Introduction and Discovery

• Latin World- Virus means Poison.
• Multiplication- Require host cells, i.e. Either Plant, animal or Bacteria.
• Cause serious diseases in crop plants, ornamental plants and forest trees.
• Cause infection in bacteria, cyanobacteria, fungi, protozoa, etc.
• Cause various diseases in variety of vertebrates including fish, amphibian, birds and animals.
• In Humans- Serious diseases like Corona, Poliomyelitis, small pox, mumps, measles, rabies, yellow fever, influenza and encephalitis.
• Viruses infect Prokaryotes- Bacteriophages and Phages.

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• Adolf Mayer (1886) – Tobacco Mosaic Disease.
• Dimitry Iwanowski (1892)- Demonstrated causative agent of Tobacco Mosaic Disease can be transmitted to healthy plants by the sap from infected plants.
• Beijerinck (1898) Discovered and coined term virus. He confirmed the filterable nature of disease causing agents and put a concept of Contagium vivum fluidum (living infectious fluid).
• W. M. Stanley (1935) crystallized TMV and got Nobel prize in 1946.
• Twort (1915) and d’Herelle (1916) discovered Bacteriophage, i.e. Bacterial virus.
• Hershey and Chase (1952) studied T2 Bacteriophage in detail.
• Holling (1978) discovered Mycophages (Parasitic virus on fungi) while Gourlay (1970) and Maniloff et al (1977)reported mycoplasmal virus.

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General Characteristics of Viruses.

• Non-cellular, self replicating agents.
• Ultramicroscopic (18-400nm), smaller than bacteria.
• Can not be filtered by bacterial filters.
• Connecting link between living and non living.
• Living characteristics- to reproduce only in living host cells and the ability to mutate.
• Non living characteristics- absence of cellular machinery, cytoplasm and cellular organelles. They can not carry out metabolism by their own and therefore replicate using the host cell’s metabolic machinery.
• Viruses 3 types- cubical, helix and complex.
• Can cause diseases in number of animals, plants and other microorganisms.
• Transmitted very easily from one organism to another organism.
• Viruses are made up of protein and one type of nucleic acid. (Either DNA and RNA).
• Nucleic acid may be single or double stranded.
• Viruses are acellular i.e. cellular machinery, protoplasm and cell organelles are absent.
• Can not be grown on artificial culture media.
• Can be cultivated in living cell culture medium.
• Antibodies cannot destroy viruses. But many viral diseases can be controlled by using antiviral drugs.
• Can be inactivated by treatment with UV rays, pyrimidine, urea and hydrogen peroxide.
• Show response towards temperature, radiation and chemical substances.

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Structure of Viruses

• Viruses shows variation in shapes and sizes. Shows different shows- spherical or cuboid, elongated, flexuous or coiled, bullet shaped, filamentous, pleomorphic etc.
• Size- 18- 400 nm in diameter.
• Complete assembly of virus is composed of virion. Virion is composed of nucleic acid core of only one type of nucleic acid (DNA or RNA) surrounded by protein coat called capsid. Capsid consist of numerous capsomeres. Shapes of virions depends upon specific arrangement of capsomeres in the capsid.
• Envelope- protein and glycoproteins.
• Viruses differ in their DNA and RNA content. DNA containing viruses are called Deoxyviruses and RNA containing viruses are called Riboviruses.
• Nucleic acids of most of the viruses are linear straight chain structure.

Viruses are classified into three groups.

• 1) Helical or Cylindrical Virus:- Protein subunits and the nucleic acids are arranged in a helix. These viruses are elongated, rod shaped, rigid or flexible. Capsid is hollow cylinder with helical structure.
• Helical viruses are divided into two groups.

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• (a) Naked capsids e.g. TMV, Bacteriophage M13 etc.
• (b) Enveloped capsid e.g. Influenza virus.

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• (2)Icosahedral virus:- protein subunits assemble into a symmetric shell that covers the nucleic acid containing core. Viruses looks like spherical in shape, but actually they are icosahedral. The icosahedron is made up of equilateral triangles fused together in a spherical shapes.
• Icosahedral viruses are divided into two groups.
• (a) Naked capsid e.g. Adenovirus, Polio virus.
• (b) Enveloped capsid e.g. Herpes simplex virus.

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• (3) Complex viruses:- Combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology.
• Viruses with complex shaped capsids are of two types.
• (a) Capsid not clearly known e.g. Pox virus (Vaccinia).
• Capsid with some attached structures (Tadpole shaped structures) e.g. T-even phages of E.coli; T2 phase has an icosahedral head, helical tail sheath, hexagonal end plate and rod shaped tail fibres.

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TYPES OF VIRUSES

• Genetic material- DNA of RNA. Viruses containing DNA- DNA Viruses, RNA- RNA Viruses
• DNA Virus – T- Phage
• The virus which infect and destroy bacterial cell are known as bacteriophage. There are three even numbered T- phages (T2, T4 and T6) and four odd numbered phages (T1, T3, T5 and T7) that infect E.coli. Commonly known as phages. Phages are discovered by Edward Twort (1915). Twort d’Herelle (1917) studied them in detail.
• The T4 phage is tadpole shaped complex virus. Capsid comprises icosahedral head, a short neck with collar bearing whiskers and a tail.
• The head is bipyramidal hexagonal prism like structure. Size of head is 95 × 65 nm which consist of about 2000 identical protein subunits (Capsomeres) enclosing a single, double stranded DNA thread. The extended portion between head and tail is called disc or collar. The tail is contractile and cylindrical which is attached to lower side of head.

