Microbiology Chapter 12 Viruses (PDF)

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