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Introduction to the Viruses: General properties of viruses: 1-They are very small in size, from...
Transcript of Introduction to the Viruses: General properties of viruses: 1-They are very small in size, from...
Introduction to the Viruses:
General properties of viruses:
1-They are very small in size, from 20-300 m.
2-They contain one kind of nucleic acid (RNA or DNA) as their
genome.
3-They are metabolically inert because they do not possess
enzyme systems necessary for the synthesis of new viral
material (ribosomes).
4-They are obligate intracellular parasites as they replicate
inside living cells.
5-They are only seen by electron microscope.
Viral Structure:
Each virus particle or virion is composed of :
A protein coat capsid
A nucleic acid core
Many viruses are naked but some are enveloped.
Viral capsid:
It is the protein coat surrounding the nucleic acid.
It is composed of small protein subunits arranged symmetrically around the nucleic acid called capsomeres.
Viral nucleic acid and viral Envelope:
Viruses contain either DNA or RNA but not both .
Most DNA viruses are double stranded, some are single stranded.
Most RNA viruses are single stranded, some are double stranded.
Virus envelope :
Many viruses are surrounded by a lipid or lipoprotein envelopes
which may be covered by spikes (glycoproteins).
Virus Symmetry
Viruses have three types of symmetry:Cubical symmetry: These viruses resemble a crystal and are called
icosahedral virus. Example: adenoviruses.
Helical symmetry: In which the particle is elongated. Most helical
viruses are enveloped . Example: influenza virus.
Complex symmetry: In which the viruses are complicated in
structure. Example: poxviruses and
bacteriophage.
Classification of Viruses:
Classical virus classification schemes have been based on the consideration of major properties of viruses:
1 -The type of nucleic acid which is found in the virion (RNA
or DNA, single stranded or double stranded(
2-The symmetry and shape of the capsid (Cubic, helical ,
complex.(
3 -The presence or absence of an envelope (enveloped, naked)
4 -The size of the virus particle.
5 -Antigenic properties .
6 -Biologic properties, including natural host range, mode of
transmission, vector relationship, pathogenicity, and tissue
tropism.
Classification of Viruses:
DNA
Icosahedral Complex
Pox virusesNaked Enveloped
Papilloma virus Herpes viruses
Hepatitis B
Viral replication: Viruses multiply only in living cells.Steps of viral replication:1- Adsorption:
Virus attaches to the cell surface.
2- Penetration (Entry):
Enveloped viruses: receptor mediated endocytosis
3- Uncoating:
Uncoating is the physical separation of viral nucleic acid from the outer
structural components.
4- Transcription of mRNA.
5- Synthesis of viral components: This involves the synthesis of viral proteins and viral genomes.
6- Assembly:
New virus particles are assembled by packaging of the genome into capsid.
7- Release:
Virus may be released due to cell lysis, or, if enveloped, may bud from the cell.
Effect of viruses on cells:
Many viruses inhibit host RNA, DNA or protein synthesis (or any combination of these).
In general, viruses could induce the following pathogenic effects:
1- Cytopathic effect (CPE):
The presence of the virus often gives rise to morphological changes in the host cell.
These may include:
1-Membrane blebbing
2-Formation of multinucleated giant cells, or fused
cells, known as syncytia
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3-Production of inclusion bodies (accumulations of
viral proteins or virions) in the nucleus or
cytoplasm
4-Rounding up and detachment of cells from the
culture dish.
5 -Cell lysis .
2-Latency:
Virus genome persists within the infected cells without
replication and the survival of infected cells.
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3- Chromosome damage:
As breakage, fragmentation, rearrangement, translocation.
4- Malignant transformation (oncogenesis):
Viruses are known to be the etiologic factors in the development
of human tumors including cervical and liver cancer.
Examples of oncogenic viruses include:
- Human papillomavirus (HPV) : Cervix Cancer
- Ebstein Barr virus (EB) : Nasopharyngeal carcinoma
- Hepatitis B and Hepatitis C viruses (HBV, HCV) : liver Cancer
- Human T cell lymphotorpic virus (HTLV): Adult T cell leukemia.
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Cytopathic effects: Left to Right :
Cytopathic effect of HSV, enterovirus 71, and RSV in cell culture .
Note the ballooning of cells in the case of HSV .
Note syncytia formation in the case of RSV.
Clinical examples on viral infection:
Skin infection: One of the most common viral infection of skin:Herpes virus infection.
Respiratory tracts infection: The most common viral infection of respiratory tracts: 1-Influenza virus infection. 2-Common cold Rhinovirus.
Digestive tracts infection:-Gastroenteritis: Rotavirus.-liver infection: Hepatitis A,B, and C.
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Sexually transmitted diseases :
The most common viruses are:
1-Genital Herpes.
2-Papilloma virus.
Nervous system Encephalitis and Meningitis:
1-Flaviviruses. (Arbovirus).
2-Rabies virus.
Diagnosis of viral infections: In general, diagnosis of viral infection can be performed by:1-Direct detection: The clinical specimen is examined directly for the presence of : A-Virus particles: Microscopy. B- Virus antigen: Microscopy. C- Viral nucleic acids: Molecular methods.
2- Virus isolation: For cultivation of viruses, the Clinical specimen is inoculated into: A- Cell culture. B- Chick embryo (fertilized eggs). C- Laboratory Animals.