viruses

viruses

INTRODUCTION OF VIRUSES

  1. Viruses are a unique group of infectious agents whose distinctiveness resides in their simple, acellular organization and pattern of reproduction.
  2. A virus particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein.
  3. Some viruses have additional layers that can be very complex and contain carbohydrates, lipids, and additional proteins.
  4. Viruses can exist in two phases: extracellular and intracellular.
  5. Virions, the extracellular phase, possess few if any enzymes and cannot reproduce independent of living cells.
  6. In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete virus particles or virions are released.
  7. viruses differ from living cells in at least three ways: (1) Their simple, acellular organization; (2) The presence of either DNA or RNA, but not both, in almost all virions; and (3) Their inability to reproduce independent of cells and carry out cell division as prokaryotes and eukaryotes do.

viruses


THE STRUCTURE OF VIRUSES

  1. Virus morphology has been intensely studied over the past 
    decades because of the importance of viruses and the realization 
    that virus structure was simple.
  2. Progress has come from the use of several different techniques: electron microscopy, X-ray diffraction, biochemical analysis, and immunology.
  3. Although our knowledge is incomplete due to the large number of different viruses. 

    Virion Size

    1. Virions range in size from about 10 to 400 nm in diameter.
    2. The smallest viruses are a little larger than ribosomes, whereas the poxviruses, which include vaccinia, are about the same size as the smallest bacteria and can be seen in the light microscope.
    3. Most viruses, are too small to be visible in the light microscope and must be viewed with scanning and transmission electron microscopes. 
    viruses

    viruses


      General Structural Properties

      1. All virions, are constructed around a nucleocapsid core (indeed, some viruses consist only of a nucleocapsid).
      2. The nucleocapsid is composed of a nucleic acid, usually either DNA or RNA, held within a protein coat called the capsid, which protects viral genetic material.
      3. Capsids are large macromolecular structures that self-assemble from many copies of one or a few types of proteins. The proteins used to build the capsid are called protomers.
      4. The most important advantage of this design strategy is that the information stored in viral genetic material is used with maximum efficiency.
      5. The various morphological types of viruses primarily result from the combination of a particular type of capsid symmetry with the presence or absence of an envelope, which is a lipid layer external to the nucleocapsid.
      6. There are three types of capsid symmetry: helical, icosahedral, and complex.
      7. Those virions having an envelope are called enveloped viruses; whereas those lacking an envelope are called naked viruses. 

                  Helical Capsids

                  1. Helical capsids are shaped like hollow tubes with protein walls.
                  2. The tobacco mosaic virus provides a well-studied example of helical capsid structure.
                  3. In this virus, the self-assembly of protomers in a helical or spiral arrangement produces a long, rigid tube, 15 to 18 nm in diameter by 300 nm long.
                  4. The capsid encloses an RNA genome, which is wound in a spiral and lies within a groove formed by the protein subunits.
                  5. Not all helical capsids are as rigid as the TMV capsid. The influenza virus genome is enclosed in thin, flexible helical capsids that are folded within an envelope.


                  Icosahedral Capsids

                  1. The icosahedron is a regular polyhedron with 20 equilateral triangular faces and 12 vertices.
                  2. The icosahedral capsid is the most efficient way to enclose a space.
                  3. When icosahedral viruses are negatively stained and viewed in the transmission electron microscope, a complex structure is revealed.
                  4. The capsids are constructed from ring or knob shaped units called capsomers, each usually made of five or six protomers.
                  5. Pentamers (pentons) have five subunits; hexamers (hexons) possess six. Pentamers are usually at the vertices of the icosahedron, whereas hexamers generally form its edges and triangular faces.
                  6. The icosahedron is constructed of 42 capsomers; larger icosahedra are made if more hexamers are used to form the edges and faces (e.g., adenoviruses have a capsid with 252 capsomers).

                  Viruses with Capsids of Complex Symmetry

                  1. Most viruses have either icosahedral or helical capsids, many viruses do not fit into either category.
                  2. The poxviruses and large bacteriophages are two important examples.
                  3. The poxviruses are the largest of the animal viruses (about 400 X 240 X 200 nm in size) and can even be seen with a phase contrast microscope or in stained preparations.
                  4. They possess an  exceptionally complex internal structure with an ovoid to brickshaped exterior.

                  viruses

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