Introduction to the Viruses:
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Transcript of Introduction to the Viruses:
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 capsidA nucleic acid coreMany 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
Classification of Viruses: a
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.
aViral replication:
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 blebbing2-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
cytoplasm4-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.
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3- Serology:
-Elevated antibodies can be detected.
-The most common methods used in hospitals.
-Can be used to differentiate acute and latent infection.
-Different methods can be used in serology such as:
1-Agglutination methods.
2-ELISA.