General Virology

1. General Virology By Dr. Emad AbdElhameed Morad Lecturer of Medical Microbiology and Immunology

2. General properties of viruses

3. Very small in size (30-200 nm). • So, they are seen only electron microscope EXCEPT pox viruses as they are large in size. • Contain one kind of nucleic acid (RNA or DNA). • Metabolically inert, they do not possess ribosomes or other organelles. • Obligate intracellular (replicate only inside the host cell). • Can not be cultivated on artificial media but grown in: • Tissue culture • Embryonated eggs • Laboratory animals

4. Size of viruses

5. Due to the small size of viruses: • Viruses can pass through the bacterial filters. • Seen only by the electron microscope. • Require high speed for sedimentation (10.000 – 30.000 rpm). This is called ultracentrifugation.

6. Structure of viruses

7. Each virus particle (virion) is composed of: • Protein coat called capsid. • Nucleic acid (RNA or DNA). • Many viruses are naked (only nucleocapsid). • But some viruses are enveloped. • In enveloped viruses, matrix proteinsare present and mediate the interaction between the capsid and envelope. • Some viruses contain internal proteins (enzymes).

8. Enveloped virus

9. Viral capsid • Composed of small protein subunits called capsomeres. • Functions of the capsid: • Protects the viral genome. • Determines the virus symmetry (cubical or helical). • Attachment of the virus to the target cells. • Capsid proteins carry the viral antigens that induce production of antibodies which neutralize the virus infectivity.

10. Viral envelope • Some viruses are surrounded by lipid envelope. • This envelope is derived from the host cell membrane during release of virus by budding. • Enveloped viruses are sensitive to ether due to their lipid content. • In some viruses, the lipid envelope is covered by glycoprotein spikes(Example: influenza virus carries two types of spikes called hemagglutinin {HA} and neuraminidase {NA}).

11. Enveloped virus with glycoprotein spikes

12. Viral nucleic acid • Viruses contain either DNA or RNA but not both. • Most DNA viruses are double stranded while most RNA viruses are single stranded. • The nucleic acid may be linear or circular. • Some RNA viruses have segmented genome like influenza virus and rota virus. • All viruses have one copy of their genome (haploid) except retroviruses which have two copies (diploid). • Functions: • It carries the genetic information for virus replication. • It is the essential infectious part of the virion.

13. Virus symmetry • Viruses have 3 types of symmetry: • Icosahedral (cubical): the virus resembles a crystal. • Helical symmetry: the capsomeres are arranged in a ribbon which is wound in the form of a helix around the nucleic acid. Note that all human viruses with helical symmetry are enveloped. • Complex symmetry: such as pox viruses which are brick shaped with ridges on the external surface.

14. Virus symmetry Icosahedral (cubical) Helical

15. Atypical virus like agents

16. Defective viruses: • Composed of nucleic acid and capsid. • But, can not replicate without a helper virus which provides the missing function. • Pseudovirions: • Contains host cell DNA instead of viral DNA within the capsid. • Prions: • Composed only of proteins + no nucleic acid. • Cause Creutzfeldt-Jakob disease. • Viroids: • Composed of single circular RNA molecule + no capsid or envelope. • Cause several plant diseases.

17. Virus replication

18. Replication cycle of the virus consists of the following steps: • Attachment (adsorption): • Interaction of the virus with specific receptors on the host cells (viral tropism). • Example: Hepatitis viruses infect the liver. • Penetration: • Non-enveloped viruses penetrate the host cell by phagocytosis. • Enveloped viruses penetrate the host cell by fusion of the envelope with the host cell membrane.

19. Uncoating: • Separation of the nucleic acid from the capsid. • Eclipse: • During this phase, the host cell is directed by the viral genome toward synthesizing viral components. • Intracellular viral synthesis: • Synthesis of the viral nucleic acid. • Synthesis of viral proteins. • Assembly: • The viral nucleic acid becomes surrounded by capsid to form the virion. • Areas of viral assembly are called inclusion bodies. • Release • In non-enveloped viruses: release occurs by rupture of the host cell. • In enveloped viruses: release occurs by budding.

20. How do viruses enter the body?

21. Aerosols Ingestion Bite Injection Sexual contact

22. Types of viral infections

23. Viral infection may be: • Superficial infection: • Infection remains local at the portal of entry. • Short incubation period • No viremia • Example: influenza • Systemic infection: • After viral entry, the virus spreads to local lymphoid tissues then causes viremia then spreads to target organs. • Example: Measles and mumps.

24. Persistent viral infections: • The virus persists in the body for a long time. • Three types are present: • Chronic infection: • Chronic disease with mild symptoms • Example: hepatitis viruses • Latent infection: • The virus persists hidden most of the time with periodic reactivation. • Example: herpes viruses • Slow virus infection: • Very long incubation period of months to years with no clinical symptoms • Example: AIDS

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