Virus definition in biology. Types and structure of viruses

virus definition
Virus (Source Wikimedia)

What is a virus? or Virus definition

In Latin, the word virus means poisonous or venomous fluid coined by Dmitri Ivanowsky (1892). He recognized certain microbes causing mosaic diseases of tobacco and are smaller than that of bacteria. These are very very minute submicroscopic biological entities. Thus virus definition is it does not have any cellular organelles but definitely has its own genetic material. For their multiplication, a it needs a host cell like bacteria, plants or animals.

The size of the virus varies between 30 to 300 nm depends upon the type of viruses. They have a specific geometric shape and filled with infected virus particles known as Virion

The general structure of the virus

A virus is mainly divided into three parts

  1. A nucleic acid genome
  2. Protein capsid
  3. Lipid envelop in case of animal viruses

A. Nucleic acid genome

The genome of viruses is composed of either single or double-stranded RNA or DNA. The genetic material is infectious in nature. The viruses which affect or infect the plants generally have single-stranded RNA. And the viruses which infect animals have either single or double-stranded RNA or DNA. Bacterial viruses have double-stranded DNA. The size of the genome varies from 5-10 kb to 100-200 kb.

Depends upon the nucleic acids present in the viruses they are further classified into DNA virus and RNA virus.

B. Protein capsid

The capsid is a protective coat of protein which wrapped or covers the nucleic acid genome. The capsid is made up of capsomeres which further having few monomers or structural units. 

The capsomere is made up of different shapes like prism, hexagonal, pentagonal, etc. depend upon the shape of capsomere in capsids viruses are further divided into following three broad categories

  1. Helical symmetry capsid
  2. Icosahedral symmetry capsid
  3. Complex symmetry capsid

1. Helical symmetry capsid

Helical symmetry capsid

Many rod-shaped or filamentous viruses have capsids with helical symmetry. Examples are tobacco mosaic viruses, influenza viruses, and bacteriophage M13.

2. Icosahedral symmetry capsid

Adenoviridae: Icosahedral Symmetry

Icosahedral means 20 sides, therefore, many viruses have a spherical, cubical or polygonal shape. Examples of icosahedral symmetry viruses are Turnip yellow mosaic virus having 32 capsomeres, polioviruses with 32 capsomeres, polyoma and papillomavirus both having 72 capsomeres.

3.Complex symmetry capsid

Complex symmetry capsid: Bacteriophage

As the name suggests the viruses with complex shapes because of the too large genome to packed inside a simply shaped capsid. Examples are poxviruses, T2 phage viruses, rabies virus, etc.

Some of the frequently asked questions

Name some common viruses?

Common cold virus, influenza virus and bronchitis virus, mumps virus these infect Humans. 

Some plant viruses are tobacco mosaic viruses and other types of mosaic viruses cause the leaf rolling, curling, yellowing and vein clearing, dwarf and stunted growth of the plants.

Can a virus be killed?

The simple medication of an antibiotic can not kill a virus there are some antiviral drugs also for certain types of viral infection. If viral infection occurs then it will take its time to cure until the body developed its immunity against it. One of the best ways to prevent the virus is vaccination.

How long does a virus last?

Depends upon the body’s immunity and the type of virus it may last from 2 days to 2 to 3 weeks.

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