Virus : Introduction

• The  term  ‘Virus’  has  been  derived  from  Latin, which  means  poison  or  venom  or  viscous  fluid.
• Viruses are defined as infectious nucleoproteins.
• Viruses are obligate intracellular parasites which can reproduce only by invading and taking over other cells as they lack the cellular machinery for self-reproduction.
• A  complete  virus  particle  is  called  virion  whose main function is to deliver its DNA or RNA genome into  the  host  cell.  So  that  genome  can  expressed (transcribed  and  translated)  by  the  host  cells.

Characteristic features of Virus

• These are submicroscopic organisms generally less than 200 mm.
• Shape  of  virus  is  variable,  eg.,  brick  shaped (influenza virus), rod shaped (TMV), tadpole like (bacteriophages). 
• They  are  obligate  parasites  i.e.,  can  live  inside living  host  only. 
• They  have either  RNA  or  DNA. 
• They  can  pass  through  bacterial  filters. 
• They  have  characteristic  mode  of  multiplication, i.e.,  once a  virus  enters  into  the  host  cell, it  takes control  of  whole  biochemical  machinery  of  host cell  and  orders  the  metabolic  machinery  to synthesize  their  own  (viral)  components. 
• Non  living  characters  of  viruses  are  – 
  • No  protoplasm 
  • No  enzyme  system 
  • No  respiration 
  • They  can  be  crystallized 
  • Do  not  grow  in  culture  medium. 
• Living  characters  of  viruses  are  – 
  • They contain nucleic acid as a result of which they  are  capable  of  synthesizing  proteins. 
  • They  can  multiply  inside  living  host  cell. 
  • They  causes  diseases. 
• On the basis of above characters it can be said that viruses form a transitional group between living and  non­living. 
• Viruses are divided into two main groups on  the basis  of  the  type  of  nucleic  acid  present  in  them :
  • Deoxyvira  (having  DNA).
  • Ribovira  (having  RNA). 
• Deoxyvira  are  further  divided  into  three  classes  – 
  • deoxyhelica (helical)
  • deoxycubica (cuboidal) and
  • deoxybinala  (binal). 
• Ribovira  is  divided  into  two  classes  ­  ribohelica (helical)  and  ribocubica  (cuboidal). 
• F.  O.  Holmes  (1948)  divided  order  ­  Virales  into three  sub­orders  – 
• Phytophaginae –  Viruses, infecting  plants 
• Zoophaginae  –  Viruses,  infecting  animals –  Phaginae  –  Viruses  infecting  bacteria 
• Structurally viruses are made up of envelope, capsid, nucleoid  and  occasionally  one  or  two  enzymes. 
• Some viruses possesses an outer thin loose covering called envelope. It is composed of proteins (from virus), lipids and carbohydrates (both from host). It  has subunits  called  peplomeres. 
• The  viruses,  which  do  not  possess  envelope,  are called  naked. 

• Capsid is the protein coat that surrounds the central protein  of  nucleoid  and enzymes (if  present). 
  • The capsid  consists  of  a  specific  number and arrangement of small sub­units called capsomeres.
• The nucleic acid present in the virus is called nucleoid. 
  • It is the infective part of the virus which utilizes the metabolic machinery of the host cell for synthesis and assembly of viral components.
  • Nucleoid represents the viral chromosomes.

• The genetic material of viruses are of 4 types – 
  • double stranded DNA (ds DNA),
  • single stranded DNA (ssDNA), 
  • double stranded RNA (dsRNA), and 
  • single stranded RNA (ssRNA).
• Double stranded or dsDNA viruses are adenovirus, herpes simplex virus, pox virus, cauliflower mosaic, coliphage lambda T (linear), hepatitis B, simian virus SV – 40, polyoma (circular or cyclic).
• Single stranded or ssDNA viruses are  coliphage MS 2, coliphage fd (linear), coliphage f × 174 (cyclic).
• Single stranded ssRNA viruses are poliomyelitis virus, foot and mouth disease virus, influenza virus, rous sarcoma virus, retroviruses, turnip yellow mosaic viruses, tobacco necrosis virus, TMV, potato mosaic virus, bean mosaic virus (all linear).
  • ssRNA is of two types – negative stranded (RNA – RNA viruses) and positive stranded (RNA – DNA viruses or retroviruses).
• Double stranded or dsRNA viruses are reo­virus, wound tumour virus (all linear).
• Only few viruses contain certain enzymes. For e.g., lysozyme in bacteriophages, reverse transcriptase in retroviruses.
• Symmetry of viruses may be –
  • Helical symmetry : Capsomeres are arranged in helical manner in the capsid, e.g., TMV.
  • Cubical symmetry : Capsomeres are arranged on the surface to form a 20 side cube, e.g., turnip mosaic virus.
  • Biosymmetrical or mixed symmetry : Bacteriophages

TMV (Tobacco Mosaic Virus)

• TMV is the most thoroughly studied virus and was discovered by the Russian worker D. Ivanowski (1892).9
• It is a rod-shaped virus measuring 300 × 18 nm and have helical symmetry.
• Have single-stranded DNA which is 330 nm in length and having 7300 nucleotides.
• Number of capsomeres in capsid is 2130.
• 5% RNA and 95% protein is present in TMV.

