12th Class Biology blank Viruses

The term 'virus' has been derived from Latin, which means poison or venom or viscous fluid. They remain inactive outside a living host but become active inside the host and multiply in it. They represent a transitional form of life between non–living and living world.

Luria (1953) defined virus as "Sub-microscopic entities capable of being introduced into specific living cells and reproducing inside such cells only. "Single virus is called 'Virion', most of the plant virus are RNA virus. Most of the animal virus are DNA virus.

(i) Important discovery of virus

Carolous causius (1576) recorded first viral disease in tulips.

1. Mayer (1886) found a disease in tobacco caused by virus and called it tobacco mosaic disease.
2. Ivanowski (1892), a Russian Botanist, discovered the infectious nature of the viruses. He was the person, who discovered the virus.

Beijerinck (1898) first used the word Virus.

1. Twort (1915) and D. Herelle (1917) discovered bacteriophages, a kind of virus which infected bacteria and destroyed them.
2. M. Stanley (1935) first time isolated tobacco mosaic virus (TMV) in crystalline form and showed that crystals were made up of proteins. Nobel prize was awarded to him for this work.

(ii) Nature of viruses : Viruses are regarded as intermediate between non-living entities and living organisms. It is very difficult to as certain whether they are living or non-living. Some characters of viruses suggest their non-living nature where as many other characters suggest their living nature. The two views are listed below –

Viruses are non-living : The following characters state that they are non-living.

(a) Viruses have no complete cellular structure. They are not surrounded by cell membrane or cell wall.

(b) They do not show cellular metabolism and lack respiration.

(c) They possess high specific gravity unlike living organisms.

(d) Viruses are active only when they are inside the living host cells. Out side the host, they are good as chemical substances. Thus, they do not have their independent existance.

(e) The viruses can be precipitated just like chemical substances.

(f) Postulates of Robert Koch are not true for the viruses. Virus cannot grow in “invitro” condition in lab.

Viruses are living organisms : The following characters state that they are living organisms –

(a) They have definite shape and morphology like that of a living organism.

(b) They possess genetic material (DNA or RNA), which determine their structure and development. Genetic material passes from generation to generation in usual manner.

(c) All viruses are intracellular obligate parasite and attack specific hosts. The bacteriophages recognise the real bacterial surface.

(d) They show property of mutation.

(e) They show irritability and respond to environmental conditions such as heat, ultraviolet rays, humidity, drought, alcohol, etc.

(f) They can grow inside the host and multiply enormously showing one of the most important property of living organisms.

(iii) Chemical composition : Chemically viruses are nucleoproteins. They are made up of central core of nucleic acid. Nucleic acid is only one, either DNA or RNA. This nucleic acid (DNA or RNA) represents the genetic characters of virus. TMV has RNA (like most plant viruses have) 10% RNA and 90% protein is present in influenza virus and PSTV (Potato Spindle Tuber Viroid) also has RNA but it does not have capsid (protein coat). Plant viruses contain RNA but in cauliflower mosaic virus contain DNA. Bacteriophages contain DNA and almost half animal viruses contain RNA and half contain DNA. But it is called that often animal viruses contain DNA. Cancer causing viruses reovirus contain both RNA and DNA, Only some enzymes are detected in viruses such as – Lysozyme in bacteriophages, transcriptase in vaccinia virus, reverse transcriptase and DNA or RNA polymerase in retroviruses.

(iv) Shape : There is variation in shapes of viruses. On the basis of shape viruses have been placed in the following categories.

(a) Straight, rigid rods with helical architecture, e.g. TMV, Barley stripe mosaic virus (BSMV).

(b) Long flexous thread–like rods, e.g. Potato latent mosaic, Wheat streak mosaic virus.

(c) Polyhedral virions, e.g. Turnip yellow mosaic, Tobacco ring spot virus.

(d) Tadpole like – Bacteriophages.

(e) Spherical – Influenza virus.

(v) Size : Viruses have a long range of size. They range from 10 mm to more than 300 mm in size. The virus of foot and mouth disease (FMD) of animals is smaller than the largest protein molecule. Largest virus is smallpox virus – variola (250 mm).

(vi) General structure of virus

Structurally viruses are made up of envelope, capsid, nucleoid and occasionally one or two enzymes.

Envelope : Some viruses possess an outer thin loose covering, called envelope. It is composed of proteins (from virus), lipids and carbohydrates (both from host). The smaller subunits of envelope are called peplomers The viruses, which do not possess envelope, are called naked.

Capsid : It is the protein coat that surrounds the central portion of nucleoid and enzymes (if present). The capsid consists of a specific number and arrangement of small sub-units called capsomeres. These sub-units possess antigenic properties.

Nucleoid : The nucleic acid present in the virus is called nucleoid. It is the infective part of virus which utilizes the metabolic machinery of the host cell for synthesis and assembly of viral components. The genetic material of viruses are of four types :

(a) Double stranded DNA (ds DNA)

(b) Single stranded DNA (ss DNA) 

(c) Double stranded RNA (ds RNA) 

(d) Single stranded RNA (ss RNA)

(vii) Life cycle : The word reproduction is not appropriate in case of viruses because they have no cellular components or cell organelles. They do not reproduce themselves but divide by a special mechanism as follows.

