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Lecture 1
Virology introduction
Viruses: are the smallest infectious agents (0.2-300nm) in size, composed of nucleic
acid surrounding by protein shell which is in some type of viruses surrounding by lipid
envelope.
-Viruses are small, sub cellular that are unable to multiply outside a living host cell. They
replicate only in living cells.
-Containing only one kind of nucleic acid (DNA or RNA) as their genome
-Nucleic acid is enclosed in a protective protein shell which may be surrounded by lipid
containing membrane
-They are inert in the extra cellular environment. -The virus is a
o
bligatory
n
intracellular
microorganism , so the virus does not live outside the cell.
-Viral nucleic acid containing information causing programming the infected host cell to
synthesize number of virus specific macromolecule for the production of virus progeny.
-The coat protein assemble together to form a capsid which stablize the virus against the
extracellular environment (such as nucleases)
that
facilitate attachment and perform
penetration of virus into the susceptible host cell.
-
The virus infection may have little or no effect on the host cell or may result in cell
damage or death
.
Capsid: The protein shell, or coat that encloses the nucleic acid genome.
Capsomeres: Morphologic units seen in the electron microscope on the surface of
icosahedral virus particles. Capsomeres represent clusters of polypeptides, but the
morphologic units do not necessarily correspond to the chemically defined structural
units.
Defective virus: A virus particle that is functionally deficient in some aspect of
replication.
Envelope: A lipid-containing membrane that surrounds some virus particles. It is
acquired during viral maturation by a budding process through a cellular membrane.
Virus-encoded glycoproteins are exposed on the surface of the envelope. These
projections are called peplomers.
Nucleocapsid: The protein-nucleic acid complex representing the packaged form of the
viral genome.
Virion: The complete virus particle. In some instances (eg, papilloma viruses, picorna
viruses), the virion is identical with the nucleocapsid. In more complex virions
(herpesviruses, orthomyxoviruses), this includes the nucleocapsid plus a surrounding
2
envelope. This structure, the virion, serves to transfer the viral nucleic acid from one cell
to another.
Types of symmetry of virus particles
Icosahedral symmetry
( cubic(: composed of
12
verticles and equal triangular
sides ,with approximate outline of sphere, e.g Herpes virus and Adenovirus.
Helical symmetry :the capsomeres are arranged like steps in a spiral strain case
or hollow, rod shaped, the helix rigid or flexible, e.g influenza and parainfluenza
viruses.
Complex viruses :e.g Poxvirus, in which there are many layers around the
capsid.
Classification of Viruses
Basis of Classification
The following properties have been used as a basis for the classification of viruses:
A- Based on chemical and physical criteria :
(1) Virion morphology, including size, shape, type of symmetry, presence or absence of
peplomers, and presence or absence of membranes.
(2) Virus genome properties, including
Type of nucleic acid (DNA or RNA).
Size of genome in kilobases (kb) or base pairs (bp).
Strandedness (single or double).
Whether linear or circular, sense (positive, negative, ambisense).
Segments (number, size).
Nucleotide sequence.
G + C content.
Positive or Negative Sense (Polarity(
(3) Physicochemical properties of the virion, including:
Molecular mass,
pH stability,
Thermal stability,
Susceptibility to physical and chemical agents, especially Ether and
other
detergents.
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(4) Virus protein properties, including: number, size, and functional activities of
structural and nonstructural proteins, amino acid sequence, modifications (glycosylation,
phosphorylation), and special functional activities (transcriptase, reverse transcriptase ,
neuraminidase and fusion acticity).
(5) Genome organization and replication, including:
gene order,
Strategy of replication (patterns of transcription, translation).
Cellular sites (accumulation of proteins, virion assembly, virion release).
(6) Antigenic properties.
(7) Biologic properties, including:
Natural host range.
Mode of transmission.
Vector relationships.
Pathogenicity.
Tissue tropisms and pathology.
B- Classification according to diseases they produce:
Generalized diseases: in which virus is spread throughout the body via blood stream
and in which multiple organs are affected Skin rashes may occur, these include
Measles, rubella, chicken pox, yellow fever and enteroviruses.
Diseases primarily affected specific organs:
a- Diseases of CNS, such as poliomyelitis, rabies, aseptic meningitis and herpes
simplex.
b- Diseases of liver, such as hepatitis type A,B,C,D,E ,yellow fever and rubella virus.
d- Diseases of skin or mucous membranes, such as herpes simplex, molluscum
contagiosum, warts and herpes zoster.
e- Diseases of Eye, such asadenovirus, herpes keratoconjunctivitis and epidemic
haemorragic conjunctivitis.
f- Diseases of the gastrointestinal tract, such as rotavirus and enteric adenviruses.
g- Sexually transmitted diseases, such as herpes, hepatitis B virus, papilloma viruses,
reteroviruses (HIV) and cytomegalovirus.
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Evolutionary Origin of Viruses
Two theories of viral origin can be summarized as follows:
(1) Viruses may be derived from DNA or RNA nucleic acid components of host cells that
became able to replicate autonomously and evolve independently. They resemble genes
that have acquired the capacity to exist independently of the cell. Some viral sequences
are related to portions of cellular genes encoding protein functional domains. It seems
likely that at least some viruses evolved in this fashion.
(2) Viruses may be degenerate forms of intracellular parasites. There is no evidence that
viruses evolved from bacteria, though other obligately intracellular organisms, eg,
rickettsiae and chlamydiae, presumably did so. However, poxviruses are so large and
complex that they might represent evolutionary products of some cellular ancestor.
Universal System of Virus Taxonomy
A system has been established in which viruses are separated into major groupings called
families on the basis of virion morphology, genome structure, and strategies of
replication. Virus family names have the suffix -viridae. And each family is divided to
subfamily –virinae within each subfamily, subdivisions called genera are usually based
on physicochemical or serologic differences. Criteria used to define genera vary from
family to family. Genus names carry the suffix -virus.
By 2000, the International Committee on Taxonomy of Viruses had organized more than
4000 animal and plant viruses into 56 families, 9 subfamilies, and 233 genera, 24 families
contain viruses that infect humans and animals.
Classification
of viruses
based on nucleic acid genome
1- DNA viruses
:
into divided
A-
E
nveloped
DNA viruses like: Hepadnaviruses (Hepadnaviridae)
and
Herpesviruses
(Herpesvridae).
B- Non
-enveloped viruses like: Parvoviridae,
,
and ePapillomavirida
Polyomaviridae
(Papovaviridae)
and Adenoviridae
.
:
C
–
Complex DNA viruses
Poxviridae
2- RNA viruses divided into :
-
A
Non
-
enveloped RNA viruses such as
,
Picornaviruses
,
Astroviruses
Caliciviruses and Reoviruses.
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-
B
Enveloped RNA viruses such as Togaviruses,
,
,Flaviviruses
Arenaviruses
v
Corona
iruses
,
,
Reteroviruses Orthomyxoviruses,
,
,Rhabdoviruse
Bunyaviruses
,
Paramyxoviruses and Filoviruses
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Viruses differ from other microorganisms in their structure, biology
and way of replication
Property
Virus
Bacteria
Size
10-300nm
1000
nm
Genome (nucleic acid(
DNA or RNA, but not both
DNA and RNA
Cell wall
Envelope present in some viruses
Have cell wall
Ribosome
No ribosome
Have ribosome
Multiplication by binary fission
(
-
)
)+(
Sensitivity to antibiotics
(
-
)
)+(
Growth in culture media
Grow only in living host cells
Grow in culture media