Medical Microbiology Chapter 6 Viral Classification, Structure, and Replication

Introduction To Viruses  Viruses are obligate intracellular parasites –they cannot reproduce outside a living cell  Viruses do not contain an ATP-generating system  Viruses do not contain any protein synthesizing machinery  What is the structure of a virus?

Introduction To Viruses (cont.)  Classification: –Genetic material  DNA viruses (double- and single-stranded)  RNA viruses (double- and single-stranded) –Presence of an envelope –Means of reproduction

Virion Structure  The virion or virus particle consists of: –nucleic acid (RNA or DNA) –surrounded by a protein coat - Capsid –envelope (if present)  Originates from the host cell’s membrane  The capsid (for animal viruses) comes in 2 basic shapes: –icosahedral - many-faced, spherical shape –helical - tube of protein –Figures 6-5, 6-6, and 6-7

Viral Structure (cont.)  Some virions also contain essential enzymes or other proteins  Reverse transcriptase in HIV  Figure 6-5  The capsid or the envelope has special attachment proteins or glycoproteins that mediate the attachment of the virus to the target cell –Virus Attachment Proteins (VAP) –Figures 6-5 and 6-7

Viral Structure (cont.)  Naked viruses – very resistant to drying, acids, detergents, etc.  Enveloped viruses – must remain in aqueous solution –They are damaged by drying, acids, detergents, etc.  Importance in Transmission  Different Types of Viruses – Figure 6-4 and Tables 6-1, 6-2, 6-3, and 6-4

Viral Replication  How viruses affect the cells they infect depends on the virus and the host cell –Some viruses will immediately begin producing new virions  if the new virions are released by the cell breaking open, the cell dies (most naked viruses)  some virions can leave the cell without killing it (most enveloped viruses) –Other viruses will insert their DNA into the host chromosome and cause a latent infection

Viral Replication  Contains six steps:  Attachment – VAPs attach to cell receptors –This determines the virus host range and tissue tropism  Epstein-Barr virus~C3d on human B-cells  rhinovirus~ICAM-1 of epithelial tissue  HIV~CD4 on helper T-cells  Influenza virus~sialic acid on epithelial cells  rabies virus~acetylcholine receptors on neurons

Viral Replication  Penetration - this occurs by: –Receptor-mediated endocytosis (most naked viruses) –Viropexis – naked viruses slip directly through the membrane –Membrane fusion – enveloped viruses  Uncoating - the capsid is lost, exposing the DNA or RNA

Viral Replication  Macromolecular synthesis of Viral Proteins and Nucleic Acids –How would this work for DNA viruses? –In what ways would the process be different for RNA viruses  Positive sense (+) RNA  Negative sense (-) RNA

Viral Replication –Retroviruses are especially complex  these are RNA viruses that convert their RNA into DNA  the enzyme reverse transcriptase is responsible for this conversion  the DNA is then incorporated into the host cell’s genome  the viral DNA is then transcribed and translated to produce more virions

Viral Replication  Assembly – once all the necessary proteins and nucleic acids are synthesized, –They automatically assemble into new virions –This happens in the cytoplasm or nucleus in naked viruses –It happens at the membrane for enveloped viruses

Viral Replication  Release – the virions exit the cell –Cell lysis or exocytosis for naked viruses –Budding for enveloped viruses

Disease of the Day  West Nile Virus – transmitted from birds to humans by mosquitoes –Details Details