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Structure & Replication of Viruses By Dr. Marwa Salah

Structure & Replication of Viruses By Dr. Marwa Salah. Types of Microbes. Virus like-agents Viruses Bacteria Fungi Protozoa Helminths. Viruses can infect all organisms in nature:. Bacteriophages: bacterial viruses. Plant viruses: include complete viruses & viroids.

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Structure & Replication of Viruses By Dr. Marwa Salah

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  1. Structure & Replication of Viruses By Dr. Marwa Salah

  2. Types of Microbes • Virus like-agents • Viruses • Bacteria • Fungi • Protozoa • Helminths

  3. Viruses can infect all organisms in nature: Bacteriophages: bacterial viruses. Plant viruses: include complete viruses & viroids. Animal viruses: infect insects or vertebrates including man.

  4. Differences between Viruses & Bacteria • Viruses are very small in size, ranging form 20-300 nm. • They can only be seen under the electron microscope (except poxviruses). • They can pass through bacterial filters. • They need ultracentrifugation for sedimentation. • Viruses contain only one type of nucleic acid (DNA or RNA), never both. • They are obligatory intracellular parasites, i.e. can only replicate inside living cells. • They can not be cultivated in the laboratory on artificial culture media. • They are not susceptible to antibacterial agents.

  5. CLASSIFICATION OF VIRUSES A- Classification by symptomatology: • It is the old classification based on diseases they produce, i.e. tropism, e.g. neurotropic viruses, enteroviruses, ... etc. B- Classification of viruses into orders, families, genera & species, based on: 1. Type of nucleic acid (DNA viruses & RNA viruses). 2. Virus size. 3. Virus symmetry. 4. Presence or absence of envelope.

  6. CLASSIFICATION OF VIRUSES • Order (…..virales) • Family (….viridae) e.g. Herpesviridae • Subfamily (….virinae) e.g. Alphaherpesvirinae • Genus (…. Virus) e.g. Herpes virus • Species e.g. HSV-1

  7. STRUCTURE & COMPOSITION OF VIRUSES • The complete virus particle (virion) is composed of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid). • The nucleic acid & the protein coat are called nucleocapsid. • Some viruses have additional lipoprotein layer called envelope, other viruses are non-enveloped (naked). • Matrix proteins between the capsid & the envelope. • DNA &RNA polymerases

  8. Viral Nucleic Acid (Genome) • It is responsible for virulence, i.e. it is the infectious part of the virus. Also, it carries the genetic information of the virus. • Only one type of nucleic acid is present in the virus, DNA or RNA, never both. • Most DNA viruses are double-stranded (ds) while most RNA viruses are single-stranded (ss). • The viral ssRNA may be positive sense strand (mRNA or +ve sense) or negative sense strand (-ve sense). It may be segmented or non-segmented.

  9. DNA viruses Enveloped Naked (ds) Pox Herpes Hepadna (ds) Polyoma Papilloma Adeno (ss) Parvo Pox, e.g., Smallpox virus Herpes, e.g., Herpes simplex virus; Hepadna, e.g., Hepatitis B virus Papilloma, e.g., Papillomavirus; Parvo, e.g., Parvovirus B19

  10. RNA viruses (+)ssRNA Via DNA (+)ssRNA (-)ssRNA dsRNA enveloped Naked enveloped Naked enveloped Picorna Noro Toga Flavi Corona Rhabdo Orthomyxo Paramyxo Bunya Arena Filo Retro Reo

  11. RNA viruses Picorna, e.g., Poliovirus, Hepatitis A virus Toga, e.g., Rubella virus; Flavi, e.g., Hepatitis C virus Rhabdo, e.g., Rabies virus Orthomyxo, e.g., Influenza virus; Reo, e.g., Rotavirus Retro, e.g., Human Immunodeficiency virus.

  12. Viral Capsid • It is a protein shell formed of polypeptide subunits called capsomeres. • Viral capsid has the following functions: • It protects the nucleic acid. • It mediates attachment to host cell (in non-enveloped viruses). • It is the antigenic part of the virus. • It is responsible for the viral morphology (or symmetry).

