This document defines viruses and summarizes their key characteristics and classification. It describes how viruses were first discovered through experiments filtering bacteria and plant extracts. Viruses are non-cellular particles that contain genetic material and invade living cells. They are smaller than bacteria, contain either DNA or RNA, and lack organelles. Viruses replicate only inside host cells and do not undergo binary fission. They have various structures depending on their nucleic acid arrangement and symmetry. Viruses are classified into groups based on their nucleic acids and ability to produce mRNA.

1. Rajendra Kr. Dangol
BSc biotech,sem II
Maitidevi,Ktm

2. Contents
Definition of viruses
Discovery of viruses
Concept of viruses
Characteristics of viruses
Structure of viruses
Symmetry of viruses
Classification of viruses

3. What are viruses ?
●
A virus is a
non-cellular
particle
made up of
genetic
material and
protein that
can invade
living cells.

4. Discovery of virusesn 1884 C. Chamber land, in Pasteur's lab, discovered that if you passed a liquid containing bacteria through an unglazed PORCELAIN tube, the bacteria were COMPLETELY RETAINED and
he solution that passed through (the FILTRATE) was sterile.
n1892 D. IWANOWSKI applied this test to a filtrate of plants suffering from TOBACCO MOSAIC DISEASE with shocking results; the filtrate was FULLY CAPABLE of producing the ORIGINAL
DISEASE in new hosts.
Filtration of a mixture of
bacteria and viruses. If a
mixture of viruses and bacteria are
filtered through a bacterial-proof
filter (red), the viruses will pass
through into the filtrate in the flask.
Filtered beer is produced by a
similar process.

5.  When repeated, filtrations produced the
same results and nothing could be seen in the
filtrates using the most powerful microscopes,
nor could anything be cultivated from the
filtrates, Iwanowski and associates concluded
that they had discovered a new pathogenic
life-form which they called by the
unimaginative, but functional, name of
"FILTERABLE VIRUS".

6. The concept of virus
Edward Jenner (1798), introduced the term
virus in microbiology.
Virus in Greek means poison.
Edward Jenner noticed that milk maids who
infected with cowpox develop immunity
against smallpox.
He inoculated a boy with the vesicle fluid taken
from the hand of infected maid.
The boy developed sustained immunity against
smallpox.

7. Edward Jenner assumed that the vesicle fluid
that has been taken from the hand of the milk
maid contained a poison ( virus ), that was
responsible for immunity.

8. General characteristics of viruses
Viruses are smaller than bacteria, they range in
size between 20-300 nanometer (nm)
Viruses contain only one type of nucleic acid,
either DNA or RNA, but never both.
Viruses consist of nucleic acid surrounded by a
protein coat. Some viruses have additional
lipoprotein envelope.
Viruses lack cellular organelles, such as
mitochondria and ribosomes.

9. General characteristics of viruses
Viruses are obligate cellular parasites. They
replicate only inside living cells.
Viruses replicate through replication of their
nucleic acid and synthesis of the viral protein.
Viruses do not multiply in chemically defined
media.
Viruses do not undergo binary fission.

10. General structure of viruses
Viruses composed of nucleic acid
either DNA or RNA, surrounded
by a protein coat called the
capsid.
The capsid is composed of small
structural units called
capsomeres.
The capsid protects nucleic acid
from inactivation by the outer
physical conditions.
CAPSOMERES

11. General structure of viruses
Some viruses have additional lipoprotein envelope ,
composed of virally coded protein and host lipid. The
viral envelope is covered with glycoprotein spikes
Some viruses have enzymes inside the virion. All ss-
RNA viruses with negative polarity have the enzyme
transcriptase ( RNA dependent RNA polymerase) inside
virions.
Retroviruses and hepatitis B virus contain the enzyme
reverse transcriptase.

12. Symmetry of viruses
Viruses are divided into three groups, based on the
morphology of the nucleocapsid and the arrangement of
capsomeres.
Cubic symmetry:
The virus particle is icosahedral in shape (almost
spherical particle ) and the nucleic acid contained inside
the capsid. The icosahedrons particle is composed of 20
equilateral triangles , 12 vertices and has 2,3,5 rotational
symmetry.

13. Helical symmetry :
The virus particle is elongated or
pleomorphic (not spherical), and
the nucleic acid is spiral.
Caposomeres are arranged round
the nucleic acid

14. complex symmetry:
The virus particle does not
confirm either cubic or helical
symmetry.

15. Baltimore classification
Viruses were divided into six groups based on the their
nucleic acid and m-RNA production.
1- ds-DNA viruses.
2- ss-DNA viruses.
3- ds- RNA viruses.
4- ss-RNA viruses with positive strands( positive
polarity).
5- ss-RNA viruses with negative strands(negative
polarity).
6- ss-RNA viruses associated with the enzyme
reverse transcriptase.

16. References
Caspar DLD: Design principles in virus particle
construction. In Horsfall FL, Tamm I (eds): Viral and
Rickettsial Infections in Man. 4th Ed. JB Lippincott,
Philadelphia, 1975 .
Mattern CFT: Symmetry in virus architecture. In Nayak
DP (ed): Molecular Biology of Animal Viruses. Marcel
Dekker, New York, 1977 .
Morse SS (ed): The Evolutionary Biology of Viruses.
Raven Press, New York, 1994 .
Palmer EL, Martin ML: An Atlas of Mammalian Viruses.
CRC Press, Boca Raton, 1988 .
Nermut MV, Stevens AC (eds): Animal Virus Structure.
Elsevier, Amsterdam, 1989 .
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