In this article we will discuss about:- 1. Meaning of Virus 2. Characters of Virus 3. Importance.
Meaning of Virus:
The name ‘virus’ came from a Latin word virus which means venom or poisonous fluid. Although plant diseases like leaf roll of potato and human diseases like yellow fever, small pox etc., were known for long time, the nature of causative agent was known to us quite later.
Adolph Meyer (1886), an agriculture chemist of Holland, observed a diseased tobacco plant showing mottling of leaf and named it mosaic. He was able to demonstrate the infectious nature of the sap of infected plant by grinding, filtering through double filter paper and then applying the sap to the healthy plants.
The infective capacity of the fluid (sap) was lost by heating at 80°C and he concluded that certain microbes are the causative agent of tobacco mosaic. D. Iwanowski (1892), a Russian scientist, was the first to demonstrate the transmission of tobacco mosaic virus disease from infected to healthy plant through sap, even the sap was filtered through Chamberland filter candle, which is sufficient enough to remove bacteria. W. M. Beijerinck (1898), a bacteriologist of Holland, demonstrated that the invisible, filterable and non- cultivable infectious sap could diffuse through an agar gel, like a fluid.
The bacteria and other organism cannot do so. From the above, Beijerinck believed that the fluid itself was alive and he called it as “contagium vivum fluidum” i.e., infectious living fluid.
Loeffler and Frosch (1898) observed that the agent of mouth and foot disease of cattle could pass through bacteriofilter. The agent could not be grown in artificial culture medium and was not visible under microscope. Walter Reed and his associates (1900) discovered the agent of yellow fever, the virus, which is the first discovered viral disease of man. Till date many viral diseases of vertebrates are known.
F. W. Twort (1915), a British scientist, was the first to describe some viruses that attack bacteria. Two years later, Felix d’Herelle (1917), a French scientist, independently observed and studied in detail about the virus attacking bacteria. He named the virus bacteriophage (bacteria- eater), commonly called phage.
Schlessinger (1933) was the first to purify virus by using differential centrifugation. Later, in 1935, Wendell M. Stanley, an organic chemist, observed that the virus could be crystallised and consisted largely of proteins.
Later, it has been established that virus contains a small amount of RNA or DNA in addition to protein. Therefore, virus is an organism of nucleoprotein and the nucleic acid is the infective agent rather than protein. A few years later, in 1946, W. M. Stanley was awarded the Nobel Prize for the above discovery.
Based on the nature of host, viruses cannot be categorised either in the animal or in the plant kingdom. The branch that deals with the viruses is called virology. The scientists who work in this branch are called virologists.
Characters of Virus:
In brief the important characters of viruses are:
(a) They are non-cellular, self-replicating agents.
(b) They can grow and multiply intracellularly as an obligate parasite (i.e., grow only in living host) or remain inert outside the host.
(c) Depending on the symmetry, they are of three types: cubical, helical and complex.
(d) The viruses consist of two parts: the centrally placed nucleic acid, covered by protein coat.
(e) The nucleic acid is either DNA or RNA, but both do not remain together.
(f) The nucleic acid may be single or double stranded.
(g) The outer covering i.e., shell or capsid is made up of protein units, called capsomeres; except some animal viruses which are with additional polysaccharides.
(h) They have no machinery of their own for protein synthesis and thereby they use host machinery for the synthesis of protein.
(i) During replication their nucleic acid directs the host cell to make different parts of virus and when these parts assemble together they form a complete infectious particle, the virion.
(j) They are transmitted very easily from one organism to another organism.
General Characters of Virus:
The viruses are non-cellular, self-replicating, obligate, intracellular parasitic agents essentially composed of a protein that covers a central nucleic acid molecule, either RNA or DNA. The amount of protein varies from 60 to 95% and the rest is nucleic acid. A simple virus particle is often called a virion. They grow and multiply only in living cells.
They cause diseases of animals including man, and plants of different groups, except bryophytes and gymnosperms. All known viruses are the pathogen of either plants or animals. They can exist outside the host cell for long period, but remain inert in free state. Again they become active after entering in a living host cell. They cannot grow or multiply in nutrient medium, supplemented with the extract of host cell in which it grows, but can grow only in intact living cell, thus they are obligate parasite.
They are the smallest among the infective agents, even much smaller than the smallest bacteria and varying over a wide range from 18-400 nm (Parvo virus– 18-26 nm, Tobacco mosaic virus – 1 7.5 nm x 300 nm, Tulip mosaic virus – 28 nm, Polio virus – 27 nm x 30 nm, Influenza virus – 80 nm x 120 nm, Small pox virus – 400 nm, etc.).
According to electron microscopic observation they are of different forms (Fig. 2.37) such as rod shaped, spherical, cubical etc. Within the host cell, the virus can grow, multiply and undergo mutation, but it does not respire. They depend completely on the enzyme system of the host cell for their activity.
