Virus : Definition, Characteristics and Classification
For proper survival of the plants and animals, they have to struggle against the environment, their different factors and other living organisms. Viruses are microscopic organisms and can be seen only under electron microscope.In nature, any living organism is affected by virus, then it will take a serious turn. Viruses are mysterious biological agents which do not show any sign of life in free-state and are seen to remain as non-living things. There are million species of virus in nature, among them, about 5000 species have been described in details. The term virus is derived from a Latin word, meaning slimy liquid or poison. This term was first given by Dutch microbiologist Martinus Beijerinck. The branch of science which deals with the virus is called Virology and the specialists of this branch are designated as Virologist.
Definition of Virus
Viruses are biological agents as they exhibit characters of both non-living and living organisms. Some definitions of viruses are given below:
Salient Features of Virus
Nature of Virus
Where They Occur
Characteristics of Virus
Virus shows the both living and non-living characteristics.
Living characteristics are:
The non-living characteristics are:
Size of virus
Virus particles are ultra-microscopic entities that can be seen only under electron microscope. They are very small and vary in size appreciably. The average diameter of a virus lies between 8-280 mµ. The foot and mouth disease virus of cattle is the smallest known virus which is nearly 8-12 mµ in size and the larger sized virus particles are noted in the vaccinia and Variola viruses (280-300 mµ).
The size of the tobacco leaf mosaic virus is 17 mµ. On the other hand, parrot fever virus (Psittacosis) Chlamydia psittaci is the largest among the viruses, which is a large as 450 mµ in diameter. According to Salle (1974), these are not virus. They can easily seen under a normal compound microscope. In this case, the larges virus will be Lymphogranuloma venereum (size: 300-400 mµ).
Shape of Virus
Virus body shows four different shapes such as spherical, rod-shaped, cuboidal and tadpole-like structure.
Spherical shaped virus: These type of virus are more or less round, like small golf ball such as influenza virus, Polio virus, Encephalitis virus, Tumor virus, etc. They show 18-150 mµ in diameter.
Rod shaped virus: This type of virus looks like small rod such as Tobacco mosaic virus (TMV), Mumps virus etc. They can grow 800 mµ in length and 15 mµ in diameter.
Cuboidal shaped virus: This type of virus is cubic shaped such as small pox viruses like Vaccinia and Variola, Canary pox, Herpes etc. Their size ranges between 210-305 mµ.
Tadpole shaped virus: This type of virus looks like spermatozoa or tadpole such as bacteriophase virus. They have head and tail in their body. The head measures 47-104 mµ and the tail measures 10-225 mµ.
Classification of Virus
On the basis of nucleic acid, virus is divided into two types such as:
On the basis of the virus host, they can be divided into:
On the basis on Genome structure, virus is classified into the following types:
Retroviruses, Rabies virus
Smallpox virus, Herpesviruses
Retroviruses, Rabies virus
Herpesviruses, smallpox virus
Herpesviruses, Rabies virus, retroviruses, smallpox virus
Many bacteriophages, Papillomaviruses
Genome with a single segment
Genome with multiple segments
On the basis on Capsid Structure, virus is classified into the following types:
Hepatitis A virus, polioviruses
Tobacco mosaic virus (TMV)
Yellow fever virus, Epstein-Barr virus, rubella virus, herpes simplex virus, HIV-1
Rabies virus, Influenza viruses, measles virus, mumps virus
Complex with many proteins; some have combinations of icosahedral and helical capsid structures
Hepatitis B virus, T4 bacteriophage, Herpesviruses, Smallpox virus
According to David Baltimore Virus is classified into following categories:
Herpesvirus (Herpes simplex)
Parvovirus (Canine parvovirus)
Rotavirus (Childhood gastroenteritis)
Positive Signed Single stranded RNA
Picornavirus (Common cold)
Negative Signed Single stranded RNA
Single stranded RNA viruses with Reverse Transcriptase Enzyme
HIV (Human immunodeficiency virus)
Double stranded DNA viruses with Reverse Transcriptase Enzyme
Hepadnavirus (Hepatitis B virus)
Advantages of Virus
Disadvantages of Virus
You may also read: Virus Structure