[Biology] Viruses and Viroids

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WHAT IS VIRUS

  • About a century ago the word “virus” was generally referred to as a poison associated with disease and death.
  • The present status of virus is completely different.
  • Now viruses are known as particles of nucleic acid often with a protein coat.
  • Viruses are defined as non-cellular infectious entities which contain either RNA or DNA, normally encased in proteinaceous coat, and reproduce only in living cells.
  • They replicate in living organisms and are responsible of many diseases such as influenza, hepatitis, small pox and AIDS.
  • The branch of science which deals with the study of viruses is known as virology.

CHARACTERISTICS OF VIRUS

Viruses are infectious particles with both living and non-living characteristics.

LIVING CHARACTERISTRICS

  • Viruses possess genetic material in the form of DNA or RNA.
  • Viruses can replicate inside a host cell and show gene mutations similar to living organisms.
  • Viruses contain a protein coat as an essential feature within its structure.
  • Viruses are capable of causing various diseases in plants and animals.

NON-LIVING CHARACTERISTICS

  • Viruses are acellular and do not possess any cytoplasm or cellular organelles.
  • They do not carry out metabolism on their own and are completely depend on host cell’s metabolic machinery.
  • Viruses can be crystalized.
  • Viruses do not carry out respiration.
  • In-vitro culture of viruses is not possible.

STRUCTURE OF VIRUS

  • Viruses are acellular in nature and ultra-microscopic in size.
  • Structurally, they consist of two components;
    • Nucleoid
    • Capsid.
  • Nucleoid is the inner core of nucleic acid and may contain either DNA or RNA but not both.
  • Capsid is the outer protein coat encasing the nucleoid.
  • Shape of capsids varies from virus to virus.
  • The capsid is made from the proteins, the capsomeres, that are encoded by viral genes within their genome.
  • The number of capsomeres is an important feature of a particular virus.
  • For example:
  • Herpes virus has 162 capsomeres
  • Adenovirus has 252 capsomeres.

CLASSIFICATION OF VIRUS

Viruses are classified differently according to various aspects such as;

  • On the basis of type of nucleic acid
  • On the basis of type of host
  • On the basis of mode on nutrition

ON THE BASIS OF TYPE OF NUCLEIC ACID

On the basis of nucleic acid viruses are divided into two major types:

  1. DNA viruses
  2. RNA viruses

DNA VIRUSES

  • These are viruses containing DNA as its genetic material.
  • DNA virus are further classified as;
    • Single-stranded (ss) DNA virus
    • Double-stranded (ds) DNA virus

RNA VIRUSES

  • These are viruses containing RNA as its genetic material.

ON THE BASIS OF TYPE OF HOST

According to the type of host being infected, viruses are categorized into four major types

  1. Animal viruses
  2. Plant viruses
  3. Bacteriophage
  4. Insect viruses

ANIMAL VIRUSES

  • The viruses which infect, live and replicate inside the animal cell including man are known as animal viruses.
  • Their genetic material is either RNA or DNA.

FOR EXAMPLE

  • Influenza virus
  • Rabies virus
  • Mumps virus
  • Poliovirus

PLANT VIRUSES

  • The viruses that invade plant cells and cause infection in plants are called plant viruses.
  • Their genetic material is RNA which remains enclosed in the proteinaceous coat.

FOR EXAMPLE

  • Tobacco mosaic virus
  • Potato virus
  • Beet yellow virus
  • Turnip yellow virus

BACTERIOPHAGE

  • Viruses which infect bacterial cells are called as bacteriophage or bacteria-eaters.
  • They contain DNA as genetic material.
  • Bacteriophage are specific in nature.
  • A particular bacteriophage attacks only one specific species or only one strain of bacteria.

FOR EXAMPLE

  • Escherichia virus T4
  • Enterobacteria phage T2
  • M13 bacteriophage
  • Bacteriophage MS2

INSECT VIRUSES

Viruses which act as pathogens of many arthropod species

FOR EXAMPLE

  • Baculoviruses
  • Granuloviruses
  • Ascoviruses
  • Polydnaviruses
  • iridoviruses

ON THE BASIS OF MODE OF TRANSMISSION

AIR-BORNE VIRUS

virus transmission occurs through air via the respiratory route

FOR EXAMPLE

  • Swine flu
  • Rhino virus      

FECAL-ORAL ROUTE

virus transmission occurs through ingestion of fecally contaminated water or food or sexual malpractices.

FOR EXAMPLE

  • Hepatitis A virus
  • Polio virus
  • Rota virus

SEXUALLY TRANSMITTED

Virus transmission occurs through sexual contact with infected individual

FOR EXAMPLE

  • HIV
  • Retro virus

TRANSFUSION-TRANSMISSION

Virus transmission occurs through blood transfusion

FOR EXAMPLE

  • Hepatitis B virus
  • HIV

ZOONOTIC TRANSMISSION

Virus transmission occurs through contact with infected animals, birds or insects.