Structure:
The virus consists of two parts:
(i) Nucleic acid (centrally placed), and
(ii) Protein coat, sometimes with additional envelope.
(i) Nucleic acid:
Viruses contain only one type of nucleic acid i.e., either DNA or RNA. The DNA containing viruses are called Deoxyviruses, whereas viruses having RNA are called Riboviruses. They vary in the structure of their nucleic acid. Most of the plant viruses have RNA either single (TMV) or double stranded (Rice ragged stunt viruses), except a few have DNA either single (Gemini viruses) or double stranded (Dahlia mosaic virus).
Animal viruses have mostly double stranded DNA or either single (Polio virus) or double (Reo virus) stranded RNA and bacteriophages contain mostly double stranded DNA, but they also have single stranded RNA (f2, R17, fr) or single stranded DNA (f1, fd, M13) (Table 2.13). Each virion contains only one molecule of nucleic acid, called genome, consisting of nucleotide pairs whose number ranges from 1000-250,000 pairs. The amount of nucleic acid of a virion usually depends on its size. The number of genes per virion ranges from 4-8 for small viruses and 100-200 for the large viruses.
(ii) Protein coat:
The protein coat surrounding the genome is called capsid and the capsid together with the enclosed nucleic acid is called nucleocapsid. The capsid is made up of a large number of protein subunits, called capsomeres (Fig. 2.39A). Many mammalian viruses have envelope made up of a bilayered lipoprotein, mainly of host cell origin that surrounds the nucleocapsid (Fig. 2.39B).
Symmetry:
The capsid is symmetrically arranged around the central nucleic acid.
Based on symmetry of capsid, the viruses are grouped into three categories:
(a) Cubical (icosahedral),
(b) Helical, and
(c) Complex.
(a) Cubical (icosahedral) capsids (Fig. 2.38A):
They have a polygon with 12 corners (vertices), 20 sides (facets) and 30 edges. Each side is an equilateral triangle. They are of two types — Pentons (pentagonal capsomeres at the corners) and Hexons (hexagonal capsomeres at the corners), e.g., herpes and toga viruses are enveloped and papova and adenoviruses are naked.
(b) Helical capsids (Fig. 2.38B):
Both nucleic acid and capsomeres are coiled together and form a spiral or helical tube. All the helical types are RNA viruses and most of them are enveloped, e.g., Tobacco mosaic virus (TMV), Influenza virus, etc.
(c) Complex capsids (Fig. 2.38C):
Viruses which do not conform to either of the above two types due to complexity of their structure are called complex capsids, e.g., pox virus and bacteriophages like T2, T4, and T6.
The viruses differ from bacteria in the following points:
(a) The viruses are very much smaller.
(b) They lack the machinery for protein synthesis.
(c) They do not have cellular organisation.
(d) They neither grow in artificial culture medium nor divide by binary fission.
(e) They have only one kind of nucleic acid.
(f) The viruses are resistant to antibiotic.
Viruses have both living and non-living characters.
Living characters of viruses:
(a) They have the nucleic acid (DNA or RNA) i.e., the genetic material that can replicate.
(b) Mutation is well-established by the availability of mutant forms in some viruses.
(c) They are sensitive to stimulants like radiation, chemical substances etc.
(d) They can multiply in the living cells of the host.
(e) The viruses have antigenic property.
(f) They can attack specific host.
Non-living characters of viruses:
(a) The viruses remain as inert material outside their host.
(b) They are autocatalytic in nature.
(c) They are devoid of cell membrane and cell wall.
(d) The viruses are devoid of cellular organelles like ribosomes, mitochondria etc.
(e) The viruses can be crystallised.
Importance of Virus:
The viruses have both harmful and useful activities:
A. Harmful activities:
(i) Viruses are responsible for various diseases of both plants (tobacco mosaic, yellow vein mosaic of lady’s finger, leaf roll of potato, leaf curl of papaya etc.) and animals (small pox, meningitis, pneumonia, mumps, bronchitis etc.). The plant viruses cause damage to different parts like root, leaf, fruit, seed etc. and cause economic losses by reducing the quality and quantity of the plant products.
(ii) Phages often kill the beneficial microorganisms during commercial production of antibiotics and milk products.
B. Useful activities:
(i) In space research, lysogenic phage cultures are used as radiation detector by Russians in the space ship (Vostok 2).
(ii) Avirulent or temperate phages help in genetic recombination (transduction) and are used widely in genetic research.
(iii) Phages are used as scavengers to eradicate the bacteria present in the polluted water.
(iv) To a limited extent phages are used in therapy and prophylaxis of some bacterial diseases.
(v) By holding both the living and nonliving characters, viruses got the importance in determining the origin of life.
(vi) Viruses are utilised in the production of vaccines, used to develop immunity against viral infection.