VIRUSES

1 EJC H2 Biology T4W10 Viruses Genetics of Bacteria and Viruses - Viruses 1. Overview of Topic Viruses are biological entities which defy the definition of life. Outside of a host cell, a virus particle is nothing more than a speck of dust. However, once a virus infects a host cell, it has the capability of reproducing at the expense of the host itself. The Spanish flu which surfaced in 1918 lead to a pandemic which claimed more lives than those lost during the 1 st World War. Despite being one of the most microscopic entities in the biological world, viruses can threaten the existence of huma n life itself on earth. In this chapter, we will learn about the different types of viruses (Bacteriophages & Animal Viruses), their reproduction cycles as well as how they undergo evolution. 2. Learning Outcomes a. Discuss how viruses challenge the cell theory and concepts of what is considered living. b. Describe the structural components of viruses, including enveloped viruses and bacteriophages, and interpret drawings and photographs of them. c. Describe the structure and organisation of viral, prokaryotic and eukaryotic genomes (including DNA/RNA, single-/double-stranded, number of nucleotides, packing of DNA, linearity/circularity and presence/absence of introns). d. Describe how the genomes of viruses are inherited through outlining the reproductive cycles of: i. bacteriophages that reproduce via lytic cycle only, eg. T4 phage; ii. Bacteriophages that reproduce via lytic and lysogenic cycles, eg. Lambda phage; iii. enveloped viruses, eg. influenza iv. retroviruses, eg. HIV. e. Describe how variation in viral genomes arises, including antigenic shift and antigenic drift. 3. References  Biology, 9th Edition, by Campbell, N.A. and Reece J.B. (2011).  Microbiology – An Introduction, 9th Edition by Tortora, Funke and Case. (2007).  Molecular Biology of the Cell, 3rd Edition, by Alberts, Bray and Lewis. (2008).

2 EJC H2 Biology T4W10 Viruses Contents 1. Overview of Topic ...................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Introduction ............................................................................................................... 3 5. Viruses challenge the cell theory and the definition of an living organism .............. 4 6. Structure of Viruses ................................................................................................... 6 a. Genome ................................ ................................ ................................ ................. 6 b. Capsid ................................ ................................ ................................ .................... 7 c. Envelope ................................ ................................ ................................ ................ 7 d. General Morphology ................................ ................................ .............................. 8 7. Viral Replication ....................................................................................................... 10 8. Bacteriophages ........................................................................................................ 11 i. Bacteriophages which undergo the lytic cycle – T4 bacteriophage ..................... 11 ii. Bacteriophages which undergo the lysogenic cycle – lambda bacteriophage ..... 14 9. Animal Viruses ......................................................................................................... 18 a. Influenza ................................ ................................ ................................ .............. 18 b. Retrovirus – Human Immunodeficiency Virus (HIV) ................................ ............. 22 10. Variation in Viral Genomes by Antigenic Shift & Antigenic Drift ............................. 28 Antigenic drift ................................ ................................ ................................ .............. 28 Antigenic shift ................................ ................................ ................................ .............. 30 11. Pathogenicity of Animal Viruses .............................................................................. 33 a. Influenza ................................ ................................ ................................ .............. 33 b. HIV ................................ ................................ ................................ ....................... 34 c. Pathogenesis of other viruses ................................ ................................ .............. 36 12. Links ..................................................................................................................... 38 13. Glossary ............................................................................................................... 38

3 EJC H2 Biology T4W10 Viruses 4. Introduction A Brief History of Virology: 1796: Edward Jenner used cowpox to vaccinate against smallpox. Jenner was the first person to deliberately vaccinate against any infectious disease. Although Jenner is commonly given the credit for vaccination, variolation, the practice of deliberately infecting people with smallpox to protect them from the worst type of the disease, had been practiced in China at least two thousand years previously. 1885: Louis Pasteur experimented with rabies vaccination, using the term "virus" (Latin, poison) to describe the agent. Although Pasteur did not discriminate between viruses and other infectious agents, he originated the terms "virus" and "vaccination" (in honour of Jenner) and developed the scientific basis for Jenner's experimental approach to vaccination. 1892: Dimitri Iwanowski described the first "filterable" infectious agent - tobacco mosaic virus (TMV) - smaller than any known bacteria. Iwanowski was the first person to discriminate between viruses and other infectious agents, although he was not fully aware of the significance of this finding. 1898: Martinus Beijerinick extended Iwanowski's work with TMV and formed the first clear concept of the virus "contagium vivum fluidum" - soluble living germ. Freidrich Loeffler and Paul Frosch demonstrated that foot and mouth disease is caused by such "filterable" agents. Loeffler and Frosch were the first to prove that viruses could infect animals as well as plants. 1915: Frederick Twort discovered viruses infecting bacteria. 1917: Felix d'Herelle independently discovered viruses of bacteria and coins the term bacteriophage. The discovery of bacteriophages provided an invaluable opportunity to study virus replication at a time prior to the development of tissue culture when the only way to study viruses was by infecting whole organisms. 1935: Wendell Stanley crystallized TMV and showed that it remained infectious (Nobel Prize, 1946). Stanley's work was the first step towards describing the molecular structure of any virus and helped to further illuminate the nature of viruses. 1939: Emory Ellis and Max Delbruck established the concept of the "one step virus growth cycle" essential to the understanding of virus replication (Nobel Prize, 1969). This work laid the basis for the understanding of virus replication - that viral particles do not "grow" but are instead assembled from preformed components. 1940: Helmuth Ruska used an electron microscope to take the first pictures of viral particles. Along with other physical studies of viruses, direct visualization of virions was an important advance in understanding virus structure. 1961: Sydney Brenner, Francois Jacob, and