Virology is the study of viruses – submicroscopic, parasitic particles of genetic material contained in a protein coat and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine.

1. VIROLOGY

2. Dr. Farzin Asghari Sana

3. Discovery of viruses
 In 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 the solution that
passed through (the FILTRATE) was sterile.
In1892 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.
Dr. Farzin Asghari Sana

4. What are Viruses?
Definition-
Viruses are noncellular particles made up of genetic material and
protein that can invade living cells.
Discovery of Viruses
Martinus Willem Beijerinck (1897) coined the Latin
name “virus” meaning poison
Dr. Farzin Asghari Sana

5. Tobacco Mosaic Virus
Wendell Stanley (1935) discovered viruses
were made of nucleic acid and protein
Dr. Farzin Asghari Sana

6. Dr. Farzin Asghari Sana

7. Dr. Farzin Asghari Sana

8. Are Viruses Living or Non-living?
Biologists consider viruses to be non-living
because:
-Are not cells
-Do not grow or respond to their surroundings
-Cannot make food, take in food, or produce wastes
-Viruses do not respond to stimuli.
They can only multiply if in another living cell Dr. Farzin Asghari Sana

10. 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.
Dr. Farzin Asghari Sana

11. 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.
Dr. Farzin Asghari Sana

12. Dr. Farzin Asghari Sana

13. Dr. Farzin Asghari Sana

14. Viroids
ss RNA genome and the smallest known pathogens.
Affects plants
Prions
Infectious particles that are entirely protein.
No nucleic acid
Highly heat resistant
Animal disease that affects nervous tissue
Affects nervous tissue and results in
Bovine spongiform encephalitis (BSE) “mad cow disease”,
Scrapie in sheep
kuru & Creutzfeldt-Jakob Disease (CJD) in humans
Dr. Farzin Asghari Sana

15. The capsid is composed of small structural units called
capsomeres.
The capsid protects nucleic acid from inactivation by the
outer physical conditions.
Dr. Farzin Asghari Sana

16. Some viruses have additional lipoprotein envelope , composed of virally coded
protein and host lipid. The viral envelope is covered with glycoprotein spikes
Dr. Farzin Asghari Sana

17. Dr. Farzin Asghari Sana

18. 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
Helical symmetry :
The virus particle is elongated or pleomorphic (not spherical), and the nucleic acid is spiral.
Caposomeres are arranged round the nucleic acid
complex symmetry:
The virus particle does not confirm either cubic or helical symmetry.
Dr. Farzin Asghari Sana

19. Dr. Farzin Asghari Sana

20. Virion structures. Nonenveloped (naked) viruses consist of a genome surrounded by a protein shell, or capsid. Shown
here is an icosahedral capsid, the most common type in nonenveloped viruses. Enveloped viruses have a membrane
that surrounds the nucleocapsid, which can have an icosahedral, icosadeltahedral, or helical shape. The helical
nucleocapsid, found only in most enveloped (−) RNA viruses, is formed by association of viral proteins, including RNA
polymerase, with the genome. Dr. Farzin Asghari Sana

21. Dr. Farzin Asghari Sana

22. Assembly of the icosahedral capsid of a picornavirus. Individual proteins associate into subunits,
which associate into protomers, capsomeres, and an empty procapsid.
Dr. Farzin Asghari Sana

23. Dr. Farzin Asghari Sana

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25. Dr. Farzin Asghari Sana

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27. 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.
• (+) sense: Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated into protein by
the host cell.
• (-) sense: Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by
an RNA polymerase before translation.
Dr. Farzin Asghari Sana

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30. Classification of major viral families based on genome structure and virion morphology. A, DNA viruses. L, linear genome; C,
circular genome. B, RNA viruses. S, segmented genome. Dr. Farzin Asghari Sana

31. Morphology and relative size of viruses. Herpesvirus, adenovirus, poxvirus, retroviruses, and
rhabdoviruses have characteristic shapes, whereas other viruses are distinguished by size,
presence of an envelope, or an icosa(delta)hedral capsid.
Dr. Farzin Asghari Sana

33. Dr. Farzin Asghari Sana

34. 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. Dr. Farzin Asghari Sana

35. VIROLOGY