Viruses, Viroids, and Prions - PDF

VIRUSES, VIROIDS, AND PRIONS VIRUSES  Where do viruses fit in the classification of life?  What domain are they in? VIRUSES Viruses are not classically included in the classification of organisms! Non-cellular Obligate intracellular parasite...

VIRUSES, VIROIDS, AND PRIONS VIRUSES  Where do viruses fit in the classification of life?  What domain are they in? VIRUSES Viruses are not classically included in the classification of organisms! Non-cellular Obligate intracellular parasites Alive or not? VIRUS ANATOMY 2 parts to every virus: Outer capsid—composed of protein subunits Inner core—either DNA or RNA Viral genome very small Some viruses have: Spikes for attachment to host cell Outer envelope VIRUS REPRODUCTION  Has a specific host cell  Once inside host cell, viral genome takes over the metabolic machinery to make more viruses  Cannot reproduce without its host VIRUS REPRODUCTION  Lytic Cycle- virus takes over immediately; results in the destruction of the infected cell and its membrane.  Lysogenic Cycles- virus waits for the right conditions to take over the host cell. LYTIC CYCLE  Virus takes over immediately  Host cell dies immediately after virus is released  5 stages: 1. Absorption 2. Entry 3. Replication 4. Assembly (maturation) 5. Release LYSOGENIC CYCLE  Virus lies dormant until triggered to enter lytic cycle  Bacteria does not die until phage enters lytic cycle  After absorption and entry, phage integrates into host chromosome as a prophage  When conditions are right, some trigger causes phage to enter lytic cycle where it will replicate, maturation, and release killing the host cell. LYTIC VS LYSOGENIC NATURAL RESERVOIRS & VECTORS Reservoir Species- living host of a virus (plant or animal) without causing disease of the host itself. Viral Vector- transmits viruses from host to host (usually insects) ENZOOTIC CYCLE VS EPIZOOTIC CYCLE Enzootic Disease– disease that regularly affects animals in a particular district or at a particular season. Epizootic Disease– disease that is temporarily prevalent and widespread in an animal population. VIRAL CLASSIFICATION  Viral classification is more complicated than living things since they evolved from different ancestors. Viruses represent “remnants” of different organisms, so even genomic or protein analysis isn’t useful. Viruses have no common genomic sequence that they all share!  In the past, biologist tried to classify them by a few different techniques:  Genome structure (Nucleotide Classification):  DNA vs RNA genome  Single-stranded vs double-stranded genome  Linear or Circular genome  Non-segmented vs segmented genomes  Capsid structure (Morphological):  Naked icosahedral  Enveloped icosahedral  Enveloped helical  Naked helical  Complex with many proteins  Group Characteristics Mode of mRNA Production Example Baltimore Viral I dsDNA* mRNA is transcribed directly from the DNA template Herpes simplex (herpesvirus) Classification II ssDNA* DNA is converted to double- stranded form before RNA is Canine parvovirus transcribed (parvovirus)  The most commonly and mRNA is transcribed from the Childhood III dsRNA gastroenteritis currently used system of RNA genome (rotavirus) virus classification Common cold IV ssRNA (+) Genome functions as mRNA  First developed by Nobel (picornavirus) Prize-winning biologist V ssRNA (-) mRNA is transcribed from the Rabies (rhabdovirus) RNA genome David Baltimore in the early 1970s. Reverse transcriptase makes ssRNA viruses with DNA from the RNA genome; DNA Human  In addition to the VI reverse is then incorporated in the host immunodeficiency virus transcriptase genome; mRNA is transcribed (HIV) differences in morphology from the incorporated DNA and genetics, groups The viral genome is double- viruses according to stranded DNA, but viral DNA is dsDNA viruses with replicated through an RNA how the mRNA is VII reverse Hepatitis B virus produced during the intermediate; the RNA may serve (hepadnavirus) transcriptase directly as mRNA or as a replicative cycle of the template to make mRNA virus. * ds= double stranded ss= singles stranded HOST DOMAIN VIRAL CLASSIFICATION  Viruses can also be classified based on the type of host they use:  Animal viruses- The viruses which infect and live inside the animal cell including man are called animal viruses. Example: influenza virus, rabies virus, mumps virus, poliovirus etc. Their genetic material is RNA or DNA.  Plant viruses- The viruses that infect plants are called plant viruses. Their genetic material is RNA which remains enclosed in the protein coat. Some plant viruses are tobacco mosaic virus, potato virus, beet yellow virus and turnip yellow virus etc.  Bacteriophages- Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as genetic material. There are many varieties of bacteriophages. Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria. EMERGING VIRUSES  Causative agent of a disease that only recently has infected large numbers of people  COVID-19, HIV, West Nile virus, SARS virus, Hantavirus, Ebola virus, Avian influenza (H5N1) virus, Zika Virus  Emerge in several ways: 1. Transported to new location (air travel) 2. Able to infect new species 3. New mode of transmission https://www.nytimes.com/2021/02/15/health/scientists-viruses.html Emerging diseases. Back again! Back again! Get’s meaner and Get’s meaner and Meaner each time. Meaner each time. Back again! Get’s meaner and Meaner each time. Huntaviru https://www.pbs.org/newshour/show/southern-china-hotbed-disease-developm ANTI-VIRAL MEDICATION  Difficult to develop since it uses its host’s cell’s own machinery  Some drugs structurally like nucleotides, interferes in the viral genome synthesis  Some block viral enzymes like reverse transcriptase in HIV  Best option is prevention!  Vaccines  Distance  Masks  Viroids and Prions Viroids  Only affects plants  Naked strands of RNA not covered by capsid  Like virus takes over cell to make more viroids  About a dozen crop diseases PRIONS  Proteinaceous infectious particles  Misshaped protein changes normal protein into more misshaped protein  Discovered as cause of kuru  Also mad cow disease and Creutzfeldt-Jakob disease Kuru- deadly prion  Affected many people of Papua New Guinea  Prior to the 1950s, a dead family member was cooked and eaten.  Brain most likely to pass kuru https://www.youtube.com/watch?v=ZHOy6hiUcyA