Viruses are ultramicroscopic, acellular parasites that can only replicate inside host cells. They are much smaller than bacteria and can only be seen with electron microscopes. Viruses infect all forms of life from animals and plants to bacteria and archaea. They contain either DNA or RNA and have protein capsids that protect their genetic material. Viruses are obligate intracellular parasites as they cannot carry out metabolism and require host cells to replicate. Examples of viral diseases include influenza, hepatitis, AIDS, and the common cold.

1. Viruses: Introduction and general
characters
Dr. Dhole N. A.
Department of Botany,
Digambarrao Bindu ACS College, Bhokar

2. Introduction:
• The branch of science that deals with the study of viruses is called
virology.
• The scientists who work in this branch are called virologists.
• Viruses are ultramicroscopic, acellular, infectious, nucleoproteinous,
obligate parasites that can only replicate inside a host cell.
• Viruses are much smaller than bacteria, their size is variable and
which can cross the bacterial filter.
• The extracellular forms of viruses are inert particles and are known as
virions.
• From a biological perspective, viruses cannot be classified either as
living organism or non-living.

3. On the basis of the host cell they infect,
• viruses infect to animals are called as zoophages (animal viruses),
• phytophages (plant viruses),
• phycophages (algal viruses),
• mycophages (fungal viruses),
• zymophages (yeast viruses),
• bacteriophages (bacterial viruses),
• cyanophages (infecting cyanobacteria),
• coliphages (infecting E. coli bacteria).

4. • Viruses are visible only under the electron microscope .
• The discovery of virus cannot be credited to any one scientist but it
was the contribution of many scientists.
• Louis Pastuer (1884): He coined the term ‘viruses’ (virus = poison)
which are smaller than bacteria. He first demonstrated that viral
diseases like rabies can be transmitted for one host to another.
• Adolf Mayer (1886): He first discovered the mosaic disease of
tobacco.
• D.J. Iwanowsky (1892)- He first discovered the Tobacco Mosaic Virus
causing mosaic disease of tobacco.
• W.M. Stanley (1935): He first isolated and crystallized TMV. For this
Stanley shared the 1946 Nobel Prize with Northrop and Sumner who
had earlier crystallized the enzyme urease for the first time. Stanley is
known as the ‘father of virology’.

5. General Characters of Viruses:
• Viruses are non-living, non-cellular structure.
• They do not have a cytoplasm or any kind of cellular organelles to
carry out any kind of metabolism.
• Viruses are able to infect all living forms including plants, animals,
human beings and even micro-organism.
• Viruses are obligate intracellular parasites they require a living cell or
organism for its multiplication.
• Viruses can be observed only under the electron microscope.
• They are 10 to 100 times smaller than bacteria and its size ranges
from 20 to 300 nm so these viruses can pass easily through
bacteriological filters. They are filterable.

6. • Size of viruses is measure with the help Ultrafiltration,
ultracentrifugation and electron microscopy.
• Viruses do not have any kind of cellular organization.
• Viruses have spikes which help the virus to attach on the host cell.
• It contains either DNA or RNA as a nuclear material.
• As we know viruses are an obligate intracellular parasite and lack the
essential enzyme for protein and nucleic acid synthesis. These viruses
depend on the host cell for its replication and multiplication.
• Viruses are unaffected by antibiotics, there are many differences
between micro-organism and viruses

7. • Viruses are of great concern in the field of medical microbiology
because they are responsible for various human diseases.
• Examples of diseases caused by viruses are Rabies, AIDS, Mumps,
Hepatitis, Influenza, Dengue, common cold and many more diseases
are caused due to viruses.
• Viron is an extracellular infectious particle of the virus.
• Viron contains essential nucleic acid which is protected by the protein
coat called as the capsid.
• The function of the capsid is to protect nucleic acid from nucleases
and other environmental factors.
• The capsid is made up of polypeptide molecules.

8. • Capsid shows two types of symmetry that are cubical or icosahedral
symmetry and helical symmetry.
• Icosahedral symmetry shows 12 vertices and 20 sides.
• The icosahedral contains two types of capsomers Pentons at the
vertices and hexons at the Facets or sides.
• Different viruses have different shapes, most of the animal viruses are
roughly spherical in shape, pox virus is brick shape, TMV is rod shape
etc.
• Viruses may be enveloped or non-enveloped.
• Envelop is made up of lipoproteins and is derived from host cell
membrane.

9. • Viruses require a living media like the embryonic egg, cell culture or
bacterial cells.
• Viruses are classified on the basis of morphology, chemical
composition, and mode of replication.

10. References:
• https://www.biologydiscussion.com/viruses/viruses-history-and-
features-of-viruses/5675
• https://byjus.com/biology/virus/
• https://open.oregonstate.education/generalmicrobiology/chapter/int
roduction-to-viruses/
• https://microbenotes.com/classification-of-virus/
• https://www.yourarticlelibrary.com/micro-biology/viruses-definition-
characteristics-and-other-details-with-figure-micro-biology/26672

11. Thank you