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Lec 35- Viruses

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University of Lethbridge
BIOL 1010
Igor Kovalchuk

Viruses Lecture 35 The discovery of the viruses  In 1883 Adolf Mayer found the mosaic disease of tobacco could be passed from plants to plant via spraying sap extracted from infected plants  In 1897, Martinus Beijernick discovered that this infectious agent could be passed from one infected plant to the other eventually it could reproduce itself  In 1935, Wendell Stanley crystallize the infectious particle now known as tobacco mosaic virus (TMV) Viruses: a Group of Intracellular Parasites  A virus is a submicroscopic infectious particle composed of a protein coat and a nucleic acid core  Viruses like cells, carry genetic information encoded in their nucleic acid, and can undergo mutation and reproduce  However, they can not carry out metabolism  Viruses are classified b the type of nucleic acid they contain, and the shape of their protein capsule  A protein shell that encloses the viral genome is called a capsid  Depending on the type of the virus capsid may be: o Rod shaped o Polyhedral o Complex  Capsid consist of capsomers  Andenovirus has 252 capsomers arranged into a polyhedral with 20 triangular faces- an isohedron o DNA virus that causes “pink eye” and colds o H1N1 has haemagglitnin and Neuraminidase  Paillomavirus (HPV) o Causes warts o These infectious particle are small, about 15 nm in diameter Viruses reproduce only within a host cell  Obligate intracellular parasite- they reproduce only within a host cell  They have no metabolism and no ribosomes; they cannot make their own proteins  Dependent on the host recognition, which occurs via the specific receptor molecule on the surface of the cell, viruses have either broad (rabies) or very narrow (some phages) host range  Eukaryotic viruses are tissue specific Factors affecting multiplication of viruses  A viral infection begins when the genome of a virus makes its way into a host cell  A virus first needs to infect a cell before it can multiply  The ability for a virus to infect a cell is affected by several factors  Host Range o The host range of a virus is defined y:  A tissue type that the virus can infect  The species it can infect and multiply in  Susceptibility o The susceptibility of a host is the cell’s or animals capacity to become infected o  Permissiveness o Infection of a permissive cell results in productive infection one in which infectious viral progeny are released o Infection of a non-permissive cells result in abortive infection, where susceptible cells may allow expression of only some viral genes. In this case, no infectious virions are produced o Infection of transiently permissive cells results in restrictive infection; only a few viral progeny are produced, viral production ceases, yet the viral genome persists in cells o The infected cells may still be transformed and suffer serious consequences to appearance and activity from the effects of the viral genes  Tissues specifically o Some viruses have host range broad enough to include several species (rabies virus) o Some infective only to one species (phages for E.coli) o Eukaryotic viruses are tissue specific: human cold virus infect only the cells lining the upper respiratory tract, ignoring other tissues The viral cycle and reproductive cycle The lytic cycle 
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