INTRODUCTION

Viruses were first discovered after the development of a porcelain filter, called the Chamberlabd-
Pasteur filter,which could remove all bacteria visible in the microscope from any liquid sample. In 1886,
Adolph Meyer demonstrated that a disease of tobacco plants, tobacco mosaic disease, could be
transferred from a diseased plant to a healthy one via liquid plant extracts. In 1882, Dimitri Ivanowski
showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had
removed all viable bacteria from the extract. Still, it was many years before it was proven that these
“filterable” infectious agents were not simply very small bacteria, but were a new type of tiny, disease-
causing particle.

Virions, single virus particles, are very small, about 20–250 nanometers in diameter. These individual
virus particles are the infectious form of a virus outside the host cell. Unlike bacteria (which are about
100 times larger), we cannot see viruses with a light microscope, with the exception of some large
virions of the poxvirus family. It was not until the development of the electron microscope in the late
1930s that scientists got their first good view of the structure of the tobacco mosaic virus (TMV) and
other viruses. The surface structure of virions can be observed by both scanning and transmission
electron microscopy, whereas the internal structures of the virus can only be observed in images from a
transmission electron microscope. The use of these technologies has enabled the discovery of many
viruses of all types of living organisms. They were initially grouped by shared morphology. Later, groups
of viruses were classified by the type of nucleic acid they contained, DNA or RNA, and whether their
nucleic acid was single- or double-stranded. More recently, molecular analysis of viral replicative cycles
has further refined their classification.

What is Virus?

Viruses are obligatory intracellular parasite very small in size and have a simple, but effective structural
organization. they usually consist of just two or three categories of components and use the
components of the Host Cell to perform their "metabolism".

-Viral infections are the most common cause of human disease, it responsible for at least 60% of the
illness.

-Antibiotic have no effect on viruses, but antiviral drugs have been developed to treat life -threatening
infections.

-Vaccine can produce lifelong immunity and prevent viral infection.

-Viruses effect on all life forms, including human, animals, plants, fungus and bacteria.

-They damage or kill the cells that they infect

-A few viruses can produced cancer.

1.Genome

The viral genome (either DNA or RNA but not both) codes for the few proteins necessary for replication.
Some proteins are nonstructural, e.g.. Nucleic acid polymerases and some are structural, i.e. they
become incorporated and form part of the virion.

2.Capsid (outer protein coat )

- many protein subunits are assembled to form a tight "shell" (capsid made up of subunits called
capsomers) inside which the nucleic acid genome lodges for protection.

- The arrangement of capsomers give the virus structure its genomic symmetry

- The capsid together with its enclosed nucleic acid is called the nucleocapsid.

3.Viral envelop (not found on all viruses).

- Some viruses acquire an outer lipoprotein coat by "budding" through the host cell membranes and are
thus called Enveloped viruses.

- The envelop is important for interaction with cellular components during the process of infection and
replication.

- Enveloped viruses are more sensitive to heat, drying, detergent and lipid solvents such as alcohol and
ether than non enveloped virus.

STRUCTURE of VIRUS

-Viruses are vary in size 20 – 300 nm in diameter

- The shape of viruses are determined by the arrangement of the repeating subunits that form the
protein coat (capsid) of the virus.

- Most virus appear as spheres or rods in the electron microscope. In addition to these forms, bacterial
viruses can have very complex shapes

- Viruses have no metabolic enzymes and cannot generate their own energy.

- Viruses cannot synthesize their own proteins. For this they utilize host cell ribosomes during
replication.

- Unlike cells, viruses do not grow in size and mass leading to a division process. Rather viruses grow by
separate synthesis and assembly of their components resulting in production of mature viruses.
A virus like particles (VLPs)

An assembly of virus structural proteins that mimics the configuration of a real virus, except that it
contains no genetic material. If a person is vaccinated with VLPs then an immune response is generated
as if the immune system has been presented with a real virus.

Subviral particles

Viroids

- Are infectious agents composed exclusively of a single piece of circular single stranded RNA which has
some double-stranded regions. They do not contain a capsid.

-Viroids mainly cause plant diseases but have recently been reported to cause a human disease.

Prions

- An infectious particle that does not contain DNA or RNA. It is a protein particle. There are no genetic
material.

- Diseases are caused by the conversion of a normal host glycoprotein into an infectious form (e.g. Mad
Cow).