  13. Viral Symmetry 1. Icosahedral symmetry:Icosahedral or isomeric or cubic viruses resemble a crystal with 20 triangular facets & 12 corners. This include all DNA viruses, except poxviruses (brick-shaped) & some RNA viruses. 2. Helical symmetry:It is formed of repeated polypeptide units (protomers)The viral nucleic acid is forming a coil-shaped helical nucleocapsid. This includes many of RNA viruses, e.g. rabies virus. 3. Complex symmetry:e.g. the brick-shaped poxviruses & bacteriophages.

  14. Icosahedral symmetry Helical symmetry

  15. Complex Symmetry

  16. Viral Envelope • It is formed of lipids or lipoproteins & is derived partly from the host cell membrane during release by budding. • It is sensitive to lipid solvents, e.g. ether. • It has glycoprotein spikes which are the organ of attachment of the enveloped virus to host cell receptors.

  17. VIRUS REPLICATION • Viruses are inert particles without metabolic activities. They depend on living host cells for providing the virus components under the information given by the virus genome. • With the exception of the poxviruses, all DNA viruses replicate in the nucleus of the host cell. • With the exception of the orthomyxoviruses & retroviruses, all RNA viruses replicate in the cytoplasm of the host cell.

  18. STAGES OF VIRUS REPLICATION • Attachment or adsorption:Adsorption of the virus by specific viral structure to specific receptor sites on the surface of the susceptible host cell (tropism). • Penetration (Internalization): This occurs either by: • Endocytosis in case of non-enveloped viruses • Fusion of viral envelope with host cell membrane in case of enveloped viruses.

  19. 3. Uncoating:The nucleic acid is released from the capsid by lysosomal enzymes & is available for replication. 4. Eclipse:This is the time from uncoating until assembly of mature viruses. During this phase, no infectious viruses can be detected in the host cell. 5. Synthesis of viral components: Synthesis of early proteins before NA synthesis and late (structural) proteins after NA synthesis

  20. Genome replication (+)ssRNA genome c(-)ssRNA (-)ssRNA genome c(+)ssRNA dsDNA Gene Transcription • Transcription of the viral nucleic acid into specific messenger RNAs according to the type of viral nucleic acid whether DNA or RNA, ds or ss, positive or negative sense strand. • Translation of messenger RNAs in the cell ribosomes to form viral components.

  21. STAGES OF VIRUS REPLICATION DNA-Viruses ------- mRNA (by transcriptase enzyme). RNA-Viruses- Positive strand (+ssRNA) ---- acts directly as mRNA. - Negative strand (-ssRNA) ------- positive RNA strand (mRNA) by RNA polymerase.

  22. STAGES OF VIRUS REPLICATION Retroviruseswhich contain +ssRNA, by the action of the reverse transcriptase enzymes, will produce complementary ss DNA (cDNA) --------ds DNA. +ssRNA reverse transcriptase (cDNA) ds DNA. • This is either integrated in host cell genome causing transformation or is transcribed to mRNA.

  23. 6. Assembly (morphogenesis):The nucleic acids are enclosed within the protein coats to form mature viruses (virions). This occurs either in the nucleus of host cell, e.g. herpes viruses or in the cytoplasm, e.g. polioviruses. 7. Release:New viruses are released either by: a- Lysis of host cell & release of new viruses in case of non-enveloped viruses. b- Budding through the cell membrane in case of enveloped viruses.

  24. C- Unclassifed viruses & agents: I- Defective virus particles: • They are small viral particles that can not replicate unless they are present with a helper virus. This is due to lack of genetic information necessary for completion of the replication cycle, e.g.: 1- Delta agent or hepatitis D virus (HDV):It requires hepatitis B virus (HBV) to replicate. 2- Adeno-associated viruses (AAV):These are particles found in adenovirus preparations. III- Viroids: circular ssRNA

  25. II- Unconventional transmissible agents (Prions) • (Infectious Proteins, no nucliec acid) • They resist inactivation by proteases • Cellular prion protein (PrPc) has alpha helical structure and is encoded by host chromosomal DNA. It is sensitive to proteases and soluble in detergents. • Scrapie prion protein ( PrPcs has high abnormal beta-sheet content. It is resistant to proteases and insoluble in detergents. It is transmissible and causes diseases.

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