Viral disease

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Viral disease

• Learning objective
• To be able to describe the structural features of
  a virus.

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What diseases do viruses cause?

• Diseases caused by viruses include chickenpox,
  smallpox, common cold, influenza, measles, mumps,
  rabies, polio, yellow fever.

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Viruses

• Viruses are the smallest and simplest of the
  microbes.
• They are acellular (not made of cells)
• Viruses are obligate parasites who can only
  reproduce inside host cells which get damaged in
  the process, leading to disease.
• Viruses are thought to have arisen from lengths
  of DNA that became separated from their cells.

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Living or not?

• Viruses lack the mitochondria necessary to derive
  energy and they cannot reproduce on their own.
• They are dependent on their host cells and are
  only classed as living organisms when they infect
  host cells.
• After reproducing, viruses cause their host's
  destruction. Viruses are described as obligate
  intracellular parasites.

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• Outside the host cell viruses are inert and
  called virons.
• By taking over the host cells metabolic machinery
  they are able to replicate.
• Reproduction is the only common characteristic
  that viruses have with other living organisms

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• 1. Why can viruses be classified as living and
  non living?
• 2. Explain why all viruses are considered to be
  parasites.

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Size does matter..

• Viruses are smaller than bacteria about 20
  400 nm

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Morphology

• Viruses consist of
• A core of nucleic acid which can be DNA or RNA
• A protein coat or capsid
• There is no protoplasm or cytoplasm.
• The capsid is a protein coat on the very outside
  to protect the genetic material.
• It is made up of protein units called capsomeres
  which link together to form a very geometrical
  shape.

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Function of capsid

• Protects the nucleic acid when the virus is not
  in a host cell.
• Helps the virus to gain entry into a host cell
  and introduce the viral nucleic acid.
• Viruses may also possess an envelope and that
  would have come from a previous host its not
  their own.

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Function of envelope

• Binds to the host cell membrane.
• It helps the viro-particles to fuse with a new
  cell., (a disguise) e.g. HIV

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Structure

• Viruses have distinct structures and they
  identify the cells which they attack by
  recognising specific cell surface receptors.
• Viruses will usually only infect one species.
• Viruses which infect bacterial cells are called
  bacteriophages, those that infect animals and
  plants are called animal and plant viruses.

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• A complete virus particle is called a virion.
• A virion is the dormant form of a virus that is
  transmitted between cells.
• Virions are too small to see with a light
  microscope and were first seen in the 1930s using
  the electron microscope.
• Once inside a host cell virions dismantle into
  their separate parts, and the virus can be
  reproduced.

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All virions contain

• Nucleic acid, which can be DNA or RNA, and single
  or double-stranded.
• Viruses are classified according to the type of
  nucleic acid they contain.
• The nucleic acid typically codes for 5-100
  proteins (by comparison, the bacterium E. coli
  has about 4000 genes).
• A protein coat called a capsid, made of subunits
  called capsomeres.
• If the capsid proteins are closely bound to the
  nucleic acid, then the combination is called a
  nucleocapsid.
• Because capsids are composed of many repeating
  subunits, they tend to have simple geometrical
  shapes, such as helix or icosahedron (20
  triangular faces).

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• Some virions have very simple structures
  containing nothing else, but many virions have
  morecomplex structures, including
• Enzymes, required to replicate the viral nucleic
  acid or incorporate it into a host.
• A lipid envelope, not made by the virus itself,
  but derived from a host cell membrane.
• Matrix proteins to attach the capsid to the
  envelope.
• Glycoproteins to allow the virus to attach to
  host cells.

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Some examples of virion structures

• Discovered in 1852 by Ivanovsky when
  investigating tobacco mosaic disease in tobacco
  plants.
• He found that sap from a diseased plant could
  still cause infection even after being put
  through a bacterial filter so he concluded there
  was a disease-causing organism smaller even than
  a bacterium.
• Named from the Greek word meaning poisonous fluid.

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3 types of capsid

• helical e.g. TMV
• icosahedral e.g. HIV
• complex which has a polyhedral head and a
  helical tail e.g. T2

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Tobacco mosaic virus (TMV)

• A coil of RNA surrounded by a helical capsid.

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Adenovirus

• A single stranded DNA surrounded by an
  icosahedral capsid.

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Bacteriophage

• Bacteriophages are viruses that infect bacteria
• An example is the T2 virus which has complex
  structures that combine icosahedral and helical
  capsids.

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HIV
The human immunodeficiency virus (HIV) is an
enveloped retrovirus. It comprises 2 copies
of single-stranded RNA together with some
enzymes, surrounded by an icosahedral capsid,
which is in turn surrounded by a sphere of matrix
proteins attached to a lipid envelope.
Protein coat
Nucleic acid strand
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VIRAL DISEASE

• What is a virus?
• What characteristics of living things do viruses
  show?
• Do you consider viruses to be alive?
• Give reasons to justify your answer.
• What does obligate intracellular parasite mean?
• Roughly what size are viruses?