what is a virus

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Virus Structure
Source: Florida University

What is Virus

The viruses are the simplest forms of life which instead of having the cellular organization (viz. plasma membrane, cytoplasm, and nucleus) similar to bacteria, blue-green algae, plants and animal, contain definite genetically determined macromolecular organization, genetic material and characteristic mode of inheritance.

Who discovered the virus? When Virus Discovered?

The viruses were first discovered by Iwanowski (1892) as extremely small micro organisms.

Virus Structure

The viruses are sub-microscopic, acellular, forms of life and are much smaller than bacteria. They range in size from 20 to 80 m. Iwanowski showed that mosaic disease in the leaves of tobacco is caused by the ultramicroscopic agents that can pass even through the pores of fine filter paper which did not allow even bacterial cells to pass through.

The viruses possess a regular geometrical and macromolecular organization. Basically, all viruses consist of a core of only one type of nucleic acid (DNA/ RNA) which remains wrapped in a coat of protein called capsid. The capsid is composed of numerous protein molecules called capsomeres. The capsomeres determine the shape of the virus particle or viron, thus the viruses may contain three types of symmetry such as cubic (eg Bacteriophase x 174, Turnip yellow mosaic virus, etc.) Helical (eg Potato spindle tuber virus) have no capsid around the nucleic acid core or viral chromosomes. There are some highly specialized viruses such as influenza virus and mumps containing a membranous envelope around the capsid.

How does Virus work?

Viruses contain only one kind of nucleic acid as the hereditary material. The DNA viruses have single DNA molecule which may be either linear (having free and) or circular (having no free end) in shape, mostly, the linear DNA molecule is double stranded (eg P 22 bacteriophage). The circular DNA molecule may be either single stranded (eg x 174 bacteriophages) or double stranded as in most animal viruses. The RNA viruses are the only biological systems known in which RNA is the genetic material mostly, the RNA is present in its usual single stranded form (eg Plant viruses, Influenza etc.) but some viruses such as Retrovirus have a core of double stranded RNA similar in properties to DNA.

Why is virus host specific?

The viruses lack in necessary energy yielding and synthetic enzyme systems. Therefore, they cannot lead a free living mode of life for the performance of fundamental life activities such as reproduction, genetically determined structures and functions, they have to lead a parasitic mode of existence. All viruses are, therefore, invariably intercellular parasites of specific hosts which may be bacteria plants, or animals. The viruses may be classified into following 3 groups according to the type of host:

Types of Viruses

Bacterial viruses or Bacteriophages

Viruses that parasitize bacterial cells are called bacteriophages. At the time of infection of T4 bacteriophage, a phage becomes anchored to the bacterial cell wall by its tail fibers. An enzyme at the core of the tail of the phage digests the part of the bacterial cell wall so as to produce a hole and finally, the phage DNA is injected into the bacterial cell.

Plant Viruses

The plant viruses parasitize the plant cells and disturb their metabolism and cause severe disease in them. All plant viruses consist of ribonucleoproteins in their organization. Some of the important plant viruses are tobacco mosaic virus (TMV) potato virus, and turnip yellow viruses (TYV).

Animal Viruses

They infect the animal cells and cause different diseases in animals and human beings. Generally, they have a spherical shape and genetic material in the form of DNA and sometimes RNA. The capsid of animal viruses is surrounded by an envelope. Some of them are Small pox virus, Influenza virus, mumps, pea virus etc. Poliomyelitis is an extensively studied animal virus which has got RNA as its genetic material

Disease caused by Virus in Human

Disease Causative Pathogen Mode of transmission Incubation
Small Pox Variola Virus Direct contact (droplets), Indirected by infected articles 12 days
Chicken Pox Varicella Virus Direct contact (droplets), Indirect by Infected object 12-16 days
Common Cold Rhinovirus Contact 2-5 days
Influenza /Flu Orthomixo-virus Contact, Virus transmitted through discharge from respiratory tracts of persons infected with disease 1-2 days
Measles Measles virus (Paramyxo virus) Direct contact, virus transmitted through air by droplets during talking Coughing and sneezing 10-14 days
Mumps Mumps virus Direct contct, virus in Saliva and Secretin of nose invades salivary glands 12-21 days
Viral encephalitis Encephalitis virus (arbovirus) Some domestic animals reservoir of virus, transmitted by mosquito bite to man 4-21 days
Poliomylitis Poliovirus Contact, housflies, fleas, food and water 7-14 days
Rabies Rabies virus Bite of a mad (Rabid) dog 2-16 days
Dengue fever Dengue virus Mosquito (Aedes) bite 4-8 days
Herpes simplex Herpes virus simplex Contact, Saliva, stools, contaminated articles
Herpes Zoster Herpes virus zoster Contact droplets 7-14 days
Acquired immuno Deficiency Syndrome (AIDS) Human T-cell Leukemia Virus (HTLV-III); also called LAV Via blood and sperm among homosexuals, heterosexuals, intravenous drug users, hemophiliacs, promiscuous individuals and prosititutes 2 months to 10 years

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