Classification of Virus

-Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA
genome surrounded by a protective, virus-coded protein coat.

-Viruses range from the structurally simple and small parvoviruses and picornaviruses to the large and
complex poxviruses and herpesviruses.

-Viruses are classified on the basis of morphology, chemical composition, and mode of replication.

-The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the
spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa
and from plants and fungi to bacteria.

The following properties have been used as a basis for the classification of viruses.

-Virion morphology, including size, shape, type of symmetry, presence or absence of peplomers, and
presence or absence of membranes.
 -Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome in kilobases (kb)
or kilobase pairs (kbp), strandedness (single or double), whether linear or circular, sense (positive,
negative, ambisense), segments (number, size), nucleotide sequence, G + C content, and presence of
special features (repetitive elements, isomerization, 5′-terminal cap, 5′-terminal covalently linked
protein, 3′-terminal poly(A) tract).

-Genome organization and replication, including gene order, number and position of open reading
frames, a strategy of replication (patterns of transcription, translation), and cellular sites (accumulation
of proteins, virion assembly, virion release).

-Virus protein properties, including number, size, and functional activities of structural and
nonstructural proteins, amino acid sequence, modifications (glycosylation, phosphorylation,
myristylation), and special functional activities (transcriptase, reverse transcriptase, neuraminidase,
fusion activities).

-Antigenic properties.

-Physicochemical properties of the virion, including molecular mass, buoyant density, pH stability,
thermal stability, and susceptibility to physical and chemical agents, especially ether and detergents.

-Biologic properties, including natural host range, mode of transmission, vector relationships,
pathogenicity, tissue tropisms, and pathology.

Classification On the Basis of Genetic Material Present

Viruses are small, nonliving parasites, which cannot replicate outside of a host cell.A virus consists of
genetic information — either DNA or RNA — coated by a protein.

Accordingly, they are classified as DNA viruses and RNA viruses.The nucleic acid may be single or
double stranded, circular or linear, segmented or unsegmented.

DNA viruses

As their name implies, DNA viruses use DNA as their genetic material.Some common examples of DNA
viruses are parvovirus, papillomavirus, and herpesvirus.DNA viruses can affect both humans and animals
and can range from causing benign symptoms to posing very serious health.

RNA viruses

The virus that possesses RNA as genetic material are called RNA viruses.Rotavirus, polio virus, yellow
fever virus, dengue virus, hepatitis C virus, measles virus, rabies virus, influenza virus and Ebola virus are
examples of RNA virus.

DNA-RNA viruses
The RNA tumor viruses called Leukoviruses and Rous’s viruses unusually contain both DNA and RNA as
genetic material.

On the basis of the presence of a number of strands:

Double-stranded DNA

It is found in pox viruses, the bacteriophages T2, T4, T6, T3, T7 and Lamda, herpes viruses, adenoviruses
etc.

Single-stranded DNA

It is found in bacteriophagesφ, X, 74 bacteriophages.

Double-stranded RNA

It has been found within viral capsid in the reoviruses of animals and in the wound tumour virus and rice
dwarf viruses of plants.

Single-stranded RNA

It is found in most of the RNA viruses eg: tobacco mosaic virus, influenza virus, poliomyelitis,
bacteriophage MS-2, Avian leukemia virus.

On the Basis of Presence of Envelope

The envelope is a lipid-containing membrane that surrounds some virus particles. It is acquired during
viral maturation by a budding process through a cellular membrane

Virus encoded glycoproteins are exposed on the surface of the envelope. These projections are called
peplomers.

Enveloped Virus

DNA viruses: Herpesviruses, Poxviruses, Hepadnaviruses

RNA viruses: Flavivirus, Toga virus, Coronavirus, Hepatitis D, Orthomyxovirus, Paramyxovirus,

Non-Enveloped Virus

DNA viruses- parvovirus, adenovirus and papovavirus.

RNA viruses- Picornavirus, Hepatitis A virus and Hepatitis E virus.

Naked icosahedral: Hepatitis A virus, polioviruses

Enveloped icosahedral: Epstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, HIV-1

Enveloped helical: Influenza viruses, mumps virus, measles virus, rabies virus

Naked helical: Tobacco mosaic virus

Complex with many proteins: some have combinations of icosahedral and helical capsid structures.
Herpesviruses, smallpox virus, hepatitis B virus, T4 bacteriophage.

