13 Viruses

• Viruses can threaten the existence of human life itself on earth
• Once a virus infects a host cell, it has the capability of reproducing at the expense of the host itself
• Viruses are biological entities which defy the definition of life
• The Spanish flu which surfaced in 1918 lead to a pandemic which claimed more lives than those lost during the 1st World War
• Chapter content

  Learn about different types of viruses (Bacteriophages & Animal Viruses), their reproduction cycles, and how they undergo evolution
• Number of confirmed cases of people living with HIV/AIDS worldwide reaches 33 million in 1999
• Smallpox was officially declared to be eradicated in 1979
• History of Virology

  1. 1796: Edward Jenner used cowpox to vaccinate against smallpox

  2. 1885: Louis Pasteur experimented with rabies vaccination and used the term "virus" to describe the agent

  3. 1892: Dimitri Iwanowski described the first "filterable" infectious agent - tobacco mosaic virus (TMV)

  4. 1898: Martinus Beijerinick extended Iwanowski's work with TMV and formed the first clear concept of the virus "contagium vivum fluidum"

  5. 1915: Frederick Twort discovered viruses infecting bacteria

  6. 1917: Felix d'Herelle independently discovered viruses of bacteria and coins the term bacteriophage

  7. 1935: Wendell Stanley crystallized TMV and showed that it remained infectious

  8. 1939: Emory Ellis and Max Delbruck established the concept of the "one step virus growth cycle"

  9. 1940: Helmuth Ruska used an electron microscope to take the first pictures of viral particles

  10. 1961: Sydney Brenner, Francois Jacob, and Matthew Meselson demonstrated that bacteriophage T4 uses host cell ribosomes to direct virus protein synthesis

  11. 1970: Howard Temin and David Baltimore independently discovered reverse transcriptase in retroviruses

  12. 1979: Smallpox was officially declared to be eradicated

  13. 1983: Luc Montaigner and Robert Gallo announced the discovery of human immunodeficiency virus (HIV)

  14. 1990: First approved human gene therapy procedure was carried out on a child with severe combined immune deficiency (SCID)
• Outbreak of Ebola epidemic in West Africa with 24,000 confirmed cases and over 9800 deaths


  2013 - present
• Nucleotide sequence of the largest known virus genome completed in 1999: Paramecium bursaria Chlorella virus 1
• Smallpox eradicated


  1979
• Viruses are obligate parasites that require a living host to support their metabolism and replication
• Release of two vaccines protecting against several cervical cancer-causing strains of human papillomavirus (HPV)

  2006
• Viruses may be regarded as non-living organisms because they are acellular, lack cellular organelles, do not carry out metabolism, cannot reproduce independently, do not grow or undergo developmental changes, do not respond to stimuli outside the host cell, and can only evolve within a host cell
• Outbreak of H1N1 influenza pandemic

  2009 - 2010
• Retrovirus vector used in gene therapy procedure on a child with severe combined immune deficiency (SCID)
• Number of confirmed cases of people living with HIV/AIDS worldwide reaches 33 million


  1999
• Outbreak of severe acute respiratory syndrome (SARS) caused by the SARS coronavirus

  2002-2004
• Viruses challenge the cell theory and the definition of a living organism
• Viruses are polyphyletic with many evolutionary origins
• Viruses may be regarded as living organisms because they possess genetic material, propagate their genetic information, direct host enzymes, self-replicate, undergo mutation, respond to stimuli, and evolve to adapt to the environment
• An intact infectious viral particle is called a virion, composed of the genome surrounded by the capsid
• No single gene is shared between all viruses or viral lineages
• Viruses are obligate parasites which require a living host to support many of their functions
• The viral genome can be single or several segmented, circular or linear molecules of nucleic acid
• Viruses may have multiple origins, possibly originating from different cell types or subcellular components
• The viral genome codes for the synthesis of viral components and enzymes for replication and assembly of a virion
• Most animal viruses have an envelope surrounding their nucleocapsid
• The capsid of a virus is a protein coat surrounding the genome, composed of protein subunits called capsomeres
• Viruses are totally dependent on a host cell for replication and contain only one type of nucleic acid: DNA or RNA but never both
• Some viruses have an additional outer layer known as an envelope, composed of phospholipids and glycoproteins
• Some viruses have an additional outer layer called an envelope, embedding viral glycoproteins
• Viruses have many evolutionary origins and do not have a common ancestor
• The sizes of viruses range from 10 to 300 nm
• Viral components must assemble into complete viruses to go from one cell to another
• A virus can have either DNA or RNA but never both
• Viral genomes were more similar to that of their host cells than to other types of viruses
• Viruses may be classified based on shape, genome structure, presence of an envelope, and mode of replication
• The capsid protects, attaches, and introduces the genome into host cells
• The number of genes present in a virus determines the degree of complexity displayed by the virus