Bacteriophages

• Viruses  infecting  bacteria  are  known  as bacteriophages or phages. Sea water can hold 100 million bacteriophages per µl.
• Bacteriophages may be virulent or avirulent.
• Prophages or non­virulent phages or non­infective phages are the phages which do not cause lysis of bacteria soon after their formation inside. Such bacterial cells which are having prophages inside them are called lysogenic bacteria.
• Virulent phages or infective phages are the phages which cause lysis of bacterial cell at once.
• Most studied series of bacteriophages is T­series (T , T , T etc.).
• T­even phages are characterized by angular head and contractile tail.
• Bacteriophages have tadpole­like structure, i.e., with head and tail. Inside the head is present nucleic acid, generally DNA.
• Head is prism­like hexagonal having length 950Å and breadth 650Å, tail is also 950Å in length, joined to head by neck and collar,  tail is having hollow core of 80Å and is surrounded by tail sheath.
• At the end of tail, end plate is present to which 6 tail fibres are attached and each is 1500Å in length.

Lytic  cycle  (T  bacteriophage)

• The multiplication process of virulent phage is called the lytic cycle.
• The main steps include adsorption, penetration, formation of new phages, and lysis.
• Bacteriophage attaches to the surface of bacterium by its tail fibers.
• Lysozymes create a hole in the host cell wall.
• The tail sheath contracts and ejects the viral chromosome/DNA into the bacterium.
• Viral DNA controls the metabolic machinery of the host cell. It produces nucleases.
• Nucleases degrade DNA and mRNAs of the host.
• Viral DNA is not affected as its cytosine bases are methylated.
• A number of copies of viral DNA are produced followed by the synthesis of capsid protein, polyamines, and lysozymes.
• The components assemble and form phage particles or viruses.
• The host cell ruptures to release viruses (lysis).

Lysogenic  cycle (l phage)

• The multiplication process of temperate phage is called lysogenic cycle.
• The phenomenon of existence of non­virulent prophage in the host cell is called lysogeny.
• The host cell in which lysogeny occurs is called lysogenic cell.
• Lysogen is a strain of bacteria carrying prophage.
• Lysogenic cycle is shown by 1 (lambda) phage which also infects E.coli bacterium.
• Lytic bacteriophage multiply in host bacterium which then undergoes complete lysis (degeneration) to release the resulting daughter phages.
• The phage attaches to the surface of bacteria by means of tail, which produces a hole in the host cell wall and injects the phage DNA.
• Phage DNA produces a repressor, (C1) becomes nonvirulent or temperate and gets integrated to bacterial chromosome at a specific site by means of enzyme integrase.
• The viral genome is now called prophage/provirus.
• It multiplies along with the bacterial genome and is passed on to the progeny.
• Occasionally the synthesis of repressor is stopped due to ultraviolet radiations or chemical factors.
• The temperate/non­virulent phage is now changed to lytic/virulent phage.
• The single strand DNA of f×174 or coliphage fd is known as plus strand. It forms its complementary or negative strand. The double strand or replicative DNA takes over the metabolic machinery of the host to synthesize plus strands DNAs and protein for assembly of new phages.

Pinocytic  reproduction

• The whole virus enters the host cell except the envelope.
• It is quite common in RNA viruses, which are of two types: RNA-RNA virus and RNA-DNA virus.
• In RNA-RNA viruses, DNA has no role in their multiplication.
• After entering the host cell, the viruses produce enzyme replicase, which helps in producing more genetic RNA over the template of the parent RNA genome.
• The latter also produces mRNAs for synthesis of viral proteins.

• RNA­-DNA viruses are also called retroviruses (Temin, 1970), e.g., Tumor/Cancer viruses, HIV. The viruses possess enzyme reverse transcriptase (Temin and Baltimore, 1972; in Rous Sarcoma Virus or RSV of Mouse). The enzyme builds DNA over RNA genome. The phenomenon is called reverse transcription or teminism.
• The copy DNA (cDNA) builds its complementary strand. The double strand copy DNA attaches to host DNA/chromosome and is now called provirus.
• Application of bacteriophages are:
  • Studying viral infection mechanism.
  • Control of certain bacterial diseases.
  • Purity of Holy Ganges is due to presence of bacteriophages.

