This seminar presentation summarizes the general characteristics and classification of viruses. It defines viruses as obligate intracellular parasites that are too small to be seen by optical microscopes and must replicate inside host cells. Viruses do not have cellular organization and contain either DNA or RNA, but not both. They lack the enzymes for protein and nucleic acid synthesis and are dependent on host cell machinery for replication. Viruses come in various shapes and sizes and have capsids made of protein that surround their nucleic acid cores. Their capsids exhibit different symmetries and some viruses have envelopes. Viruses are cultivated using techniques like animal inoculation, embryonated egg culture, and cell culture.

1. Seminar Presentation
on
General characteristics &
Classification of viruse

2. What are viruses?
Viruses are submicroscopic (ranging from about 20 to 300 nm in
diameter), obligate intracellular parasites.
They are too small to be seen by optical microscopes, and they have no
choice but to replicate inside host cells.

3.  Viruses do not have a cellular organization.
 contain only one type of nucleic acid, either DNA or RNA but never
both.
 lack the enzymes necessary for protein and nucleic acid synthesis
and are dependent for replication on the synthetic machinery of host
cells.
 multiply by a complex process and not by binary fission.
 unaffected by antibacterial antibiotics.
MAIN PROPERTIES OF VIRUSES

4.  Viruses must be infectious to endure in nature.
 Viruses must be able to use host cell processes to produce their
components (viral messenger RNA, protein, and identical copies of
the genome).
 Viruses must encode any required processes not provided by the cell.
 Viral components must self-assemble.
Consequences of Viral Properties

5. Size
 Viruses are much smaller than bacteria and they are filterable.
 Size of virus is measuresd by using;
 Passing them through Collodion Membrane
 Electron Microscopy
 Ultracentrifugation
 Comparatative Measurements
 Size of viruses ranges from 20nm to 300nm.
 Examples
 Size of parvovirus is 20nm – smallest virus
 Size of pox virus is 300nm - largest virus
MORPHOLOGY OF VIRUSES

6.  Shape of the virus particle varies in different groups of viruses.
 Most of the animal viruses are roughly spherical, some are
irregular and pleomorphic.
Shape of the Virus
Rabies virus is bullet-shaped
Tobacco mosaic
virus rod-shaped
Pox virus is brick-shaped

7. STRUCTURE AND CHEMICAL COMPOSITION OF
THE VIRUSES
Viral Capsid
Virus Symmetry
Viral Envelope
Viral Nucleic Acids

8. Viruses consist of nucleic acid core
surrounded by a protein coat called
capsid.
Viral Capsid
The capsid with the enclosed nucleic
acid is known as nucleocapsid.

9.  The capsid is composed of a
large number of capsomers
which form its
morphological units.
Chemically capsid are
polypeptide molecules which
are arranged symmetrically to
form an impenetrable shell
around the nucleic acid core.

10. Function
of
Capsid
Binding
sites
Vehicle of
transmission
Antigenic
Structural
symmetry
Host’s
defence
Protection

11. Icosahedral symmetry
Helical symmetry
Complex symmetry
Viral
symmetry

12. Icosahedral
symmetry
•Ubiquitous among
spherical viruses
12 vertices or corners and
20 facets or sides.
• Each facet is in the shape
of an equilateral triangle.
• Two types of capsomers
constitute the icosahedral
capsid.
• Pentagonal capsomers at
the vertices (pentons)
• Hexagonal capsomers
making up the facets
(hexons).
• E.g. Adenoviruses, cowpea
chlorotic mottle virus
(CCMV)
Virus Symmetry

13. Helical
symmetry
• Capsomere and nucleic
acid are wined together
to form helical or spiral
tube like structure.
• Most of the helical
viruses are enveloped &
all are RNA viruses.
• Typical virus with
helical symmetry is
tobacco mosaic virus
• RNA virus with 2130
identical Capsomeres
arranged in a helix
Helical Symmetry

14. Complex Symmetry
Complex
symmetry
• Not purely helical or
icosahedral but with
some extra structure.
•Pox virus –Largest
animal virus
• Large
bacteriophage
• – binal Symmetry
•Head resembles
icosahedral, tail is
helical

15. Non- enveloped virus Enveloped Virus
• Lack Lipid membrane
• Resistant to heat
• Adenovirus usually causes upper
respiratory tract infections. Poliovirus,
rotavirus, and norovirus are other
examples of non-enveloped viruses.
• Enclosed in Lipid Membrane
• Sensitive to heat
• Influenza virus that causes seasonal flu
symptoms. The herpes simplex virus, the
chickenpox virus, and even the recent
Ebola virus, are considered as enveloped
viruses.

