Virus – Characteristics, History, Shape, Classification

The word virus is originally derived from the Latin language – which means “poison.” Till now, many viruses have been reported. All viruses live a parasitic life. They cause many dangerous diseases in plants, animals, and human beings.

A unique feature of viruses is that they possess some characteristics of living organisms and some of non-living things. Therefore, strictly speaking, they cannot be called only living, nor only non-living, but are considered to exist between the two.

Characteristics of Viruses as Living Beings –

1. They possess the power of reproduction.
2. They show a special type of heredity and variation.
3. The changes occurring in them resemble mutations.
4. Their chemical composition resembles that of proteins; therefore, like proteins, they also perform chemical reactions. They are capable of carrying out complex activities of living cells and, like microorganisms, they grow and multiply. On the basis of these features, their reproduction resembles that of living cells and microorganisms.

Characteristics of Viruses as Non-living Beings –

1. They do not have a respiration process.
2. They do not have a cellular structure.
3. They lack enzymes.
4. They do not perform metabolic activities.
5. From a chemical point of view, since the viral particles are non-living, they can be preserved for an indefinite period.

Thus, biologists, on the basis of these available characteristics, have placed viruses in a position between living and non-living.

Some Special Characteristics of Viruses

• A virus contains only one type of nucleic acid. Some contain Deoxyribonucleic acid (DNA), while others contain Ribonucleic acid (RNA), whereas in other living organisms, both types of nucleic acids are found.
• The reproduction of viruses occurs through their single nucleic acid, while in other organisms it occurs through chromosomes made of nucleic acids.
• The microscopic particle itself is called a “virus.” Viruses do not grow by binary fission.
• Viruses lack a metabolic system, i.e., the enzyme-driven processes through which potential energy is transformed are absent.
• Viruses are considered absolute parasites because they use the enzymes of their host cells for the process of protein synthesis.

Brief History

The virus was first discovered by Ivanovsky in 1892, who stated that these ultra-microscopic organisms are fatal agents and carriers of diseases in animals and plants. Ivanovsky identified the Tobacco Mosaic Virus (TMV).

Bejernik (1899) named this infection fluid ‘Contagium Fluidum’. At the same time viruses were also discovered as responsible for disease in animals.

The first definite proof of insect vector was given by Japanese scientist Takalvid (1901). Hanger (1905) and Freiberg (1917) showed that virus disease is caused by enzyme. According to Smith (1929), “Insects which eat diseased tobacco transmit the virus from one plant to other healthy plants.”

Ivanowski (1903) carried out cytological study of the affected tissues and studied the unknown structure X-bodies.

W.M. Stanley (1935) in his important discovery obtained the tobacco mosaic virus in paracrystalline form. Dube (1943) showed that virus particles originated from the mitochondria of plants and its derivatives. According to Darlington (1944), “Viruses have evolved from the proteins of the cell.”

Shape

The structure of the particles of viruses found in animals and plants is different, like in some it is spherical, in some it is rod shaped, in some it is brick-shaped, in some it is sperm or tadpole shaped.

Apart from these shapes, other shapes can also be seen in the virus world.

Viruses of plants and animals are generally considered non-motile. Bacterial viruses, which are generally called bacteriophages or simply phages, have a tadpole-shaped structure. Its head is usually round and the tail is slender. The length of the head and tail can be equal or small or large. It is not yet known whether the tail contributes to locomotion or not.

The structure of a typical virus particle consists of an inner portion of nucleic acid that is partially or completely covered by a protein sheath. Some viruses have been isolated in pure form and have been obtained in crystalline form. All the viruses that have been isolated have molecules of very large size and weight.

The complete particle of a virus is called a “virion”, the protein shell is called a capsid and each sub-unit of the protein is called a capsomere.

The nucleic acid which is surrounded by the capsid is called nucleo-capsid. The shape found in some virus particles is shown in the following table:

Organization

According to modern thinking, viruses are proteins of high molecular weight. Inside which there is nucleic acid. According to Bowden (1959), the study of X-rays gives information about the state of nucleic acid in the rod-like tobacco mosaic virus. Whether the virus is of animals or plants, the presence of nucleoprotein is very important. It mainly contains two proteins and nucleic acid. RNA forms the center which is covered by a protein sheath.

All viruses contain protein units arranged in a regular sequence. The ratio of nucleic acid and protein varies in different viruses. The spherical particles are 18-43% nucleic acid and purines and pyrimidines. Plant viruses along with nucleic acid and core usually contain RNA or sometimes both DNA and RNA or only RNA.

Classification

Generally, classification of viruses is a difficult task, but the classifications presented from time to time are as follows:

I. Provisher (1957) divided viruses into the following four groups-
(1) Plant Viruses
(2) Mammalian Viruses
(3) Insect Viruses
(4) Bacterial Viruses or Bacteriophage

II. On the basis of host – It includes those viruses which attack the host. This classification has been presented on the basis of these characteristics. Along with classifying viruses into species, they are also divided into serological types. The classification of hosts is as follows-
(1) In humans
(2) In animals
(3) In fishes
(4) In insects
(5) In plants
(6) In bacteria

III. On the basis of fibers, Franvisher (1959) classified viruses on the basis of affecting different types of fibers. It is as follows –
(1) Neurotropic virus – This type of virus infects only nerve cells.
(2) Dermatotropic virus – This type of virus infects the skin cells. For example – small pox, measles, cow pox, etc.

(3) Viscerotropic viruses – This type of virus also affects visceral tissues and cells of internal organs. For example – yellow fever etc.

(4) Pantropic viruses – These viruses infect many types of tissues.

(5) Miscellaneous virus – This type of virus includes miscellaneous viruses. For example – mumps, influenza, common cold etc.

4. Based on the fundamental properties of the virus – Brands and Works (1975) have classified the long rod-shaped viruses of plants. Using morphological as well as serological relationships, the long rod-shaped viruses of plants have been divided into 6 different groups as follows–

(1) Tobacco rattle virus group
(2) Tobacco mosaic virus group
(3) Potato virus X-group
(4) Potato virus S-group
(5) Potato virus Y-group
(6) Beet yellows virus group

Difference between Plant Virus and Animal Virus

Those viruses that infect invertebrate animals are called invertebrate viruses.
The viruses that infect vertebrates are called vertebrate viruses.
Some such viruses have also been observed that infect both animals and plants; such viruses are called dual host viruses.

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