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Renin Angiotensin Mechanism.

Viruses: Introduction & Classification.

Introduction:

  • A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism, having the capacity to infect all types of life forms, from animals and plants to microorganisms.

  • It has the smallest size in the microorganisms 20-300nm.

  • They lack essential metabolic machinery to synthesize their own essential proteins hence has to depend on the hosts.

  • Viruses hence are called obligate intracellular parasites.

  • The viral genetic material is either RNA or DNA but not both.

  • This genetic material is enclosed in a specialized protein coat when the virus is outside of the host cell.

  • The protein coat plays a vital role in the entry of viruses inside the host cell.

  • The structurally complete and infectious virus is called “Virion”.

  • As they lack machinery of bacterias they are insensitive to antibacterial antibiotics.

  • Viruses cause a number of diseases in all animals, in humans they cause many diseases ranging from common cold to AIDS, COVID-19, Ebola etc.


  • Viruses only multiply in specific species and thus they are divided into three major classes as follows,

    • Bacterial Viruses (Phagineae), DNA, infect bacteria.

    • Plant Viruses (Phytophagineae),  RNA, infect higher plants.

    • Animal Viruses (Zoophagineae), DNA, RNA, infect humans and other animals like dogs, parrots, horses, etc.

Classification of Viruses:

  • Viruses are classified on the basis of morphology, chemical composition, and mode of replication.

  • Viruses are classified on the basis of physicochemical properties discussed as follows,

A) Primary Characteristics:

  1. Chemical nature of nucleic acid: DNA or RNA. single or double stranded or segmented genome, (+) or (-) sense, Mol. wt.

  2. Structure of Virion: Helical, icosahedral or complex, naked or enveloped, complexity, diameter of nucleocapsids for helical viruses.

  3. Site of replication: Nucleus or cytoplasm.

B) Secondary CHaracteristics:

  1. Host Range: Host species, specific cell or tissues.

  2. Specific surface structures: e.g. antigenic properties.

  3. Mode of transmission: e.g. feces, air, water etc.

  1. Classification Based on Genetic Material:

  • Accordingly, they are classified as DNA viruses and RNA viruses.

  • The nucleic acid may be single or double stranded, circular or linear, segmented or unsegmented.

  1. DNA viruses:

  • As the name indicates, DNA viruses use DNA as genetic material.

  • e.g. parvovirus, papillomavirus, and herpesvirus.

  • Can affect both humans and animals.

  1. RNA viruses

  • As the name indicates, RNA viruses use RNA as genetic material.

  • e.g. Rotavirus, polio virus, yellow fever virus, dengue virus, hepatitis C virus, measles virus, rabies virus, influenza virus and Ebola virus are examples of RNA virus.

  1. DNA-RNA viruses

  • These are considered as exceptions as they contain both DNA and RNA as genetic material.

  • RNA tumor viruses are called Leukoviruses and Rous’s viruses.

2) On the Basis of the Type of Host

  • Viruses can also be classified on the basis of the host as follows,

    • Animal viruses

    • Plant viruses

    • Bacteriophage

  1. Animal Viruses:

  • The viruses which infect and live inside the animal cell including man are called animal viruses. Eg; influenza virus, rabies virus, mumps virus, poliovirus etc.

  • Their genetic material is RNA or DNA.

  1. Plant Viruses:

  • The viruses that infect plants are called plant viruses. 

  • Their genetic material is RNA which remains enclosed in the protein coat. 

  • e.g.  tobacco mosaic virus, potato virus, beet yellow virus and turnip yellow virus etc.

  1. Bacteriophages:

  • Viruses which infect bacterial cells are known as bacteriophage. 

  • They contain DNA as genetic material.

  • There are many varieties of bacteriophages.

  •  Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria.

3) Classification of Virus on the Basis of Mode of Transmission.

  1. Virus transmitted through respiratory route:

Eg, Swine flu, Rhinovirus      

  1. Virus transmitted through faeco-oral route:

Eg. Hepatitis A virus, Polio virus, Rota virus

  1. Virus transmitted through sexual contacts:

Eg. Retrovirus

  1. Virus transmitted through blood transfusion:

Eg. Hepatitis B virus, HIV

  1. Zoonotic virus: Virus transmitted through biting of infected animals;

Eg. Rabies virus, Alpha virus, Flavivirus

4) Baltimore Classification.

  • The most commonly used system of virus classification was developed by Nobel Prize-winning biologist David Baltimore in the early 1970s.

  • In addition to the differences in morphology and genetics mentioned above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

  • Group I :

    • viruses contain double-stranded DNA (dsDNA) as their genome. 

    • Their mRNA is produced by transcription in much the same way as with cellular DNA.

  • Group II:

    • viruses have single-stranded DNA (ssDNA) as their genome. 

    • They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur.

  • Group III:

    • viruses use dsRNA as their genome. 

    • The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.

  • Group IV: 

    • viruses have ssRNA as their genome with a positive polarity. 

    • Positive polarity means that the genomic RNA can serve directly as mRNA. 

    • Intermediates of dsRNA, called replicative intermediates, are made in the process of copying the genomic RNA. 

  • Group V: 

    • viruses contain ssRNA genomes with a negative polarity, meaning that their sequence is complementary to the mRNA. 

    • As with Group IV viruses, dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. 

    • Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.

  • Group VI: 

    • viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse transcriptase, to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome. 

    • Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome.

  • Group VII: 

    • viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.
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