Viroids :

• Viroids  are  sub­viral  infectious  agents, which contain only very low molecular weight RNA and not a protein coat. Viroids cause potato spindle tuber disease (PSTV), citrus exocortis, etc. The only human disease known to be caused by a viroid is hepatitis D.
• Interferons are protein molecules that prevent viral multiplication. These are produced by cells in mammals, rodents, birds, etc. and provide resistance against viruses.

Diseases caused by viruses

• Plant diseases caused by viruses are:
  • Tobacco mosaic disease
  • Leaf curl of papaya
  • Yellow vein mosaic of bhindi
  • Potato leaf roll
  • Vein bandings mosaic disease of potato
  • Grassy shoot of sugarcane
  • Bunchy top of banana
  • Tungro disease of rice
  • Tomato leaf curl
• Human diseases caused by viruses are:
  • Chickenpox: Varicella virus
  • Smallpox: Variola virus
  • Measles: Rubeola virus
  • Rabies: ssRNA (Rabies virus)
  • AIDS: A RV (Aids associated retrovirus)
  • Yellow fever: Transmitted by Aedes aegypti mosquito
  • Dengue fever: Transmitted by Aedes aegypti mosquito
  • Polio: Transmitted through food, water, contact
  • Hepatitis-B: Transmitted through contact and body fluid.

Transmission  of  viruses  occurs  by –

1. Polluted air, water, and foodstuff can facilitate the spread of viruses in both humans and animals. Flies and mosquitoes often act as carriers, serving as vectors for these viruses.
2. Vectors, such as sap-sucking insects like aphids and whiteflies, play a crucial role in transmitting virus diseases to plants.
3. Additionally, virus diseases can also be transmitted through stem cuttings, seeds, tubers, agricultural implements, and other means.

Disease  can  be  controlled  by 

1. Removal of diseased plants and plant parts.
2. Using disinfested seed.
3. Testing the germplasm and selecting virus-free seed.
4. Destruction of alternative hosts.
5. Destruction of insect vectors.
6. Practising crop rotation.
7. Growing disease-resistant varieties.

Virus : Important Invention

1. Viruses were discovered after Chamberland developed bacterial filters (1884).
2. Meyer (1886) described tobacco mosaic disease but he couldn’t isolate the causal organism. Further, he said that the disease is transmissible and infectious.
3. Russian botanist D. Ivanowski (1892) discovered the causal organisms of tobacco mosaic disease, and these causal organisms could pass through the filters which retained bacteria.
4. Beijerinck (1898) confirmed the earliest studies and named these organisms as “Contagium vivum fluidum”, living infectious fluid.
5. F.W.T’wort. (1915) and F.H.d’Herelle (1917) discovered certain viruses which infect bacteria or bacteria eaters, i.e., bacteriophages or phages.
6. Stanley (1935) crystallized tobacco mosaic virus and stated that these crystals retain their infectivity for a long time if kept in bottles.
7. Bawden and Pirie (1936) firstly studied the chemical nature of viruses and stated that these are nucleoproteins.
8. Edward Jenner (1796) discovered vaccination against small pox.
9. Louis Pasteur (1880) discovered vaccination against rabies.
10. Loeffler and Frosch (1898) discovered the first animal pathogenic virus (Foot and mouth virus of cattle).
11. S. Luria, M. Delbruck, and Lwoff (1942-48) discovered the mechanism of replication in bacteriophages.
12. A. Harshey and M. Chase (1952) said that nucleic acids are infective and proteins are non-infective parts of a bacteriophage.
13. T. O. Diener (1971) discovered new infectious agents, which are still smaller than viruses.
14. Stanley B. Prusiner discovered certain infectious agents or slow viruses which contain only proteins. Prusiner got the Nobel prize for this work in 1997.
15. Alliac Issacs and Lindeman (1957) gave the term interferons to the chemical substances responsible for viral interference.
16. Mycophages, viruses infecting fungi, were discovered by Sinden (1957) in Agaricus bisporus. These have double-stranded RNA and are spherical or polygonal in shape.
17. A. Salk (1957) invented vaccination against Polio.
18. Lu Montagnier et. al. (1893); R. Gallo et. al. (1984) discovered the AIDS virus for HIV.
19. Safferman and Morris (1963) discovered cyanophages (viruses infecting cyanobacteria like Lynggya).

