Introduction to virology: Viral
structure and classification
PROF A.O. ABODERIN
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History
• By the last ½ of 19th C. existence of a diverse
world of bacteria, fungi, and protozoa was well
established.
• 1840 – Jacob Henle hypothesized the concept
of submicroscopic agents
• 3 major advances –
– spontaneous generation did not occur,
– R. Koch demonstrated the causes of anthrax and
TB,
– importance of sterile fields – Lister
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Discovery period
• Adolf Mayer, a German scientist discovered
Tobacco mosaic disease
• Dimitri Ivanofsky, a Russian scientist, reported
that “the sap of leaves infected with TMD
retains its infectious properties even after
filtration through Chamberland filters”.
Operational definition of viruses
• Martinus Beijerinck, a Dutch soil microbiologist
showed that the filtered sap could be diluted
and then regain its “strength” after replication in
living growing tissue of the plant. “contagium
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vivum
fluidum” – a contagious living liquid. 3
A new concept
• Mayer, Ivanofsky and Beijerinck contibuted to
this:
– a filterable agent
– too small to be observed in the LM
– but able to cause disease by multiplying in living
cells.
• Loeffler and Frosch rapidly described and
isolated the first filtrable agent in animals – the
foot-and-mouth disease virus.
• Walter Reed and his team in Cuba recognized
the first human filterable virus, yellow fever
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virus
in 1900
Virus
• The term virus (from the Latin for slimy liquid or
poison)
• In the first decades of 20th C. the infectious
entities were referred to as filterable agents i.e.
operational definition
• Virus late became restricted to agents that
fulfilled the criteria of the trio above and that
were the 1st agents to cause a disease that
could not be proven using Koch’s postulates
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Developments
• By the end of 1930s many viruses have been
identified
– tumour viruses,
– bacteriophages,
– Mumps virus,
– influenza viruses
– many arboviruses
• The process of developments continues to the
present:
– human retroviruses,
– several new hepatitis viruses and
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– herpesviruses
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Basic characteristics of Viruses
1- Structure unique: Although viruses are very
heterogeneous, there is a unity of structure, basically protein
and nucleic acid.
2-Replication: Note, not binary division, but burst of virus
particles.
3-Small size: Viruses are "filterable" agents ranging from
20nm – 300nm in diameter.
4-Obligate (genetic) parasites: Dependent on host cell
genetic and protein synthesizing machinery.
5-Genome: Virus genome is either DNA or RNA not both.
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Control measures for viruses include capitalizing on our knowledge of:
Growth on
artificial media
Division by binary fission
Whether they have
both DNA and RNA
Whether they have
ribosomes
Their sensitivity to
antibiotics
Bacteria
Yes
Yes
Yes
Yes
Yes
Mycoplasma
Yes
Yes
Yes
Yes
Yes
Rickettsia
No
Yes
Yes
Yes
Yes
Chlamydia
No
Yes
Yes
Yes
Yes
Viruses
No
No
No
No *
No
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Viral Structure
• Characteristics of virus:
– Size
– Nucleic acid
– Metabolic activities
• Components of a typical virus
– Virion - the intact virus particle
– Capsid- The protein coat
– Capsomeres
– Nucleic acid
– Envelope
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VIRUS STRUCTURE
1.
2.
3.
4.
5.
All viruses are constructed according to certain
basic principles. All viruses contain
a nucleic acid genome (RNA or DNA) and
a protective protein coat (called the capsid).
The capsid is made of similar subunits known
as the capsomeres. The nucleic acid genome
plus the protective protein coat is called
the nucleocapsid which may have
icosahedral, helical or complex symmetry. In
addition, viruses may or may not have
an envelope.
Viruses range in size from 20-300 nanometers
in diameter to several hundred nanometers in
length in the case of the filoviridae.
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Enveloped virus
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Herpes virus
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Enveloped virus
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Nucleic acid
•
•
•
•
•
Composed of either RNA or DNA
Size ranges from 3kb (e.g. hepadna-) to 300kb
Encodes from 3-4 to several 100s proteins
Single or double stranded, circular or linear
RNA genomes comprise either a single
molecule of NA or multiple discrete segments
• RNA Segments vary from as few as two in
Arenaviridae to 12 in Reoviridae
• It may have a positive sense or negative sense
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The capsid (Virus coat)
• Made up of proteins, arranged in multiple
almost identical units - capsomeres
• Is antigenic and specific for each virus type
and so used for virus identification in serological
tests.