FOR EXAMPLE

  • Rabies virus
  • Alpha virus
  • Flavi virus

REPLICATION CYCLE OF VIRUS-BACTERIOPHAGE

  • Viruses cannot replicate on their own
  • They depend on host’s biosynthetic machinery to undergo replication.
  • In order to study the replication of viruses, we take bacteriophage as an example
  • Replication of bacteriophage in host involve following steps:
    • Attachment
    • Penetration and un-coating
    • Replication and assembly
    • Release

ATTACHMENT

  • The virus attaches to a particular receptor site on the host cell membrane through attachment proteins in the capsid
  • As a result of attachment, a week chemical union between virus and receptor site of the host takes place.
  • The attachment of virus and host is specific in nature; as only specific virus infect specific host.

PENETRATION AND UNCOATING

  • The tail of the bacteriophage releases the enzyme lysozyme
  • Lysozyme dissolves a part of the bacterial cell wall.
  • The tail sheath contracts and tail core is penetrated into the cell through cell wall and cell membrane.
  • The virus inserts its genetic material into the cell just as a syringe is used to inject vaccine.
  • The protein coat, which constitute the phage head and tail structure of virus, remains outside the cell.
  • However, many animal viruses enter the host cell as a whole. Once they enter the cell, the viral capsid is degraded and the viral nucleic acid is released, which is then available for replication and transcription.

REPLICATION AND ASSEMBLY

Replication in bacteriophage may occur in any of the two cycles:

  • Lytic cycle
  • Lysogenic cycle

LYTIC CYCLE

  • As soon as the virus enters the host cell, the viral nucleic acid gains the control of the host’s biosynthetic machinery.
  • It induces the host cell to synthesize necessary viral components (DNA, proteins), and starts multiplying.
  • Within no time of infection, new bacteriophages are formed in a great number.
  • As a result, bacterial cell bursts i.e. it undergoes lysis.
  • The phage which is responsible of the lysis of the host cell is known as lytic or virulent phage.

LYSOGENIC CYCLE

  • In this cycle, viral DNA becomes incorporated into the bacterial chromosome, instead of taking over the control of host’s machinery.
  • In this state the phage is called as prophage and this process is termed as lysogeny.
  • Under such condition, the bacterium continues to live and reproduce normally.
  • When viral DNA becomes a part of bacterial chromosome it passes to each daughter cell in all successive generations.
  • But sometimes, the viral DNA gets detached from the host’s chromosome and lytic cycle starts abruptly
  • This process is called induction.
  • The phage which is responsible for lysogeny is called temperate (lysogenic) phage

RELEASE

  • At last, the new viruses produced in the host organism are released
  • After the release, the new viruses start infecting adjacent cells and replication cycle is repeated.
  • Some viruses are released when the host cell dies.
  • Other viruses can leave infected cells by budding through the membrane.

SOME IMPORTANT VIRAL DISEASES

CORONAVIRUS

  • It is an extremely infectious disease which is caused by the coronavirus.
  • It causes flue like symptoms at first but may result in severe respiratory tract infections.
  • Every age group is susceptible to this disease; however, it is especially dangerous in individuals above 60 years of age or individuals with predisposing conditions.

SMALL POX

  • Smallpox is caused by pox viruses.
  • It is an ancient disease that is known to have caused epidemics in China.
  • Until the early twentieth century, small pox became a common disease throughout the world.
  • In small pox, raised fluid-filled vesicles are formed on the body which turn into pustules later on and form pitted scars, the pocks.
  • However, through Immunization and other control measures, smallpox has been eradicated from the world.

POLIOMYELITIS

  • Poliomyelitis is caused by polio virus and is found all over the world.
  • It may affect any age group but occurs mostly in children
  • The polioviruses are the smallest viruses to be known
  • They contain RNA as genetic material enclosed in spherical capsid

INFLUENZA

  • The causative virus is Influenza virus, which is an enveloped RNA virus.
  • Influenza is an epidemic in various parts of the world.
  • Influenza virus causes flue like symptoms.

HERPES

  • Herpes is caused by Herpes simplex virus.
  • This disease is characterized by herpetic lesions in the epithelial layers of ectodermal tissues.
  • Most commonly this disease affects the mouth, lips, and other skin sites.

AIDS

  • AIDS is short for Acquired immunodeficiency syndrome.
  • It is a chronic and potentially life-threatening disease caused by the human immunodeficiency virus (HIV).
  • HIV completely damages the immune system, thereby interfering with our body’s ability to fight infection and disease.

REFERENCES

  • Arihant’s handbook of biology. Biological classification. Page no: 7-23.
  • NCERT biology; textbook for class 11. Biological classification. Page no: 16-28.
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