Viruses – History, Characteristics , Structure, & Classification

Viruses – The Borderline of Livings and Non-living

Viruses are at the borderline of living and nonliving. Due to their crystalline nature, they are thought about as non-living. They are acellular i.e. they do not have cellular organization yet reveal some characters of living organisms (e.g. they possess DNA).

Viruses contain either RNA or DNA, typically enclosed in a protein coat. They reproduce just in living cells, where they trigger a number of diseases. They are not considered as organisms and thus are not added to the five-kingdom classification system. Prions and viroid are likewise acellular particles and are not included in the five-kingdom classification system.

History of Viruses

About a century back at the time of Louis Pasteur (1822-1895) and Robert Koch (1843-1910), the word “virus” was typically described as a poison related to disease and death. Today concept of virus is entirely different. Now viruses are acknowledged as particles of nucleic acid typically with a protein coat. They replicate in living cells and trigger lots of diseases such as influenzas, hepatitis, small pox, and AIDS.

Virology: The branch which deals with the study of viruses is called virology.


Etymology of Word Virus

The word “virus” is derived from the Latin word “venom” meaning harmful fluid. It can be defined as non-cellular infectious entities that consist of either RNA or DNA, normally framed in proteinaceous coat, and reproduce only in living cells. Viruses utilize the biosynthetic equipment of the host for its own synthesis and then move efficiently to other cells.

Discovery of Virus


In 1884, one of Pasteur’s partners, Charles Chamberland, discovered that bacteria cannot pass through porcelain filters, while agent responsible for rabies (a disease which is moved to human by bites of wild dogs, foxes, felines, bats, and other animals) can travel through these filters.

As in those days, the word virus was loosely used to describe any hazardous compound that caused disease, those hidden filterable agents of disease were referred to as filterable viruses. In 1892, Ivanowski discovered that the agent which caused tobacco mosaic disease was filterable. He obtained bacteria-free filtrate from the ground up infected plants and positioned it on healthy leaves of tobacco.

He observed that filtrate produced the disease in healthy plants. After that, the existence of comparable filter-passing, ultramicroscopic agents were seen in the victims of lots of diseases, consisting of foot and mouth disease (1898) and yellow fever (1901).

The filterable agents were first purified in 1935 when Stanley was successful in crystallizing the tobacco mosaic virus. Chemical analysis of these particles revealed that they included just nucleic acid and protein. This suggested that, unlike other kinds, viruses are of a simple chemical structure.

Characteristics of Viruses

Viruses are extremely little infectious agents, which can just be seen under an electron microscope. They range in size from 250 nanometer (nm) of poxviruses to the 20 nm of parvoviruses. They are 10 to 1000 times smaller than many bacteria, so they can pass through the pores of the filter, from which bacteria cannot pass. Viruses can not be grown on artificial media.

They can replicate only in animal and plant cells or in bacteria, where they reproduce by replication (a procedure by which lots of copies or replicas of virus are formed). Hence the infections are obligate intracellular parasites. Viruses do not have metabolic equipment for the synthesis of their own nucleic acid and protein.

They depend upon the host cell to carry out these essential functions. During replication in the host cells, viruses might cause illness and diseases. All viruses are typically resistant to broad range of readily available antibiotics such as penicillin, streptomycin and others.


Structure of Viruses

The complete, mature, and infectious particle is referred to as virion. The virions are made up of a central core of nucleic acid, either DNA or RNA, which is likewise called the genome and is surrounded by a protein coat, the capsid. Capsid gives a definite shape to virion. The capsid is made up of protein subunits referred to as capsomeres.

The number of capsomeres is the characteristics of a particular virus. For instance, 162 capsomeres exist in the capsid of the herpes virus and 252 in the capsid of adenovirus which causes some acute colds. In some animal viruses, the nucleocapsid (nucleic acid and capsid) is covered by another membrane stemmed from the host cell, the envelope. Non enveloped viruses are referred to as naked virions. Animal and plant viruses may be polyhedron (having many sides), helical (Spiral), enveloped, or complex.


Do you know???

The recently discovered (1983) and least understood microorganisms are the prions, which might be contagious proteins. Their nature is very questionable. They are composed of protein-only that contains the information that codes for their own replication. All other organisms contain their genetic details in a nucleic acid(DNA or RNA). Prions are accountable for mad cow infection and mystical brain infection Inman.

Classification of Viruses

Viruses are categorized by factors such as their core content, capsid structure, presence of outer envelope, and how mRNA is produced.

Viruses can also be classified by the style of their capsids. Isometric viruses have shapes that are approximately spherical, such as poliovirus or herpesviruses. Enveloped viruses have membranes surrounding capsids. Animal viruses, such as HIV, are often enveloped. Head and tail viruses infect bacteria and have a head that resembles icosahedral viruses and a tail shape like filamentous viruses. Capsids are categorized as naked icosahedral, enveloped icosahedrally, enveloped helical, naked helical, and complex. For instance, the tobacco mosaic virus has a naked helical capsid. The adenovirus has an icosahedral capsid.


Baltimore Classification

The most commonly-used system of virus classification was established by Nobel Prize-winning biologist David Baltimore in the early 1970s. The Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Viruses can contain double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), single-stranded RNA with positive polarity (ssRNA), ssRNA with a negative polarity, diploid (two copies) ssRNA, and partial dsDNA genomes. Positive polarity implies that the genomic RNA can serve directly as mRNA and a negative polarity implies that their sequence is complementary to the mRNA.



Viruses are non-cellular infectious entities that contain either RNA or DNA, enclosed in a protein coat and reproduce only in living cells. They lack their own synthetic machinery for their own nucleic acids and proteins.

They use the machinery of other livings for the production of their own nucleic acids and for their reproduction. They are called obligate intracellular parasites. The complete mature and infectious particle is known as a virion.

Virions are composed of genome and capsid. The genome is central core of nucleic acid either DNA or RNA. The capsid is the protein coat and gives shape to the virion. The capsid of virus is made of smaller units called capsomeres.

Retroviruses are single-stranded RNA viruses, having reverse transcriptase which can convert single-stranded RNA into double-stranded DNA.