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Today we will discuss about the Virus An Enigma to Life

Virus An Enigma to Life


  1. It is the bridge between living and non-living
  2. It is living when it is present inside the body of the host and non-living when it is outside the body of the host
  1. It can be crystallised
  2. It may contain DNA and RNA as a genetic material
  3. Pox-virus, Maesles-virus, etc .Define virus.

Virus is defined as ultramicroscopic, acellular, antigenic, obligatory parasitic, infectious particle containing only one type of nucleic acid, either DNA or RNA as their genome and having the power to multiply only within the specific living host cell, and it is regarded as intermediate between inanimate and animate object.

  1. Mention the salient features of Virus.
    Visible only under electron microscope, a kind of organism lying in between living and nonliving.
    b. Absence of cytoplasm, hence ‘noncellular’.
    c. Contains only one type of nucleic acid – DNA or RNA.
    d. They are infectious, obligatory parasites.
  2. Why virus is called intermediate between nonliving and living?
    Ans. Virus shows different phases:
    I) Extracellular phase – Virus behaves completely as inanimate object and can be converted into crystals.
    Nonliving characters of virus:
    a) Virus molecules can be converted to crystals.
    b) Absence of cell membrane and cytoplasm.
    c) Unresponsive to external stimuli.
    d) Remains inactive outside the host cell etc.II) Intracellular phase – Viruses behave like animate objects and synthesize their structural material, and multiply with the help of host enzymes.
    Living characters of virus:
    a) Presence of DNA or RNA and related proteins.
    b) Capable of reproducing within host cell.
    c) Cell division takes place by binary fission.
  3. Explain why viruses are called acellular.Ans. Viruses are called acellular because of the following reasons:
    a) Size of virus is 100 Angstrom – 200 Angstrom whereas a minimum space of 500 Angstrom is necessary for different cellular functioning.
    b) They have no power to grow independently.
    c) They have no power to work independently.
    d) Enzymes, cytoplasm and other cellular organelles are totally absent.
  4. Who first proposed the name Bacteriophage?
    Ans. Felix D’ Herelle
  5. What are the following?
    a. Cyanophage
    b. Mycophage
    c. Bacteriophage
    a) Cyanophage: Virus which attacks and kills blue green algae (Cyanophycean cells).
    b) Mycophage: Virus which attacks and kills fungi is called Mycophage.
    c) Bacteriophage: Virus which attacks bacteria and multiplies within the growing bacteria and finally kills them with bacterial cell lysis.
  6. Name the virus which was discovered first.
    Ans. Tobacco Mosaic Virus8. Describe the reproduction of a Bacteriophage. Does it differ from viral replication at any stage?
    Ans. Stages:
    I) Adsorption
    II) Penetration
    III) Eclipse phase
    IV) Maturation
    V) Release of virus
    [Explain the stages briefly from any standard textbook]

In viral replication, the virus undergoes uncoating (stripping the virus of its outer layers and capsid so that nucleic acid is released) after the stage of penetration. But bacteriophage does not undergo this process during reproduction.

  1. What is ‘eclipse phase’?
    Ans. It is the third phase in the process of viral replication during which the synthesis of components of progeny virus takes place. During this phase, the virus seems to disappear i.e. from the stage of penetration till the appearance of daughter virions, the virus can not be demonstrated inside the host cell.
  2. What is ‘lysozyme’?
    Ans. It is the enzyme secreted by the tail fibres of bacteriophage during the phase of penetration. It helps to form a hole over the host cell surface and through this hole the genetic materials pass into the host cell.
  3. What are the types of life cycles exhibited by a Bacteriophage?
    a) Virulent or lytic cycle
    b) Temperate or lysogenic cycle
  4. What do you mean by the following?
    a) Lysogenic conversion
    b) Transduction
    a) Lysogenic conversion: The process by which the prophage (phage nucleic acid which is integrated in bacterial chromosome) confers certain new properties on the lysogenic bacterium is called lysogenic conversion.
  5. b) Transduction: The process by which the bacteriophage acts as a carrier of genes from one bacterium to the other is called transduction. Transduction is of two types like Restricted transduction and Generalized transduction.
  6. Discuss briefly the importance of viruses.
    a) Origin of life: Viruses are intermediate between living and nonliving organisms and throw some definite light on the origin of life in the Earth.
    b) Interferon development: Viruses help in the generation of interferon, that is an antiviral protein formed in animals, helping in the destruction of those specific animal viruses against which the interferon is produced.
    c) Production of vaccine: The vaccination process is an effective method of producing immunity against viral disease. Viruses are cultured in their host cells, the Myxovirus, Vaccinia viruses are grown in the chicken embryos, and the vaccine is prepared by high speed centrifugation of the amniotic fluid. The polio-vaccine is produced from the kidney cell culture of Rhesus monkey and purified in the same way.
    d) Biological control: Control of some pathogenic bacteria can be done by bacteriophage.
    e) Genetic Engineering: Viruses act as vectors in the transmission of genes in the development of genetic engineering techniques in the field of medicine and molecular biology.
    f) Viruses also have negative importance because they induce disease in plants (TMV) and animals like Hepatitis, Influenza, Herpes, some of which are highly contagious and cause epidemics, some may be virulent enough to cause death.
  7. What are Lipovirus?
    Ans. Lipid-enveloped virus is called lipovirus. In these viruses, the envelope situated outside the capsid is composed together of viral protein and lipid derived from the host cell. The existence of lipids keeps the envelope loose and flexible.
  8. What do you mean by the following terms?
    a) Virion
    b) Viroid
    c) Elementary bodies
    d) Cytopathic effect of virus
    e) Prion
    f) Inclusion bodyAns.
    a. Virion: The inert phase of virus when it is present outside the host’s body, contains DNA or RNA.
  9. Viroid: These are ‘subviral’ or ‘virus like’ agents characterized by the apparent absence of an extracellular dormant phase and by a genome much smaller than those of known virus. The infective agent is a protein free, low molecular weight RNA, resistant to heat and organic solvents but sensitive to nucleases.
  10. Elementary bodies: Some of the larger virus (eg. pox virus) can be seen under the light microscope when suitably stained. The virus particle seen in this manner are called elementary bodies.
  11. Cytopathic effect of virus: Many viruses cause morphological changes in culture cells in which they grow. These changes can be readily observed by microscopical examination of the cultures. These changes are known as cytopathic effects. Eg. Measles virus produces syncytium formation.
  12. Prion: It is an unconventional virus like agent, without any detectable nucleic acid, resistant to heat, uv rays and nucleases and sensitive to proteases. They are proteinaceous infectious particles. Eg. Causative agent of Scrapie Kuru.
  13. d) Inclusion body: These are structures with distinct size, shape, location and staining properties that can be demonstrated in virus infected cells under the light microscope. Negri bodies in brain cells of animals infected with Rabies.
  14. What is interferon? What are its biological effects? What is anitviral compounds?
    Ans. Interferons are a family of host-coded proteins produced by cells (fibroblasts, lymphocytes, leucocytes) on induction by viral or nonviral inducers.