• It protects the viral genome from inactivation by
adverse environmental factors and
• Helps to determine the symmetry of the virus.
• It facilitates attachment and entry of virus into
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the
cell.
Virus symmetry
• Repetition of capsid subunits leads to structural
arrangements with symmetric features
• All but the most complex viruses exhibit either
helical or icosahedral symmetry
– Helical – repeating protein subunits are bound at
regular intervals along a helical spiral formed by NA.
All animal viruses with helical symmetry have RNA
genomes (B)
– Icosahedral – viruses with icosahedral symmetry
usually have a spheric shape, with 2-fold, 3-fold, and
5-fold axes of rotational symmetry (A)
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Virus symmetry
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Icosahedral symmetry
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Adenovirus
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Virus Taxonomy
• Progression in classification
Filterable agents – size.
Common pathogenic properties, common organ
tropisms, and common ecological and
transmission properties: e.g. hepatitis viruses,
enteroviruses, arboviruses.
Since the 1950s Morphologic & physicochemical
criteria determine classification
More recently – genetic relatedness. There is
availability of nucleotide sequences of genomes.
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Virus Taxonomy
• In the 1950s and 1960s, there was an explosion
in the discovery of new viruses
• The International Committee on the
Nomenclature of Viruses (ICNV) was formed in
1966 and became ICTV in 1973.
• ICTV operates under the auspicies of the
Virology Division of the International Union of
Microbiological Societies. ICTV has 6
committees, 45 study groups and >400
participating virologists
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Virus Taxonomy
• The 6th report of the ICTV, published in
1995, records a universal taxonomy
scheme comprising 1 order, 71 families,
11 subfamilies, and 164 genera, including
many floating genera, and more than
4,000 member viruses.
• Virus nomenclature does not involve the
use of Latinized binomial terms
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Criteria for Classification
•
•
•
•
•
Virus classification involves naming and placing
viruses into a taxonomic system.
Virus classification is based mainly on phenotypic
characteristics, including
morphology,
nucleic acid type,
mode of replication,
host organisms, and
the type of disease they cause.
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Current classification
• The type and structure of the viral NA and
the strategy used in its replication
• The type of symmetry of the virus (helical
versus icosahedral)
• The presence or absence of a lipid
envelope
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ICTV Classification System
Viral classification starts at the level of order and follows
as thus, with the taxon suffixes given in italics:
• Order (-virales)
– Family (-viridae)
• Subfamily (-virinae)
– Genus (-virus)
» Species (-virus)
Names of orders and families are italicized.
Species names generally take the form of [Disease] Virus.
Recognition of orders very recent and deliberately slow; to date, only
three have been named, and most families remain unplaced.
Approximately 80 families and 4000 species of virus are known.
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Virus-Cell Interactions
• Viruses require intact cell to replicate
• Viruses must disassemble the infecting
particle.
• Stages in Virus-Cell Interaction
– Attachment
– Penetration
– Disassembly
– Transcription
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- Translation
- Replication
- Assembly
- Release
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Attachment
• single capsid components that extend from
the virion surface, such as the attachment
proteins of adenovirus, reovirus, rotavirus
• surface glycoproteins of enveloped viruses
such as influenza virus and HIV.
• CD4 for HIV I, acetylcholine receptor for
rabies virus, or intercellular adhesion
molecule I for rhinovirus.
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Penetration & Disassembly
• Enveloped viruses such as paramyxoviruses
and retroviruses enter cells by fusion of the
viral envelope with the cell membrane.
• Other viruses enter cells by receptormediated endocytosis.
• nonenveloped viruses, must traverse cell
membranes without a fusion mechanism
involving a viral envelope.
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Genome Replication
• Translation-competent genomes e.g.
alphaviruses, flaviviruses & picornaviruses: plus
sense, (+)sense
• (-)sense RNA genomes paramyxoviruses,
orthomyxoviruses & rhabdoviruses
• Reoviruses
• Retroviruses
• DNA viruses except poxviruses replicate in the
nucleus
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Thank you for listening
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