Microbiology Chapter 12 Viruses (PDF)
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Uploaded by UncomplicatedConnemara657
University of British Columbia Okanagan Campus
2021
John Foster, Zarrintaj Aliabadi, Joan Slonczewski
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Summary
This document is Chapter 12 of a Microbiology textbook, focusing on the topic of viruses. It explains the nature of viruses, their relationship to cells, their genomes, and the different types of viral structure. The text also discusses the theories behind viral origin.
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CHAPTER 12 Viruses Copyright © 2021 W. W. Norton & Company, Inc. Viruses Chapter Objectives ▪ Define the nature of a virus and explain how a virus differs from a cell. ▪ Describe the major classes of viral genomes and cite an example of each. ▪ Describe the major components making the vir...
CHAPTER 12 Viruses Copyright © 2021 W. W. Norton & Company, Inc. Viruses Chapter Objectives ▪ Define the nature of a virus and explain how a virus differs from a cell. ▪ Describe the major classes of viral genomes and cite an example of each. ▪ Describe the major components making the virions’ structure. 2 12.1 Virus Infection and Viral Genomes Section Objectives ▪ Describe how viruses infect cells and cause disease. ▪ Define transmission, host range, and tissue tropism. ▪ Describe the different classes of viral genomes and give an example of each. 3 ▪ Viruses are omnipresent, infecting every taxonomic group of organisms including bacteria, eukaryotes, and archaea. ▪ Viruses cause important human diseases like influenza and the common cold. From https://patientser.com/difference-common-cold-influenza/ 4 ▪ Obligate Intracellular bacteria (ex. Chlamydia spp.) develop inert spore-like forms that survive outside the host → metabolize only during growth in a host cell → comparable to virus particles (except that they possess ribosomes). Infectious Agents and Neurodegeneration - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Chlamydiae-are-Gram-negative-bacteria-They-are-obligate- intracellular-parasites-because_fig3_230685741 [accessed 15 Oct 2024] Ryu WS. Virus Life Cycle. Molecular Virology of Human Pathogenic Viruses. 2017:31–45. doi: 10.1016/B978-0- 12-800838-6.00003-5. Epub 2016 May 6. PMCID: PMC7158286. Virus origin - Theories 1.- Virus-First Model Definition: Proposes that viruses predate or co-evolved alongside cellular life. Key Concept: Viruses are ancient, possibly arising before cells. Virus origin - Theories 2.- Reduction Model (Regressive Model) Definition: Suggests viruses were once small parasitic cells that lost complex functions over time. Key Concept: Viruses evolved by regressing from free-living organisms. Gradually lost genes necessary for independent life → obligate intracellular parasites. Virus origin - Theories 3.- Escape Model (Progressive Model) Definition: Proposes that viruses originated from genetic elements that "escaped" from cells. Key Concept: Viruses evolved from pieces of RNA or DNA that gained the ability to move between cells (ex. Plasmids). Mughal, F., Nasir, A. & Caetano-Anollés, G. The origin and evolution of viruses inferred from fold family structure. Arch Virol 165, 2177–2191 (2020). https://doi.org/10.1007/s00705-020-04724-1 Viruses Replicate in Host Cells ▪ Viral infection → Viral replication → Viral disease ▪ Effects: Debilitation and/or death of the host cell. Wagner, Robert R. and Krug, Robert M.. "virus". Encyclopedia Britannica, 10 Oct. 2024, https://www.britannica.com/science/virus. Accessed 15 October 2024. 9 Viruses Replicate in Host Cells ▪ Host range: The capacity of the virus to infect a particular group of host species, known as the. ▪ Tissue tropism: The range of tissue types a virus can infect. ▪ Both depend on various host factors, most importantly the surface receptor molecules. 10 Viruses Replicate in Host Cells ▪ In all cases, viruses use a relatively small number of virus- encoded proteins to commandeer the metabolism of their hosts: Antiviral agents are hard to discover. Antiviral agents have severe side effects. Viral genomes mutate fast, even faster than bacteria. 11 Viral Genomes – 1 ▪ A viral genome: either DNA or RNA. double- or single-stranded. ▪ The capsid keeps the viral genome intact outside the host. ▪ Envelope formed out of host membranes with embedded viral envelope proteins in some species. 12 ▪ The Baltimore model classification of viral groups is based on: The genome is composed of RNA or DNA single- or double-stranded if single-stranded, whether the strand directly encodes proteins or requires synthesis of a complement that encodes proteins (+ sense or – sense respectively). Viral Genomes – 2 ▪ The Baltimore model 14 12.2 Viral Structure and Diversity Section Objectives ▪ Describe the different types of viral structure, and give an example of each. ▪ Explain the significance of viral genome size. ▪ Describe the nature of viroids and prions. ▪ Explain why viruses evolve so fast. 15 ▪ The International Committee on Taxonomy of Viruses (ICTV) classifies viruses based on these factors: ▪ Capsid form (icosahedral or filamentous). ▪ Envelope (present or absent). ▪ Host range. Virus Structure: Capsid and Envelope – 1 ▪ Icosahedral capsids Radial symmetry; based on an icosahedron, a polyhedron with 20 identical triangular faces. – Example: herpes simplex virus (HSV) 17 Virus Structure: Capsid and Envelope – 2 ▪ Envelope derived from the host membrane. ▪ The envelope bristles with virus-encoded spike proteins that plug the membrane onto the capsid. 18 Viral Structure: Capsid and Envelope – 3 ▪ Filamentous Helical symmetry A helical tube around the genome Generating a flexible filament Examples: – Ebola virus 19 Viral Structure: Capsid and Envelope – 4 ▪ Complex-tailed bacteriophages Head – Icosahedral protein package – Contains genetic material Tail – Injects genetic material into host cells 20 Viral Structure: Capsid and Envelope – 5 ▪ Amorphous or complex viruses No symmetrical form Flexible “core wall” – Contains genetic material Viral envelope – Studded with spike proteins Examples: – Smallpox virus 21 Viral Genome Size ▪ RNA Small genomes Retroviruses ▪ DNA Large genomes Herpes simplex virus 22 Viral Diversity and Evolution – 1 ▪ Antigenic drift: rapid mutation New strains of virus – Changes in the genome – Natural selection No longer recognized by antibodies – Example: Influenza virus continually generates new strains requiring repeated immunizations. 23 Chapter 12 Animations Influenza Virus Replication HIV Replicative Cycle https://digital.wwnorton.com/michum2 26 Summary ▪ Virus Structure & Classification Structure: Capsid: Protein shell (Icosahedral, Helical, Complex) Genome: DNA or RNA (single or double-stranded) Envelope: Lipid layer (optional) with glycoproteins Enzymes: Reverse transcriptase, integrase (optional) Classification: By Genome: DNA vs. RNA viruses Baltimore Classification: 7 groups based on replication method By Host: Animal, Plant, Bacteria (phages) Tropism: Target tissues/cells Questions?? :D