Viruses Chapter 19 HIV virus Ebola virus H1N1 flu virus T4 bacteriophage Smallpox virus

 Virus: an infectious particle consisting of little more than gene packaged in a protein coat.  Why are they important?  Basic molecular biology concepts were discovered using viruses.  DNA and mRNA  Translation and transcription  Enhancers  Viruses can be used as experimental systems.  The study of viruses has led to the development of techniques that enable scientists to manipulate genes and transfer them from one organism to another (gene therapy).  Biological control (population control).  Studying viruses helps scientists understand how viruses cause disease. A Borrowed Life

 Polio  HIV (Retroviridae)  Influenza/Avian flu (Orthomyoviridae)  Ebola (Filoviridae)  West Nile (Flaviviridae)  Hepatitis (several families)  Herpes (Herpesviridae)  HPV (Papillomaviridae)  SARS (Coronaviridae)  Smallpox (Poxviridae)  Common Cold Diseases Caused by Viruses

 In 1883, Adolf Mayer discovered that he could transmit tobacco mosaic disease from plant to plant by rubbing sap from diseased leaves onto healthy leaves.  Suggested caused by small bacteria that were invisible under a microscope.  In 1892, Dmitri Iwanowsky passed infected sap through a filter designed to remove bacteria. Sap still caused disease.  Iwanowsky still clung to the idea that bacteria caused the disease. Discovery of Viruses… Using tobacco mosaic disease (stunts growth and gives leaves a mosaic coloration)

In 1899, Martinus Beijerinck filtered infected sap through a porcelain filter known to trap bacteria; sap still caused disease. He also noted that the agent that caused disease could not be cultivated on a nutrient plate.  He had the courage to champion a new concept.  Called the agent Contagium vivum fluidum (contagious living fluid).

Beijerinck’s suspicions were confirmed in 1935 when the American scientist Wendell Stanley crystallized the infectious particle, now known as the tobacco mosaic virus.  Won the Noble Prize in  This data freaked scientists out; a chemical thing was behaving as if it was living.

 Smallest viruses are 25nm in diameter.  Smaller than a ribosome.  Parvovirus  Largest viruses are 400 nm in diameter.  Barely visible under a light microscope.  Mimivirus Size of Viruses

Nucleocapsid (virion) Definition: viral genome and capsid Viral Genomes  Genetic Material  Double stranded DNA (dsDNA)  Single stranded DNA (ssDNA)  Double stranded RNA (dsRNA)  Single stranded RNA (ssRNA).  Genome is usually single linear or circular molecule of nucleic acid.  Genomes can consist of 3 to several hundred genes. Viral Capsids  The protein shell enclosing the viral genome is called a capsid.  Capsids are built from a large number of protein subunits called capsomers.  Shapes: helical (rod-shaped), icosahedral, complex.

 Proteins arrange helically.  Allows equivalent bonds between proteins and nucleic acids.  Bonds are not strained.  Proteins have a groove to interact with the nucleic acid.  Example: Tobacco Mosaic Virus Helical Viruses

 20 sided solid, in which each side is an equivalent triangle.  Most stable solid approaching a sphere.  Energetically stable shape.  Least distortion of protein subunits or bonds between proteins.  Example: Adenovirus (respiratory virus of animals). Icosahedral Virus

 Combination of icosahedral and helical features.  Example: bacteriophage T4  Elongated icosahedral head.  Tail has helical symmetry. Complex Virus

Relevance of Virus Structure  Molecular nature of surface components.  Important for vaccine development.  Viruses initiate entry into the cell by attaching to receptors.  Nature of biomolecular interactions. What determines virus structure?  Protein-protein interactions.  Protein structure  protein sequence  viral gene  Protein-nucleic acid interaction. Hepatitis B Virus West Nile Virus