16. Influenza virus carries two
kinds of peplomers,
 hemagglutinin -which is a
triangular spike and
 neuraminidase - which is a
mushroom-shaped structure.
Peplomers
A peplomer is a glycoprotein spike on a viral capsid or viral envelope.
 will only bind to certain receptors on the host cell; they are
essential for both host specificity and viral infectivity.

17. Functions of
Peplomer
Mediate attachment: Peplomers mediate attachment of the
virus to the host-cell receptors to initiate the entrance of the virion
in to the cell
Attach to receptors: Viral glycoproteins also attach to
receptors on red blood cells, causing these cells to agglutinate
(hemagglutination).
Enzymatic activity: Other glycoproteins possess
enzymatic activity like neuraminidase which cleave
neuraminic acid from host cell glycoproteins.
Major antigens: Glycoproteins are also major antigens for
protective immunity.

18. • Single stranded
• Double StrandedDNA
• Double Stranded
• Single Stranded
• Positive Strand
• Negative Strand
RNA
Viral Nucleic Acids

19. SUSCEPTIBILITY TO PHYSICALAND CHEMICALAGENTS
Viruses are very heat labile
Except Rhinovirus
Inactivated within seconds at 56°C
Enveloped viruses are much more heat-labile
Stable at low temperatures.
storage, they are kept frozen at –70°C.
Heat
and
Cold

20. pH
• Viruses vary greatly
in their resistance to
acidity.
• Viruses are stable
between pH values of
5.0 and 9.0.
• All viruses are
destroyed by alkaline
conditions.
Stabilization of
Viruses by Salts
• Molar concentrations
of certain salts
(MgCI2 Na2 SO4) also
protect some viruses
(for example
poliovirus) against
heat inactivation.
• The stability of
viruses is important
in the preparation of
vaccines.
Radiation
• Ultraviolet
• X-ray
• High-energy particles
inactivate viruses.
SUSCEPTIBILITY TO PHYSICALAND CHEMICALAGENTS

21. Disinfectants
Bacteria are killed
in 50 percent
glycerol saline but
this acts as a
preservative for
many viruses (for
example, vaccinia,
rabies)
Phenolic
disinfectants: are
only weakly
viricidal
Oxidising agents:
most active
antiviral
disinfectants are
oxidizing agents
such as hydrogen
peroxide,
potassium
permanganate and
hypochlorites.
SUSCEPTIBILITY TO PHYSICALAND CHEMICALAGENTS

22. Organic iodine compounds
• actively viricidal.
• Chlorination of drinking water kills most viruses.
• Some viruses (such as hepatitis virus, polio viruses) are relatively
resistant to chlorination.
Formaldehyde &Beta propiolactone
• Formaldehyde and beta propiolactone are actively viricidal and are
commonly employed for the preparation of killed viral vaccines.
SUSCEPTIBILITY TO PHYSICALAND CHEMICALAGENTS

23. Lipid Solvents
• Enveloped viruses which possess lipid-containing envelope are
sensitive to lipid solvents such as ether, chloroform and bile salts
and the naked viruses are resistant to them.
Antibiotics
• Antibiotics active against bacteria are completely ineffective against
viruses. This property is made use of in eliminating bacteria from
clinical specimens by antibiotic treatment before virus isolation.
SUSCEPTIBILITY TO PHYSICALAND CHEMICALAGENTS

24. The genetic information necessary for viral replication is contained in the viral
nucleic acid but lacking biosynthetic enzymes, the virus depends on the synthetic
machinery of the host cell for replication
VIRAL REPLICATION

25. Mechanism of replication in different types viruses

26. Incomplete
Viruses
• Incomplete viruses are due to defective assembly
and are seen in large proportions.
• The virus yield will have a high hemagglutinin
titer but low infectivity and this is known as the
‘von Magnus phenomenon.
Abortive
Infections
Viruses in which maturation and assembly seems
to be defective however in nonpermissive cells,
viral components may be synthesized
Defective
viruses
These are genetically defective in that when they
infect cells, they are unable to give rise to fully
formed progeny
Yield of progeny virions occurs only if the cells
are simultaneously infected with a helper virus,
which can supplement in the genetic deficiency
• Rous sarcoma virus (RSV)
ABNORMAL REPLICATIVE CYCLES

