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GENERAL VIROLOGY

GENERAL VIROLOGY. Definition of virus. Smallest infectious agent with a size ranging from 20 to 300 nm, genome, a single nucleic acid either DNA or RNA, but never both. Differences between Bacteria & Viruses. Morphology of viruses. Size: 20 – 300 nm.

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GENERAL VIROLOGY

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  1. GENERAL VIROLOGY

  2. Definition of virus • Smallest infectious agent with a size ranging from 20 to 300 nm, • genome, a single nucleic acid either DNA or RNA, but never both.

  3. Differences between Bacteria & Viruses

  4. Morphology of viruses • Size: 20 – 300 nm. • Filterable – ability to pass through the filters that hold back bacteria. • Ultramicroscopic – too small to be seen under the light microscope. (Except poxviruses which can be seen under the light microscope when suitably stained).

  5. Relative size of viruses

  6. Structure • A single viral particle is called a VIRION • Nucleic acid is surrounded by protein coat calledCAPSIDwhich is made up of structural subunits called CAPSOMERES. • The Capsid and the nucleic acid together are called NUCLEOCAPSID

  7. STRUCTURE OF VIRUS

  8. Shape - Capsid symmetry • Icosahedral (Cubic)- polygon with 12 vertices and 20 facets in the shape of equilateral triangle. E.g.; Papova, picorna and adenoviruses

  9. HELICAL - Capsomeres and nucleic acid are wound together to form a helical or spiral tube. E.g.: Rabies virus, Influenza virus

  10. Complex - Symmetry is complex and not fully understood. E.g: Pox viruses

  11. Virus structures Naked icosahedral Naked helical Enveloped icosahedral spikes (glycoprotein) capsomer protomer envelope nucleic acid (protein, lipid) Enveloped helical

  12. Viral envelope: Viruses may be enveloped or non – enveloped. • Envelope is the outer covering of the viruses derived from the host cell membrane when the progeny virus is released by budding. • Envelope is a lipid bilayer with virus encoded proteins on the surface.

  13. Protein subunits may be seen as projecting spikes on the surface of the envelope, which are called peplomers(peplums – envelope). • A virus may have more than one type of peplomers. • Influenza – Haemagglutinin (HA) & Neuraminidase (NA).

  14. NUCLEIC ACID: Genome of the viruses can be double stranded (ds) or single stranded (ss). • All DNA viruses are double stranded except Parvoviruses. • All RNA viruses are single stranded with exception of Reoviruseswhich is double stranded •  Single stranded RNA viruses can be • Positive stranded – Genome acts directly as messenger RNA (mRNA) • Negative stranded – Genome is complimentary to mRNA.

  15. Susceptibility • Temp - most viruses are heat labile. Hepatitis B virus - 60C for 1 hour • Stable at low temp at -70 C • pH - 5 to 9. Enteroviruses are resistant to acidic pH. • Detergents – enveloped viruses are susceptible • Disinfectants – H2O2, Hyphochlorite, BPL; resistant to phenol

  16. REPLICATION OF VIRUSES • Viruses do not have the enzymes - depend on the synthetic machinery of the host cell for replication. • The replicative cycle can be divided into 7 steps • Adsorption or Attachment - The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of the cell. • e.g.; Influenza virus Haemagglutinin binding to sialic acid on respiratory epithelium • gp 120 of HIV binding to CD4 on T cells

  17. Virus attachment

  18. 2. Penetration - after adsorption, the coat of the enveloped viruses may fuse with the host cell membrane and release the virus nucleocapsid into the host cytoplasm. • Other viruses may enter the cell by a process of endocytosis,which involves invagination of the cell membrane to form vesicles in the cell cytoplasm.

  19. Un-coating - Outer layers of the virion including the Capsid are removed and the nucleic acid is released into the host cell. • Process occurs with the help of lysosomal enzymes of the host cell.

