Introduction to Virology PDF

Introduction to Virology Joshua Anzinger Objectives Describe the major structural components of a virus Describe how viruses are classified Demonstrate the general replication steps necessary for all viruses Link mutation and adaptation Gain a basic understanding of viral diagnostic methodologies Wh...

Introduction to Virology Joshua Anzinger Objectives Describe the major structural components of a virus Describe how viruses are classified Demonstrate the general replication steps necessary for all viruses Link mutation and adaptation Gain a basic understanding of viral diagnostic methodologies Where did viruses come from? Regressive evolution (parasitism) – Viruses degenerated from previously independent life forms – Lost many functions – Retain only what they needed for parasitic lifestyle Cellular origins – Viruses derived from subcellular functional assemblies of macromolecules that gained the capacity to move from cell to cell Independent entities – Evolution on course parallel to that of cellular organisms – Evolved from primitive, pre-biotic self-replicating molecules Viruses are Abundant Numbers on earth: – Humans: 7.4 x 109 – Insects: 10 x 1018 – Bacteria: 5 x 1030 – Viruses: 1 x 1031 (likely an underestimate) 219 viruses can infect humans (as of 2012) Most newly discovered human pathogens are viruses In theory, how many genes would the “simplest” virus have? a) 1 b) 2 c) 3 d) 10 In theory, how many genes would the “simplest” virus have? a) 1 b) 2 c) 3 d) 10 Virus = small (10-400 nanometers), acellular, non-living, infectious particle containing nucleic acid and capsid that cannot metabolize, grow or reproduce without the aid of a host Virus Genetics DNA dsDNA ssDNA RNA dsRNA ssRNA Virus Size Major Differences Between Cellular & Viral Pathogens Cell Virus Intracellular Parasite Some Yes Membrane Yes Some Possess both DNA & RNA ATP Production Yes No Yes No Ribosomes Yes No Visible by Light Microscopy Yes No Structure Helical Icosahedral Complex Virus Classification Order is the highest classification but rarely used. Ends in -virales Family often the highest classification. Ends in -viridae Many families have subfamilies. Ends in –virinae Genus closely related. Ends in –virus morphology Based on: composition replication Species. Ends in –virus Examples: Dengue virus HCV Baltimore Class IV (+ssRNA) Family Flaviviridae Genus Flavivirus Species Dengue virus Baltimore Class IV (+ssRNA) Family Flaviviridae Genus Hepacivirus Species Hepatitis C virus Virus Classes – Baltimore Classification DNA RNA Reverse Transcription (RNA -> DNA) (+)ssRNA (-)ssRNA Complementary ssRNA must be made first Permissive = capable of allowing the complete replication cycle Non-Permissive = incapable of allowing the complete replication cycle Viral Tropism = virus specificity for particular cell type, tissue or species Early Phase Attachment & Penetration Fusion Capsid Rearrangment The cytoplasm can limit diffusion Ribosome (~25 nm) Human adenovirus (~80 nm) Cellular Microbiology (2006) 8(3), 387–400 Late Phase Cellular Microbiology (2006) 8(3), 387–400 Release Lysis NAKED NUCLEAR, ER, or GOLGI MEMBRANE PLASMA MEMBRANE Exocytosis Budding ENVELOPED NAKED/ ENVELOPED Viral Replication and Medicine 1. Adsorption 2. Vaccine Penetration 3. Enfuvirtide (HIV) Uncoating 4. Adamantanes (Influenza) Synthesis 5. Polymerase drugs (HIV, Herpes viruses, HBV, HCV) Assembly 6. Non-structural protein (HCV) Release Zanamavir & oseltamavir (Influenza) Mutations – base change Physical/chemical Spontaneous – X-ray – UV – Formation of different base (tautomeric) – Polymerase error Mutations - Polymerase Error DNA viruses – polymerase proofreading – 10-6-10-8 errors/base RNA viruses – NO polymerase proofreading – 10-4-10-6 errors/base 10,000 bases 1 x 10-7 errors virus base = 0.001 error virus 1 error every 1,000 viruses produced 10,000 bases 1 x 10-5 errors virus base = 0.1 errors virus 1 error every 10 viruses produced Diagnostic Methods Direct examination of specimen for: – The presence of virus particles (e.g., electron microscopy) – Viral antigen (e.g., ELISA) – Viral nucleic acid (e.g., PCR) Indirect examination of specimen – Virus isolation (culture) & identification Serological diagnosis – Assay of antibodies - developed during the course of infection Electron Microscopy—Direct Method Electron Microscopy (EM) (time: ½ day-1 day) Virus particles are identified by morphology (magnification X50-100,000) Transmission Electron Microscopy (TEM) 2D sections Useful for diagnostics Transmission Electron Microscope ELISA—Direct Method Break apart doublestranded DNA 1 cycle ~95ºC Primers anneal quicker than denatured strand ~50-60ºC Polymerase active ~72ºC PCR—Direct Method Cell Culture—Indirect Method No sample inoculation Sample inoculation Serology Method Recent infection Presence of IgM A 4-fold or more increase in IgG antibody titre between acute (1-3 days) and convalescent (10-21 days) blood Past infection Presence of IgG and absence of IgM Positive IgG titre less than 4fold increase END