Biological effects are:
I) Antiviral effects: Induction of resistance to infection.
II) Antimicrobial effects: Resistance to intracellular infections (eg. Toxoplasma, Malaria)
III) Cellular effects: Inhibition of cell growth and proliferation of DNA and protein synthesis.
IV) Immunoregulatory effects: Modulation of antibody formation, activation of suppressor T cell etc.


The antiviral compounds are interferon and zovirax

17. Classify viruses on the basis of their replication mechanism.
Class I : Fully double stranded DNA
Class 2 : Single stranded DNA
Class 3 : Double stranded RNA
Class 4 : Single stranded RNA with positive strand
Class 5 : Single stranded RNA with negative strand
Class 6 : Single stranded RNA with reverse transcriptase

  1. What is the full form of ELISA?

Ans.  Enzyme linked immunosorbent assay


  1. Name two viruses with both RNA and DNA.

Ans.  The cancer viruses the Semian virus and Rous-sarcoma virus contain RNA outside host’s body but within the host, they synthesize DNA with the help of reverse transcriptase enzyme.

  1. Why Chlamydia psittasi is not a virus techinically?

Ans.  The Chlamydia psittasi is not a virus because it contains both DNA and RNA at the sametime.

(previously it was known as largest animal virus attacking the parrot )


  1. What are the smallest and largest virus?

Ans.  The smallest virus is Rhino virus (10 nm)

The largest virus is Closter virus (plant virus) and Lymphogranuloma venereum (300-400 nm) (animal virus)


  1. Name an animal virus with RNA

Ans.  Poliomyelitis virus


  1. Name a single stranded DNA and double stranded RNA virus.

Ans.  Coliphage fx 174 and Reo-virus.


  1. What is the disposable microsyringe stage?

Ans.  The stage in which the capsid remain outside the bacterial cell and the DNA is injected inside the bacterial cell is called disposable microsyringe stage.


  1. What is capsomere?

Ans.  The unit of capsid that makes up the covering of virus, made of protein molecules.


  1. What is eclipse phase?

Ans.  It is the phase where the DNA/RNA enters into the bacterial cell, after which the bacterial metabolism becomes totally paralyzed and the viral DNA takes complete metabolic control over the bacterial cell.


  1. What is peplomers?

Ans. The structural units of the lipid coating of lipovirus (analogous to capsomere).


  1. What is provirus?

Ans.  The attachment of phage DNA (intracellular virion) with the bacterial DNA.


  1. Name enzyme secreted by virus?

Ans. Lysozyme, secreted from the tail fibres of bacteriophage which eats into the bacterial cell wall and cause a hole for the virus to inject in the nucleic acid.


  1. Why virus cannot be grown in synthetic cultures?

Ans.  As they are obligatory parasites, they cannot survive ouside.  They constantly require a host.


  1. Give examples of Virus containing (4×1)
    a) Single stranded DNA
    b) Double stranded DNA
    c) Single stranded RNA
    d) Double stranded RNA
    a) Parvovirus
    b) Herpes virus, Poxvirus etc
    c) Rhabdovirus
    d) Reovirus
  2. What is retrovirus? Give two examples. (2+2)
    Ans. Retrovirus is RNA virus containing reverse transcriptase enzyme, which prepares a DNA copy of the RNA genome of the host cell.
    Examples: Human immunodeficiency virus (HIV) and Human T cell leukaemia virus (HTLV)

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Hi viewers My name is Partha Pratim Goswami, (PpG) I am a teacher by profession. I have been teaching biology, chemistry and physics, sometimes mathematics also for last sixteen years. I would like to share my entire experience with all the viewers across the world through YouTube, which I think is an amazing platform to share all my experiences.

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