  20. Virus entry & uncoating

  21. 4. Biosynthesis of viral Nucleic acid & protein • Transcriptionof mRNA from the viral nucleic acid. • Translationof the mRNA into ‘early proteins’ – early or non-structural proteins are enzymes which initiate and maintain synthesis of virus components. • They may also induce shutdown of host protein and nucleic acid synthesis. • Replication of viral nucleic acid. • Synthesis of ‘late’ proteins – late or structural proteins are components of daughter viron capsids.

  22. Replication of ssDNA Viruses- ssDNAdsDNA mRNA  Proteins • First single stranded DNA is converted into dsDNA by producing a complimentary stand. This dsDNA acts as template for replication & synthesis of mRNA which are translated into viral proteins. • Replication of dsDNA Viruses– dsDNA mRNA  proteins • Only a part of DNA is transcribed into mRNA which encodes for early proteins required for DNA synthesis.

  23. Hepatitis B Life Cycle Virus particle (+) strand (-) strand DNA synthesis (+) strand DNA synthesis (-) strand Cell entry 3.5kb mRNA Uncoated DNA genome Packaging & export of infectious virus mRNA 3.5kb 2.4kb Viral proteins Translation of mRNAs 2.1kb 0.7kb

  24. Replication of RNA Viruses • In positive stranded ssRNA viruses, viral RNA directly acts as a template for production of complimentary strand which acts a s a template for synthesize of viral RNA. • Negative stranded ssRNA virusescarry their own polymerases for mRNA transcription. The viral RNA produces complimentary strands which act both as mRNA & template for synthesis of new viral RNA

  25. In dsRNA viruses the viral RNA is transcribed to mRNA by viral polymerases. • Retrovirusesexhibit a unique replicative cycle. ssRNA is converted into DNA by reverse transcriptase which forms RNA DNA hybrid, which later gets integrated into host cell genome and is called provirus.

  26. Positive (+) RNA virus replicative cycle + AAA nucleus - Viral proteins off + strand + Viral particles AAA

  27. Negative-strand virus replication 3’ (-) Strand 5’ 5’ 3’ (-) (+) (+) (+) Viral genome (-) coded off + strand RNA Protein synthesis from + strand RNA 5’ (+) 3’ Infectious virus particles

  28. Assembly / Maturation: • After synthesis of viral proteins and replication of viral nucleic acid the virons are assembled to form daughter virons. • RNA viruses are assembled in the cytoplasm, whereas DNA viruses (except Pox virus) are assembled in the nucleus.

  29. Release: • Non-enveloped viruses are released by cell lysis. • Enveloped viruses are released by budding (without cell lysis) during which they acquire their lipoprotein envelop from cell membrane

  30. Virus Assembly

  31. Eclipse phase of viruses: • From the stage of penetration of virus into the host cell till the appearance of first infectious virus progeny particle, the virus cannot be demonstrated inside the host cell. • This period is known as eclipse phase.

  32. Click after each step to view process VIRAL LIFE CYCLE ATTACHMENT HOST FUNCTIONS PENETRATION UNCOATING Transcription Translation REPLICATION ASSEMBLY (MATURATION) RELEASE MULTIPLICATION

  33. classification • Hierarchical. Families have suffix viridae. Genus have suffix virus. Species is important definition.

  34. DNA VIRUSES Parvo Papova Adeno Herpes Pox Hepadna RNA VIRUSES Picarno Calici Reo Arbo Toga Flavi Arena Corona Retro Bunya Orthomyxo Paramyxo Rhabdo

  35. Viroids: Single stranded circular RNA molecules that lack a protein coat. They are plant pathogens • Prions:They are infectious agents without any nucleic acid. • They are highly resistant to heat, UV rays and nucleases. • They cause slow infections with long incubation period. • Example for prion diseases – Scrapie of sheep, spongiform encephalopathy, Kuru and Creutzfeldt-Jakob disease.

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