VIROLOGY

• Virus


  An infectious, obligate, intracellular parasite
• Viral genome


  Comprises DNA or RNA
• Viral genome directs


  Synthesis of viral components by cellular systems within an appropriate host cell
• Infectious progeny virus particles (virions) are formed

  By de novo self-assembly from newly synthesized components
• Progeny virion


  The vehicle for transmission of the viral genome to the next host cell or organism, where its disassembly initiates the next infectious cycle
• Viruses


  • Lack the complex energy-generating and biosynthetic systems necessary for independent existence, but are not the simplest biologically active agents
• Viroids


  Comprise a single small molecule of noncoding RNA
• Prions


  Are thought to be single protein molecules
• Although viruses have been known as distinct biological entities for little more than 100 years, evidence of viral infection can be found among the earliest recordings of human activity, and methods for combating viral disease were practiced long before the first virus was recognized
• Evidence of viral diseases in ancient records

  • The Greek poet Homer characterizing Hector as "rabid" in The Iliad
  • Mesopotamian laws outlining responsibilities of owners of rabid dogs from before 1000 B.C.
  • Egyptian hieroglyphs illustrating consequences of poliovirus infection
• Variolation


  Inoculation of healthy individuals with material from smallpox pustule into a scratch made on the arm
• Variolation, widespread in China and India by the 11th century, was based on the recognition that smallpox survivors were protected against subsequent bouts of the disease
• The consequences of variolation were unpredictable and never pleasant: serious skin lesions invariably developed at the site of inoculation and were often accompanied by more generalized rash and disease, with fatality of 1 – 2%
• Edward Jenner


  An English country physician who recognized the principle on which modern methods of viral immunization are based, even though viruses themselves were not to be identified for another 100 years
• Jenner's experiments with cowpox

  Inoculation with extracts from cowpox lesions induced only mild symptoms but protected against far more dangerous diseases
• Vaccination


  The term coined by Louis Pasteur in 1881 to honor Jenner's accomplishments
• Louis Pasteur's first deliberately attenuated viral vaccine

  Inoculated rabbits with material from the brains of cow suffering from rabies, then used aqueous suspensions of dried spinal cords from these animals to infect other rabbits, resulting in preparations that caused mild disease yet produced effective immunity against rabies
• The first report of a pathogenic agent smaller than any known bacterium appeared in 1892
• Dimitrii Ivanovsky


  The Russian scientist who observed that the causative agent of tobacco mosaic disease was not retained by the unglazed filters used at that time to remove bacteria from extracts and culture media
• Martinus Beijerinck


  The Dutch scientist who independently made the same observation as Ivanovsky and conceptually deduced that this must be a distinctive agent, because it was so small that it could pass through filters that trapped all known bacteria
• Friedrich Loeffler and Paul Frosch


  Two former students and assistants of Koch who deduced in 1898 that such infectious filterable agents comprised small particles
• Contagium vivum fluidum


  The term Beijerinck used to emphasize the infectious nature and distinctive reproductive and physical properties of the submicroscopic agent responsible for tobacco mosaic disease
• Ultrafilterable viruses


  The term for agents passing through filters that retain bacteria, appropriating the term "virus" from the Latin for "poison"
• The pioneering work on tobacco mosaic and foot and mouth disease viruses was followed the identification of viruses associated with specific diseases in many other organisms
• Important landmarks from this early period

  • Identification of viruses that cause leukemias or solid tumors in chickens by Vilhelm Ellerman and Olaf Bang in 1908 and Peyton Rous in 1911
  • The study of viruses associated with cancers in chickens, particularly Rous sarcoma virus, eventually led to an understanding of the molecular basis of cancer
  • The fact that bacteria could also be hosts to viruses was first recognized by Friedrick Twort in 1915 and Flix d' Herelle in 1917 (Bacteriophages)
• Capsid


  The protein shell, or coat, that encloses the nucleic acid genome
• Capsomeres


  Morphologic units seen in the electron microscope on the surface of icosahedral virus particles, representing clusters of polypeptides

  
• Defective virus


  A virus particle that is functionally deficient in some aspect of replication
• Envelope


  A lipid-containing membrane that surrounds some virus particles, acquired during maturation by a budding process through the cellular membrane. Virus-encoded glycoproteins are exposed on the surface as projections called Peplomers.

  
• Nucleocapsid


  The protein-nucleic acid complex representing the packaged form of the viral genome, commonly used when the nucleocapsid is a substructure of a more complex virus particle

  
• Structural units


  The basic protein building blocks of the coat, often a collection of more than one nonidentical protein subunit, also referred to as a protomer
• Subunit


  A single-folded viral polypeptide chain

  
• Virion


  The complete virus particle, serving to transfer the viral nucleic acid from one cell to another. In some instances it is identical to the nucleocapsid, in more complex virions it includes the nucleocapsid plus a surrounding envelope.

  
• The 1930s saw the introduction of the electron microscope, which rapidly revolutionized virology by allowing direct visualization of virus particles for the first time
• Images of many different virus particles confirmed that these agents are very small and that most are simpler in structure than any cellular organism, with many appearing as regular helical or spherical particles

  
• Viral genetic material


  Encodes the structural proteins of the capsid and other viral proteins essential for other functions in initiating virus replication
• Virion


  The entire structure of the virus, comprising the genome, the capsid, and - where present - the envelope

  
• Viruses come in many shapes and sizes, and structural information is necessary for virus classification and for establishing structure–function relationships of viral proteins

  
• Knowledge of virus structure furthers our understanding of the mechanisms of certain processes such as the interaction between viruses and their host cells

  
• The Structural Simplicity of Virus Particles