On the Basis of Shapes of the Viruses

Most of the animal viruses are roughly spherical with some exceptions.

Rabies virus: Bullet shaped

Ebola virus: Filamentous shaped

Poxvirus: Brick shaped

Adenovirus: Space vehicle shaped

Classification of Virus on the Basis of Structure

Cubical virus: They are also known as icosahedral symmetry virus

Eg. Reo virus, Picorna virus.

Spiral virus: They are also known as helical symmetry virus

Eg. Paramyxovirus, orthomyxovirus.

Radial symmetry virus: eg. Bacteriophage.

Complex virus: eg. Pox virus.

On the Basis of the Type of Host

The virus can be classified on the basis of the type of host. They are:

Animal viruses

Plant viruses

Bacteriophage
Animal Viruses

The viruses which infect and live inside the animal cell including man are called animal viruses. Eg;
influenza virus, rabies virus, mumps virus, poliovirus etc. Their genetic material is RNA or DNA.

Plant Viruses

The viruses that infect plants are called plant viruses. Their genetic material is RNA which remains
enclosed in the protein coat. Some plant viruses are tobacco mosaic virus, potato virus, beet yellow
virus and turnip yellow virus etc.

Bacteriophages

Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as
genetic material. There are many varieties of bacteriophages. Usually, each kind of bacteriophage will
attack only one species or only one strain of bacteria.

Classification of Virus on the Basis of Mode of Transmission

Virus transmitted through respiratory route:eg, Swine flu, Rhino virus

Virus transmitted through faeco-oral route:eg. Hepatitis A virus, Polio virus, Rota virus

Virus transmitted through sexual contacts:eg. Retro virus

Virus transmitted through blood transfusion:eg. Hepatitis B virus, HIV

Zoonotic virus:

Virus transmitted through biting of infected animals;eg. Rabies virus, Alpha virus, Flavi virus.

Classification of Virus on the Basis of Replication Properties and Site of Replication

Replication and assembly in cytoplasm of host:All RNA virus replicate and assemble in cytoplasm of host
cell except Influenza virus

Replication in nucleus and assembly in cytoplasm of host:Influenza virus, Pox virus

Replication and assembly in nucleus of host:All DNA viruses replicate and assemble in nucleus of host
cell except Pox virus.

Virus replication through ds DNA intermediate:All DNA virus, Retro virus and some tumor causing RNA
virus replicates through ds DNA as intermediates.
Virus replication through ss RNA intermediate:All RNA virus except Reo virus and tumor causing RNA
viruses.

Baltimore Classification of Viruses

The most commonly used system of virus classification was developed by Nobel Prize-winning biologist
David Baltimore in the early 1970s.

In addition to the differences in morphology and genetics mentioned above, the Baltimore classification
scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by
transcription in much the same way as with cellular DNA.

Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded
genomes into a dsDNA intermediate before transcription to mRNA can occur.

Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a
template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.

Group IV viruses have ssRNA as their genome with a positive polarity. Positive polarity means that the
genomic RNA can serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates, are
made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity
(complementary to the positive-stranded genomic RNA) are formed from these intermediates, which
may then serve as templates for the production of RNA with positive polarity, including both full-length
genomic RNA and shorter viral mRNAs.

Group V viruses contain ssRNA genomes with a negative polarity, meaning that their sequence is
complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of
the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly
to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the
production of the negative-stranded genome.

Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme
reverse transcriptase, to dsDNA; the dsDNA is then transported to the nucleus of the host cell and
inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was
integrated into the host genome.

Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are
also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.

Conclusion
In conclusion, the classification of viruses is a critical framework for understanding their structure,
behavior, and impact on various hosts and ecosystems. By examining their genetic material, replication
strategies, capsid symmetry, host range, and other criteria, scientists can effectively group viruses into
meaningful categories. The Baltimore classification system,in particular, provides a robust method for
categorizing viruses based on their genome type and replication mechanism, enabling clear distinctions
between different virus families.

This classification aids in advancing scientific research, improving diagnostics, guiding vaccine and
antiviral developments, and controlling viral outbreaks. For instance, knowledge of the genetic structure
and replication pathways of viruses like SARS-CoV-2 and HIV has been instrumental in creating effective
therapeutic solutions. Despite the challenges posed by emerging viruses and mutations, the ongoing
study and refinement of viral classification systems remain pivotal in combating viral diseases and
understanding the complex interplay between viruses and their hosts.

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