Attachment : The bacteriophage gets attached to bacterial cell wall with the help of caudal fibres.

Penetration : Bacteriophage dissolves the bacterial wall by an enzyme Lysozyme and makes a pore in cell wall. Through this pore DNA molecule enters in the cell after contraction of head protein, entire protein coat remains outside.

Latent period : Phage DNA controls hosts cellular machinery. Instead of formation of bacterial protein, phage protein formation begins. Cellular DNA and RNA is broken down and from this cellular DNA, phage DNA is formed. Now protein covers the DNA fragments to form a kid virus.

Maturation : This young virion is changed into an adult virus hence this process is called maturation.

Release : The viruses are mature, cell wall of bacterial cell is weakened by enzyme lysozyme. The release of viruses takes place by bursting of host cell and these are again ready for next infection or attack on other bacteria.

(viii) Economic importance of viruses

Uses of viruses

(a) Specific viral strains are cultured and attenuated to be used as vaccines against specific diseases.

(b) The addition of cyanophages LPP-1 and SM-1 are useful in controlling water blooms.

(c) Bacteriophage was used by Hershey and Chase to prove that DNA is the chemical basis of heredity.

(d) Bacteriophages are of interest to geneticists because these bring about transduction.

(e) Water of river Ganga is believed to have phages which destroy bacteria. That is why its water does not get spoiled.

Tobacco mosaic virus (TMV) : It was discovered by the Russian worker D. Ivanowski. Franklin etal (1957) described the ultrastructure of (TMV) – It is a rod–shaped virus having a central core of RNA surrounded by protein coat (capsid) to form the nucleocapsid. The nucleocapsid may be naked or may be surrounded by a loose membranous envelope. The protein coat (capsid) consists of 2130 identical subunits (capsomeres). The protein is 94% and RNA is only 6%. In the entire length a single RNA molecule runs in the form of spiral coils.

Bacteriophage : The viruses which attack bacteria are called bacteriophages. In outline they look like tadpole or sperm. The body can be divided into a hexagonal head neck and a tail. The hexagonal head has a central core of DNA, which is surrounded by protein coat. The DNA is double helix. The cylindrical tail is hollow and is entirely made up of proteins. At the end of this, there are six long threads called tail fibres or caudal fibres. These fibres help the virus while attaching to bacteria. Bacteriophage contain lysozyme enzyme.

Cyanophages : Generally some of the viruses are found which attack on blue green algae. Sofferman and Morris (1963) reported 11 filamentous forms of blue green algae (Lyngbya, plactonema and phormidium, hence called LPP-1) which were attacked by viruses. These viruses are usually called cyanophages. Cyanophages contain DNA as their genetic material. These viruses resemble with bacteriophages in morphology and behaviour.

Mycophages : Some fungi such as, Mushrooms, Penicillium, etc have also been found to be infected by viruses. These are isometric in shape and contain double stranded RNA.

Phycophages : These are virus which attack on Algae.

Viroids : Diener and Raymer (1967) discovered very simple smallest infectious agents called Viroids. Viroids consist of RNA only and capsid is lacking. Viroids contain only very low mol. weight. Diener and Raymer reported that causal agent of potato spindle tuber disease was a free RNA and no viral nucleoprotein particles were present in the infected tissue. T.O. Diener (1971) termed it viroid. Viroids are single–stranded, covalently closed circular as well as linear RNA molecules. Transmission is mechanical. The symptoms on host plants are almost similar to those of viruses. Viroids cause persistent infections. A number of other diseases caused by viroids are – Cadang Cadang of coconut, Cucumber pale fruit, Chrysanthemum stunt, Avacado sunblotch, etc.

Prions : Prusiner (1982) discovered it as a human disease causal agents. Stanley B. Prusiner discovered infectious agents which were prions. Prions are proteinaceous particles thought to cause a number of diseases including the slow virus diseases, therefore also called as slow viruses. They are made of proteins molecules only. Genetic material (DNA and RNA) is absent in prions. Kuru, a disease of central nervous system found in few canniblastic tribes of New Guinea is caused by prions. Other such disease is Creutzfeld–Jacob disease of humans and animals, similar to scrapie, gerstmann – strassler – scheinker syndrome. These all are diseases of central nervous system.

Interferons : G.M. Findley and McCallum (1937) reported a phenomenon called viral interference in which the cell infected with one type of virus becomes resistant to super infection by other viruses. Alliac Issacs and Lindeman (1957) gave the term interferons to the chemical substances responsible for viral interference.

(i) Interferons are produced by cells in mammals, rodents, birds, etc., and provide resistance against viruses.

(ii) Interferons are protein molecules or polypeptides of low molecular weight which prevent viral multiplication.

 Families of animal viruses, grouped by type of nucleic acid

ss RNA that can serve as mRNA (+ strand RNA)

ss RNA that is a template for mRNA  (- strand RNA)

Important plant diseases caused by viruses

Important human diseases caused by viruses