Structure, Characteristics and classification


  • Virus is a parasite of sub-microscopic level on all the organisms. They infect all type of life forms like plants, animals, bacteria.
  • They are ambiguous in nature i.e. weather living or non-living, they are non-living in free-state but behave like living organism in a host.
  • In 1892  Dmitri Ivanovsky’s described about virus when he was working on bacterial pathogen infecting tobacco plants.
  • When virus invades a host organism it forced to replicate rapidly to produce thousands of copies of the virus. In contrast when virus is not inside a host or in the process of infecting it, virus exists in independent form known as ‘virions’ consists of a genetic material (RNA or DNA), a protein coat – capsid and the outer envelope of lipids.
  • Virus can be of various shapes ranging from helical to the icosahedral form.
  • Virus are much smaller than bacteria, size of the most virus that have been studied is between 20-300 nm.
  • They contain only one type of nucleic acid at a time either DNA or RNA but never both.
  • Virus are responsible for various infectious diseases in plant and animals rabies, AIDS (HIV), avian influenza, Ebola virus disease. Transmission of viral disease is through ‘vector’ (diseases bearing organism).


Virus shows a wide variety of shape and size, a complete virus particle know as virions contain nucleic acids surrounded by the protein coat capsid, capsid is made up of small subunits of protein knows as capsomers and an outer layer called envelope made up of lipid which is derived from host cell membrane.

The main morphological structures are:


  • Composed of single type of capsomeres stacked around the central axis of helical structure having central cavity results in the formation of rod shape structure which may be short and rigid or long and flexible
  • Genetic material (typically ssRNA or ssDNA) bound to the protein helix by the interaction between the negatively charged nucleic acid and the positively charged capsomer protein.

Example: tobacco mosaic virus


  • Most animal viruses exhibit icosahedral symmetry. The minimum no of triangular faces is three which gives rise to the 60 more capsomers. Rotavirus has more than 60 capsomers. Regular icosahedron is the optimum way to form closed shell symmetry with identical subunits.
  • Capsomers attaches with 5 other capsomers at the apices called pentons and on the triangular faces attaches with 6 capsomers called hexon.
  • Pentons and hexon may be of same protein or may be of other proteins.


  • Elongation of icosahedron along the fivefold axis, it is common in bacteriophage and composed of a cap with cylinder structure.


  • In some species, the virus modify their cell membranes to envelope them self it may be either the outer membrane of the host or the internal nuclear membrane or endoplasmic reticulum making a outer lipid bilayer known as viral envelope.
  • The infectivity are depend on the envelope of the virus, the membrane i
  • Has protein coded by the viral and host genome and lipid membrane with any carbohydrates originate by the host.


  • Non- cellular organism enclosed by a protective envelope
  • Virus attaches to their host by the help of spikes
  • Having nucleic acid (DNA and RNA) in the core which is surrounded by the protein coat
  • Considered as both living and non-living i.e. inactive when present outside the host and became active within the host cells
  • Virus uses host mechanism and enzymes to reproduce itself.

Classification :

On the basis of genetic material:

DNA virus:

  • DNA as genetic material
  • They attack on both humans and animals
  • Example: papillomavirus, parovirus and herpesvirus

RNA virus:

  • RNA as genetic material
  • Example: polio virus, ebola virus, hepatitis C virus

DNA-RNA viruses:

  • Having both DNA and RNA as genetic material
  • Example: leukoviruses, rous’s viruses

On the presence of number of strands:

Double-stranded DNAherpes viruses, adenoviruses
Single-stranded DNAbacteriophagesφ, X, 74 bacteriophages
Single-stranded RNAinfluenza virus, poliomyelitis, bacteriophage MS-2
Double-stranded RNArice dwarf viruses, wound tumour virus

On the basis of envelope:


DNA viruses:  poxviruses, herpesviruses

RNA viruses: toga virus, coronavirus


DNA viruses: adenovirus, papovirus

RNA viruses: hepatitis A and E virus

On the basis of capsid structure:

Naked icosahedralPoliovirus, hepatitis A virus
Enveloped icosahedralRubella virus, HIV-1
Naked helicalTMV
Enveloped helicalMumps virus, measles virus
ComplexSmallpox virus, hepatitis B virus

On the basis of shape:

Space vehicle shapedAdenovirus
Filamentous shapedEbola virus
Brick shapedPoxvirus
Bullet shapedRabies virus

On the basis of type of host:

Animal viruses:

  • Viruses infect and live inside the animal cell
  • Example: rabies virus, mumps virus

Plant viruses:

  • Their genetic material is RNA remains enclosed in the protein coat and infect plants
  • Example: potato virus, TMV, turnip yellow viruses


  • Virus infect bacteria are known as bacteriophage, contain DNA as genetic material

On the basis of mode of transmission:

Transmitted throughExample
Respiratory routeRhino virus, swine flu
Faeco-oral routePolio virus, rota virus
Blood transfusionHIV, hepatitis B virus
Sexual contactRetro virus
Zoonotic virusAlpha virus, flavi virus

Baltimore Classification:

  • In early 1970s Nobel prize winner David Baltimore developed the most commonly used virus classification system
  • Baltimore focus on how mRNA is produced during replication and classify virus into various groups

Group I: mRNA is produced as the same way as their cellular DNA by transcription, contain ssDNA as their genetic material

Group II: convert their ss genome dsDNA before transcription of mRNA occur, have ssDNA as genetic material

Group III: uses RNA dependent RNA polymerase to generate mRNA from the one of the strand used as template, genome is dsRNA.

Group IV: genetic material is ssRNA . Genomic RNA is with positive polarity means that is directly serve as mRNA

Multiple full length RNA strand with negative polarity are formed from the intermediate of dsRNA (replicative intermediates) made in the genomic copying process.

These serve as template form the production of positive polarity RNA

Group V: contain ssRNA with negative polarity; means that sequence is complementary to mRNA. Negative-strand is converted into mRna

Group VI: virus have two copies of genome i.e. ssRNA  conveted using reverse transcriptase enzyme to dsDNA

dsDNA transported to the host’s nucleus in inserted in its genome viral DNA is produced by transcription with host DNA

group VII: genetic material is dsDNA which make ssRNA as intermediate acts as mRNA and also converted back to dsDNA  by reverse transcriptase enzyme.

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