Precautions Against Virus Infections

1. Hygiene: Maintain good personal hygiene by regularly washing your hands with soap and water, especially before and after handling food, using the bathroom, or being in public spaces.
2. Sanitization: Use alcohol-based hand sanitizers to disinfect your hands when soap and water are not readily available.
3. Avoid Close Contact: Practice social distancing and avoid close contact with individuals who are sick or showing symptoms of illness.
4. Cover Mouth and Nose: Use a tissue or your elbow to cover your mouth and nose when coughing or sneezing, and dispose of tissues properly.
5. Stay Informed: Stay updated on the latest information and guidelines from health authorities and follow their recommendations.
6. Vaccination: Consider getting vaccinated to protect yourself from specific viral infections.
7. Avoid Contaminated Areas: Be cautious in areas with known virus outbreaks and follow any travel advisories and guidelines.
8. Proper Food Handling: Practice proper food handling and cooking techniques to prevent foodborne illnesses.
9. Regular Cleaning: Keep your living and working spaces clean by regularly sanitizing surfaces and frequently touched objects.
10. Seek Medical Attention: If you suspect you are infected with a virus or are feeling unwell, seek medical attention promptly and follow the advice of healthcare professionals.

Benefits Of Viruses

Are you shocked ? Yes there are benefits of viruses :

• Genetic Research: Viruses have been instrumental in genetic research, providing insights into gene therapy, gene editing, and molecular biology.
• Biotechnological Tools: Certain viruses have been employed as biotechnological tools in areas such as protein expression, vaccine development, and gene transfer.
• Ecological Balance: Viruses play a role in regulating ecosystems by controlling the populations of various organisms, thus contributing to ecological balance.
• Disease Targeting: Viruses have been engineered to target and destroy cancer cells, offering potential treatments for various types of cancer.
• Phage Therapy: Bacteriophages, viruses that infect and kill bacteria, have been explored as an alternative to antibiotics in combatting bacterial infections.
• Viral Predation: Viruses help in controlling the population sizes of various microorganisms, thereby preventing overgrowth and maintaining ecological equilibrium.
• Biological Control: Some viruses are harnessed for biological pest control, targeting and regulating pest populations in agricultural and environmental settings.
• Microbial Diversity: Viruses contribute to the diversity of microbial communities, influencing the composition and functioning of various ecosystems.
• Discovering New Species: Viral research has led to the discovery of novel viral species, expanding our understanding of microbial life and genetic diversity.
• Viral Interference: Research on viral interference has provided insights into antiviral defense mechanisms and immune responses in organisms.

Practice Questions

1. What is the origin of the term ‘virus’?
   

   The term ‘virus’ has been derived from the Latin word, which means poison, venom, or viscous fluid.

2. What are the main characteristics of viruses?
   

   Viruses are submicroscopic organisms generally less than 200 nm in size. They come in various shapes, are obligate parasites, contain RNA or DNA, and have a unique mode of multiplication. They possess both living and non-living characteristics, drawing a transitional line between the two.

3. How are viruses divided based on the type of nucleic acid they contain?
   

   Viruses are divided into two main groups: Deoxyvira (containing DNA) and Ribovira (containing RNA). Deoxyvira is further classified into three classes, while Ribovira is divided into two classes.

4. What is the structure of a virus?
   

   Structurally, viruses are composed of envelope, capsid, nucleoid, and occasionally one or two enzymes. Some viruses possess an outer thin loose covering called an envelope, while others are ‘naked’. The capsid is the protein coat that surrounds the central protein of the nucleoid and enzymes.

5. What are the different types of genetic material found in viruses?
   

   The genetic material of viruses can be double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), and single-stranded RNA (ssRNA). Each of these types is associated with various specific viruses.

6. What are the important inventions or discoveries related to viruses?
   

   Numerous discoveries and inventions have contributed to our understanding of viruses, including the development of bacterial filters, the crystallization of tobacco mosaic virus, the discovery of bacteriophages, and the identification of specific diseases and their causative agents.

7. What are the precautions individuals can take against virus infections?
   

   People can protect themselves from virus infections through practices such as maintaining good personal hygiene, sanitizing hands regularly, avoiding close contact with sick individuals, staying informed about health guidelines, seeking vaccination, and exercising caution in contaminated areas.

8. What are the benefits of viruses?
   

   Despite their negative impact on living organisms, viruses have proven to be beneficial in various aspects. They have contributed to genetic research, served as biotechnological tools, played a role in ecological balance, and offered potential treatments for diseases like cancer.

MCQ For NEET :

Must Read :

1. Chapter 1 : Systematics (Read in English // हिन्दी में पढ़ें)

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