Virus, History of viruses, Types and Morphology of virus


It is a non-cellular/ acellular infection agent, which contain DNA or RNA, cover with protein coat and reproduce only living host.

Discovery and History of Viruses:

The word virus come from Latin word venome which mean poisonous substance that cause disease in living things.

First time viruses were demonstrating by L. Pasture in 1884. According to pasture viruses consist of nucleic acid and protein coat.

In 1886 Mayer study the mosaic viruses in tobacco plants. According to Mayer they transfer from one host to another host.

In 1892 Lewandowsky prepare a tobacco extract from mosaic affected leave and pass through the filter to prevent the bacteria passage, the filtrate was able to infect the healthy tobacco leaf. Due to this reason viruses also known as filterable disease-causing viruses.

According to Beijernick viruses can reproduce that is common in living things. He further founds that these disease-causing agents multiply only host cell.

F. d Herelle and F. Twort:

According to him some virus only infects bacteria. So, these viruses know as bacteriophage (eater of bacteria). These viruses’ groups infect the animal, plants and bacteria.

In 1935 a biologist Stanley deeply study the structure of TMV. He crystallized it and then he found that its largely composed of protein.  Later study shows that small amount of protein in crystal and constant amount of RNA present in it. Study show that both DNA and RNA are combinedly not present in a virus. viruses are containing protein, nucleic acid, DNA or RNA.

General Characteristics of viruses:

It causes diseases in living host.

It contains DNA or RNA, protein and nucleic acid.

A Cellular in nature.

Act as a non-living without host and living in host.

They are small in nature, pass through filter paper.

Invisible in light microscope.

They do not multiply in culture medium to support growth of bacteria.

The DNA or RNA act as a viral genome. Those viruses contain RNA they have the permanent property of genetic information encoded.

Viruses cause disease in animals, bacteria and plants. Based on disease causing viruses divide into different types on the base of host.

General Types of viruses:    

  1. Animal virus
  2. Plant virus
  3. Gemini viruses
  4. Retroviruses

Animals Viruses:

These viruses cause infection in human and other animals. They consist of DNA core that protected with capsid.

Plant Viruses:

This type of viruses cause infection in ferns (plants). A ferns virus contains RNA core that was protect with protein coat. e.g. TMV

Gemini viruses:

These are also plants viruses but due to DNA structure it places into different type. It consists of single stand DNA type.


This is a special type of viruses that also cause disease in humans. in which RNA founded with genetic material. They cause ADIS in humans and animal.

Size and Shape of viruses

These range in size from 20-350nm. Most viruses are so small from 10-to300nm. Yellow fever, foot and mouth virus and poliomyelitis are very small in nature. Its diameter about 25 nm.

Size of viruses

Size of some viruses:

  1. Vaccinia virus:  200-250nm
  2. Mumps virus:   150 to 250nm
  3. Herpes simplex: 100-150nm
  4. Influenzas virus: 80-120nm
  5. Adenoviruses: 60-90nm
  6. Reovirus: 60-80nm
  7. Papaya virus: 40-60nm
  8. Polio virus:28nm
  9. Turnip and mosaic virus: 28nm
  10. Wound tumor: 55-60nm
  11. Alfalfa mosaic virus: 38-60nm
  12. Tobacco mosaic virus: 18-300nm
  13. Potato X virus: 10-500nm
  14. T2 phage: 65-210nm

Shape of viruses:

The viruses have verities of shape structure such as,

Rod like shape:  Alfalfa and mosaic viruses,

Elongate shape: Tobacco mosaic viruses

Rounded: Mumps: Herpes and influenza viruses

Bullet shape: Rabies virus

Tadpole: bacteriophage

Bacteriophage exist in different shape but most of them possess a polyhedral head and tail that help in infect the host.

Shape of viruses

General structure of a virus:

General a virus consists of capsid, capsomere and viral genome. A virus consists of a DNA, RNA filament forming core.


A DNA of virus protect by a layer of protein is known as capsid. Nucleic acid and capsid combinedly termed as nucleocapsid. Capsid is protein in nature, e.g. histidine in adenovirus, polyamines in bacteriophage type of virus. Lipids and carbohydrate are also part of virus composition such as phospholipids, neutral fats, fatty acid and cholesterol.


Capsid are built up multiple identical unite called capsomere. Due Highly symmetrical structure that can be crystallized help to study the structure through X-Ray crystallography microscope. Most of animal virus have additional lipoprotein envelope derived from the surface membrane of the host cell.

Viral Genome:

  • Bacteriophage and animal virus contain DNA.
  • Plant viruses contain RNA.
  • Both RNA and DNA never exist in same virion.
  • Virion that contain single copy of the nucleic acid are called haploid, that have two RNA single stand they known as diploid.

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