• The tail is almost equal to the length of the head. The tail is made up of thick and hollow midpiece, a hexagonal base plate to which six spikes and six long tail fibers are attached. Midpiece consist of a central hollow core (through which DNA passes to the host cell during infection) and a spring like contractile sheath. This contractile sheath comprises 24 rings of hexamers helically arranged around the core.

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RNA Virus – Tobacco Mosaic Virus (TMV).

• TMV mosaic on tobacco leaves. It is RNA virus having single stranded RNA. It is the most resistant virus of which thermal death point is 90° C for 10 minutes.
• TMV- discovered by D. Iwanowski (1892). W.M. Stanley (1935) isolated it from infected plant and crystalized. Franklin et al. (1957) have described the structure of TMV.
• TMV is rod shaped helical virus with rough surface. It looks like a bunch of Banana. Each rod is about 3000 A° (300nm) long and 180 A° (18nm) in diameter. Each rod consists of two parts, the protein coat or capsid and nucleic acid. The capsid consist of 2130 protein subunits of identical size.
• The nucleic acid is single stranded RNA molecule consisting of 6500 nucleotides. The capsomeres are closely arranged helically around RNA helix forming a hollow cylinder of 40 A° (4nm). There are about 16 1/3 protein subunits (capsomeres) in each helical turn and three turns of the helix contain about 49 capsomeres.
• The rod contains about 95% proteins and 5% RNA. The RNA provides a code which directs the synthesis of specific viral protein in the host cell.

ECONOMIC IMPORTANCE OF VIRUSES

• [1] Harmful activities:-
• (a) Responsible for killing beneficial micro- organisms during commercial production of antibiotics and milk products.
• (b)Responsible for various plant diseases. Can 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. There are the following viral plant diseases.
• (i) Tobacco Mosaic Virus (TMV): TMV attacks on the leaves and stem of plant. TMV infect tobacco, tomato and other members of family Solanaceae. Can transmitted by rubbing of leaves, handling and contact.
• (ii)Mosaic Virus of Cucumber and Lettuce: These viruses are mostly spread by pest like aphids and leaf hoppers.

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• (iii) Yellow disease of potato: It infects the leaves, stems and roots of plant and destroys the plant.
• (iv) Tomato bunchy top disease: It is caused by infectious particles called viroid. The tomato plants become dry and fells down.
• (v) Cotton leaf curl viruses: it is spread by white fly named Bemisia tabaci. Its symptoms are thickening of sub-veins in young leaves and upward curling of the leaves.
• (vi) Necrotic diseases: Disease occurs in potatoes and tomatoes. The virus kills the leaf of plant by necrosis.
• (vii) Tumours: Many viruses cause tumours in tomatoes, cucumber and tobacco plants.

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• (c) Viruses are responsible for various animal diseases. There are following animal (human) viral diseases.
• (i) HIV (Human immunodeficiency Virus): AIDS is caused by HIV virus. The HIV virus affects T- lymphocytes, which are major component of the immune system. The infected person is attacked by many diseases.
• (ii) Cancer: Some tumour producing retroviruses have special genes called oncogenes. These viruses inject these genes into host cells. Thus, divisions start in the host cells. It produces tumour and cause cancer.
• (iii)Viral Hepatitis: The inflammation of the liver due to viral infection is called hepatitis. It has different types like Hepatitis A, B, C,D etc. The common diseases caused by viruses are jaundice, abdominal pain, Liver enlargement, Fatigue and sometimes fever. It is water borne and transmitted via contaminated drinking water and food.

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• (iv) Polio: Poliomyelitis is caused by polio virus. It occurs mostly in childhood. Polio virus affects nervous system. Children are genetically affected.
• (v) Influenza: It occurs in epidemic form in world. It is disseminated by contact and air - borne transfer through coughing and sneezing of infected person. It is an enveloped RNA virus. It infects the respiratory tract.
• (vi)Small pox: caused by pox virus. DNA enveloped virus. One of the oldest diseases. Raised fluid filled vesicles are formed on the body. These vesicles changes into pustules forming pitted scars called pocks.
• (vii) Rabies: The virus is transmitted through the bite of infected animals. It affects spinal cord and brain. Rabies virus can infect a wide variety of mammals.

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USEFUL ACTIVITIES:

• (a) Viruses are utilized in the production of vaccines, used to develop immunity against viral infection.
• (b) Bacteriophages are used to destroy the bacteria in polluted water.
• (c) Bacteriophages are used in therapy and prophylaxis of some bacterial diseases.
• (d)Viruses are useful for the rapid destruction of harmful algae blooms e.g. addition of cyanophages LPP-1 and SM-1.
• (e)Lysogenic phage cultures are used as radiation detection by Russians in the space research.
• (f) Avirulent or temperate phages help in genetic recombination (transduction) and are used widely in genetic research.
• (g)Viruses are considered as very important for determining the origin of life due to presence of both the living and non living characters.
• (h)Viruses are useful in the regulation of salt water and fresh water ecosystems.
• (i)Virotherapy uses virus as a vectors to treat various diseases e.g. treatment of cancer and in gene therapy.

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• (j) can be used as an alternative to the antibodies because of the high level of antibiotic resistance now found in some pathogenic bacteria.
• (k) In nanotechnology, viruses have been used as templates for organizing materials on the nanoscale due to their size, shape and well- defined chemical structures.
• (l) In genetic engineering, viruses are sometimes used to transfer genes from one organism to another. These viruses are known as vectors.
• (m) Pesticides: Some insect pests are controlled with baculo viruses.
• (n) Sources of enzymes: A number of enzymes used in molecular biology are virus enzymes e.g. reverse transcriptases from retroviruses and RNA polymerases from phages.
• (o) Treatment of genetic diseases: Children with severe combined immunodeficiency (baby in the bubble syndrome) have been successfully treated using retroviruses.