Microbiology: A Microbiology: A Systems Approach, 2 Systems Approach, 2 nd nd ed. ed. Chapter 6: An Introduction to the Chapter 6: An Introduction to the Viruses Viruses 6.1 The Search for the Elusive 6.1 The Search for the Elusive Virus Virus  Viruses were too small to be seen with the first Viruses were too small to be seen with the first microscopes microscopes  The cause of viral infections was unknown for The cause of viral infections was unknown for years years  Louis Pasteur first proposed the term Louis Pasteur first proposed the term virus virus  1890s 1890s  Ivanovski and Beijerinck showed that a disease in Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus tobacco was caused by a virus  Loeffler and Frosch discovered an animal virus that Loeffler and Frosch discovered an animal virus that causes foot –and-mouth disease in cattle causes foot –and-mouth disease in cattle  Many years of experimentation showed what we Many years of experimentation showed what we know today and by the 1950s virology had grown know today and by the 1950s virology had grown 6.2 The Position of Viruses in the 6.2 The Position of Viruses in the Biological Spectrum Biological Spectrum  Can infect every type of cell Can infect every type of cell  Cannot exist independently from the host cell, so Cannot exist independently from the host cell, so aren’t considered living things aren’t considered living things  However, since they can direct life processes they However, since they can direct life processes they are often considered more than lifeless molecules are often considered more than lifeless molecules  Referred to as infectious particles, either active or Referred to as infectious particles, either active or inactive inactive  Obligate intracellular parasites Obligate intracellular parasites  Cannot multiply unless they invade a specific host Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic cell and instruct its genetic and metabolic machinery to make and release new viruses machinery to make and release new viruses [...]... Core of a Virus Genome- the sum total of the genetic information carried by an organism  Number of viral genes compared with a callquite small  They only have the genes necessary to invade host cells and redirect their activity  Some viruses are exceptions to the rules re: DNA and RNA    Parvoviruses contain single-stranded DNA Reoviruses contain double-stranded RNA DNA Viruses  ssDNA  dsDNA ... linear circular RNA Viruses  Mostly single-stranded   Positive-sense RNA: genomes that are ready for immediate translation into proteins Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins  Segmented- individual genes exist on separate pieces of RNA Other Substances in the Virus Particle  Other Substances in the Virus Particle    Can contain enzymes for. .. operations within the host cell Polymerases to synthesize DNA and RNA Replicases to copy RNA 6.4 How Viruses are Classified and Named  Main criteria     International Committee on the Taxonomy of Viruses, 2000     Structure Chemical composition Similarities in genetic makeup 3 orders 63 famillies 263 genera “-viridae” “ -virus Some virologists use a species naming system, but it is not an official... proteins form a binding layer between the envelope and the capsid  Glycoproteins remain exposed as spikes (peplomers)- essential for attachment Functions of the Viral Capsid/Envelope  Protects nucleic acids  Help introduce the viral DNA or RNA into a suitable host cell  Stimulate the immune system to produce antibodies that can protect the host cells against future infections Nucleic Acids: At the... official designation 6.5 Modes of Viral Multiplication  The host cell is absolutely necessary for viral multiplication Figure 6.11 Multiplication Cycles in Animal Viruses  Adsorption  Penetration  Uncoating  Synthesis  Assembly  Release Adsorption  Virus encounters susceptible host cells  Adsorbs specifically to receptor sites on the cell membrane • Because of the exact fit required, viruses have... Figure 6.6  Icosahderal   Three-dimensional, 20-sided figure with 12 evenly spaced corners Although they all display this symmetry, there are wide variations Figure 6.7 Figure 6.8 Figure 6.9 Figure 6.10 The Viral Envelope  Enveloped viruses take a bit of the host cell membrane in the form of an envelope  In the envelope, some or all of the regular membrane proteins are replaced with viral proteins... have a limited host range Figure 6.12 Penetration  Flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid  Endocytosis: entire virus engulfed by the cell and enclosed in a vacuole or vesicle  The viral envelope can also directly fuse with the host cell membrane Figure 6.13 Uncoating  Enzymes in the vacuole dissolve the envelope and capsid  The virus is now uncoated . showed that a disease in Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus tobacco was caused by a virus  Loeffler and Frosch. DNA and RNA DNA and RNA  Parvoviruses contain single-stranded DNA Parvoviruses contain single-stranded DNA  Reoviruses contain double-stranded RNA Reoviruses