Viruses

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Viruses
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Viruses

• no cellular structure, no cytoplasm, organelles
  or cell membranes
• no metabolism on their own, cannot grow or
  respire
• Therefore are not classified as living things

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Viruses continued

• Viruses can reproduce, a basic characteristic of
  life
• They can take over control of a host cell
• Can direct cell to make new virus particles
• Mobile genes that attack cells

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Parts of a virus

• Most viruses consist of 4 parts
• Core - located in the center of the virus and
  contains the virus DNA or RNA wrapped together
  with some proteins
• 2) Capsid - made of protein and forms a shell
  around the core, protects DNA from enzymes of
  host cell
• 3) Matrix - forms a layer between the capsid and
  the envelope
• 4) Envelope - consists of lipids stolen from the
  cell membrane
• of the host cell that the virus is growing
  in
• Note some viruses do not have an envelope and
  others do not have
• a capsid

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Classifying Viruses

• Often viruses are named for
• Type of disease caused (poliovirus)
• Those who discovered them (Epstein-barr virus)
• Site of replication (rhinovirus or tobacco mosaic
  virus)
• Where they were isolated (sendai-virus)
• Classification is now based on
• type of nucleic acid, shape,
• size etc.

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Shape and Size --gt determined by the type and
arrangement of proteins in the capsid

• Cylindrical
• Polyhedral
• Spherical

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Site of Replication

• Specific organism and cell
• Wide variety of organisms
• e.g. Tobacco mosaic virus

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Type of disease they cause

• 21 different groups of viruses infecting humans
  differ in set of genes (genome) and method of
  replication
• e.g. influenza virus

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Type of nucleic acid

• DNA viruses
• Less virulent
• follow the lytic cyle
• Eg. Warts, shingles, chicken pox
• RNA viruses
• Virulent
• Follow lysogenic cycle
• AIDS, rubella, rabies, measles

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How big are viruses?

• Video

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Viral Replication

• It depends on the metabolism of a prokaryotic or
  eukaryotic cell to replicate its DNA or RNA and
  to make protein coats foe each newly formed virus
  particle
• Attaches to specific receptors on the plasma
  membrane of the host cell and can only enter
  particular cells with specific receptor sites
• Enter host in one of two ways
• a) the virus injects its nucleic acid into host
  cell (lytic cycle)
• b) membrane of host engulfs virus creating a
  vacuole inside host
• Host metabolism replicates the viral DNA or RNA
  and protein coat and new virus particles are
  assembled

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• New particles are released from the host cell in
  one of two ways
• The host cell breaks open (Lysis) and releases
  the new viruses which then infect neighbouring
  cells Host cell is destroyed in process
• The host cell releases new viruses without being
  destroyed

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Bacteriophage

• Virus that infects a bacteria
• Video
• Same video different site

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T4 Bacteriophage

• Specific to E.coli
• Head ? capsid containing DNA
• Sheath ? support and pump to
• move DNA
• Base plate ? cut or bite a hole in
• cell
• Tail fibres ? find site for
• attachment
• Core ? pipeline to conduct DNA
• from head to cytoplasm of
• cell
• Collar ?attach tail to head

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Viral ReplicationLytic and Lysogenic Cycles

• Video

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Viroids

• Very small infectious pieces of RNA
• responsible for some serious plant
• diseases
• Differ from viruses as RNA does not
• code for proteins
• May interfere with the normal formation and
  functioning of RNA in host cell
• Eg. Viroid outbreak killed more than
• 10 million coconut plants in
• Philippines

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Prions

• Abnormally shaped infectious protein responsible
  for some brain diseases in mammals, including
  humans
• When tissues are eaten by another animal, prions
  enter blood stream and go to its brain
• Prions interact with normal proteins causing them
  to become abnormal and infectious
• E.g. Bovine spongiform encephalopathy (BSE)
• (Mad Cow Disease)
• Humans - Creutzfeldt-Jakob Disease
• (CJD)