27.  Viruses lack its independent metabolism
 can replicates inside host cell
 So viruses cannot be cultured in nonliving medium as bacteria
or fungi
CULTIVATION OF VIRUSES

28. Techniques
of virus
cultivation
Animal
inoculation
Cell
culture
Embroyonated
egg culture

29.  Mice infant (suckling) mice, rats monkeys, chickens, rabbits, ferrets
are used for inoculation
 Growth of the virus in inoculated animals may be indicated by death,
disease or visible lesions
Animal Inoculation
i. Primary isolation of certain viruses
ii. For the study of pathogenesis, immune response and epidemiology of viral
diseases
iii. For the study of oncogenesis.

30.  Used for the cultivation of viruses by Goodpasture (1931) and Burnet.
 The embroyonated egg offers several sites for the cultivation of viruses
such as choriallontoic membrane, amniotic sac, allantoic cavity or
yolk sac depending upon types of virus.
Embryonated Eggs

31. Cell culture ( tissue culture)Cellculture/tissueculture
Organ culture
Small bits of organ from human or animal is in tissue culture medium
used in specific purpose only
To culture corona virus tracheal ring culture is done
Explant culture
Small fragments of tissue is extracted from human or animal
Rarely used
Cell line culture
Used in lab for virus culture ,isolation and identification
First growth media is prepared by maintaining balanced salt
concentration ,essential amino acids glucose buffering agent,
antibiotics serum.

32.  cell culture is the sole system for virus isolation in most laboratories
 Based on their original , chromosomal characters and the number of generations
through which they can be maintained
Cell
culture
Primary
cell
culture
Diploid
cell
culture
Continous
cell lines
Cell culture

33. Primary cell culture
• Prepared directly from
animal or human
tissues
• Can be sub-cultured
only once or twice
• ( monkey kidney,
• human embryonic
kidney cell cultures)
• Useful for the isolation
of viruses and their
cultivation for vaccine
production
Diploid cell cultures
• Derived from human
foetal tissue
• Can be subculture 20-
50 times
• human diploid
fibroblast - MRC-5,
wi-38
• derived from normal
embryonic lung tissue
• Useful for the isolation
of some fastidious
pathogens
• Useful for the
production of viral
vaccines
• poliomyelitis ,rubella,
rabies, CMV
Continuous cell lines
• Cells of a single type,
usually derived from
cancer cells
• Capable of continuous
serial cultivation
indefinitely
• Hela
• Human carcinoma of
cervix cell line
• HEp-2
• Human epithelioma
of larynx cell line
• Vero
• Vervet monkey
kidney cell line
• Kb
• Human carcinoma of
nasopharynx cell line

34. CLASSIFICATION OF VIRUSES
Viruses are classified on following criteria:
 Structure
 Chemical composition
 Similarities in genetic makeup
 International committee on the taxonomy of viruses, which includes:
 3 orders
 63 families – “ viridae
 263 genera - “ virus”

35. TYPES OF CLASSIFICATION
3 types of systems were proposed to classify the viruses
Baltimore classification
Clinical system classification
Genetic classification

36. Baltimore Classification of Viruses
 David Baltimore based on the genomic nature of the viruses.
 The central theme of Baltimore system of virus classification is
 ‘all viruses must synthesize positive strand mRNAs from their
genomes, in order to produce proteins and replicate themselves’.
 is based on the central role of translational machinery and placed
mRNA in the center, and described the pathways to form mRNA from
DNA or RNA genomes.

37. Baltimore scheme encompassed six classes of viral genome.
Subsequently the gapped DNA genome of hepadnaviruses (e.g.
hepatitis B virus) was discovered and incorporated in a new scheme

38. Classification based on host
Animal viruses
 viruses of animal host
 Rabies , polio, mumps, chicken pox, small pox, & influenza
Plant viruses
 Viruses which show their live characteristics when attached to
plants.
 Tobacco mosaic virus, banana streak viruses,
 Bacterial viruses: bacteriophages T1 T2 T3 & T4

39. Classification based on host

40. CLASSIFICATION BASED ON GENETIC BASIS
Genetic
basis
DNA
viruses
RNA
viruses

41. Consists of DNA genome
Complete their activity by transcription
Most of then attack on organisms on similar genome
DNA VIRUSES

42. Consists of RNA genome.
Complete their activities by reverse transcriptase
RNA VIRUSES