diseased tobacco leaf extract will remain healthy.
• But it turns out the plant remains infected with mosaic diseases.
• Based on the results of these experiments, Ivanowsky concluded that mosaic disease is caused by bacteria that are very small in size. So this bacteria can get away from bacteria filter.
• A few years later, Martinus W. Beijerinck did the same research with Ivanowsky.
• Beijerinck assumes that there are other infectious (not bacteria) capable of reproducing and attacking healthy tobacco plants. However, these infections are different from bacteria because they can not be cultured in agar medium.
• The Beijerinck hypothesis is reinforced by Wendell Stanley's research. He is an American scientist who can first crystallize infectious agents in tobacco-ridden tobacco plants.
• Wendell Stanley named the infectious agent as Tobacco mosaic virus (TMV)
• Viruses can not be classified as cells because the virus has no nucleus and cytoplasm.
• Viruses are said to be living objects if they are inside a host cell (living organism) and capable of reproduction.
• However, if it is outside the living organism, the virus is a dead object. This virus is also called virion. Virions are microscopic particles of viruses that can crystallize.

The Characteristics of Virus 

• Viruses are organisms that contain only one type of nucleic acid - DNA or RNA.
• Can only live on the network of living organisms.
• It is not a cell so it has no cell membrane, cytoplasm, and the cell nucleus.
• RNA or DNA is covered by protein capsule.
• It is very small (smaller than bacteria) so it can not be seen using a light microscope. Typically, the virus size is between 20 - 300 nm.
• Has a variety of body sizes and shapes.
• The virus can not reproduce outside its host cell.
• The virus is parasitic.
• Can be crystallized.

Virus Structure

1. Body Shape
- The virus has a varied body shape such as round, stem, polyhedral, oval and T shape.
- Examples of viruses that are round are influenza virus and Human Immunodeficiency virus (HIV).
- An example of a rod-shaped virus is Tobacco mosaic virus (TMV).
- Example of an oval-shaped virus is Rabies virus.

2. Body size
Viruses are very small microorganisms. Virus size ranges from 20 - 300 nm. Because of its super small size, the virus can only be seen using a special microscope such as an electron microscope.

3. Body parts of the virus

• The virus has a very simple structure.
• Generally, the virus consists of nucleic acids wrapped in a protein sheath (capsid).
• Each virus has only one type of nucleic acid. Viruses containing DNA such as smallpox viruses and viruses containing RNAs such as influenza and HIV.
• The capsid is a nucleic acid wrap. The capsid is composed of proteins called capsomers.
• There are helical-shaped helmets and some are polyhedrals.
• The polyhedral virus has an additional portion of the tail casing, the base plate, and the tail fiber. Its function is to attach to the infected cell.
• Some viruses like influenza and Herpes viruses have envelopes composed of lipids and glycoproteins.
• Viruses that do not have covers are called "naked" viruses.

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