Viruses

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Viruses

• Chapter 21.1

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Virus

• Latin word meaning poison
• Acellular infectious agent
• Do not produce ATP, do not grow
• Do not carry on homeostasis, respiration,
  fermentation or photosynthesis
• Needs a host cell to reproduce it is not a
  living cell on its own
• They lack ribosomes so they must use the host
  cell to translate viral RNA to proteins
• They use the host cell for amino acids,
  nucleotides and lipids

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More facts

• Not given scientific names. Given names of the
  diseases it causes e.g. rabies virus
• Since viruses lack cell walls they are not
  affected by antibiotics which break down the cell
  wall.
• Viruses are specific to their host cells because
  they need specific proteins to bind to the host
  cells.
• Smaller than bacteria 20-250 nm
• Seen with electron microscope

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Bacteriophage

• Viruses that attack bacteria are bacteriophages
• Has a protein coat capsid- surrounding its
  genetic material.
• All viruses contain either RNA or DNA
• Flu, aids and rabies are RNA or retroviruses
• Chickenpox is a DNA virus
• A virus particle outside a host is called a viron
• Shaped as either a rod (helical) or sphere
  (polygonal). This one is a rod

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Capsid

• The capsid is the protein coat that encloses the
  nucleic acid
• Function
• to protect nucleic acids from digestion by
  enzymes
• Provides proteins that enable viron to penetrate
  the host cell
• Spherical virus is shown

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Spheres and Rods

• Flu virus on top
• Has a membrane envelope surrounding the capsid.
  Composed of lipids and glycoproteins
• Tobacco mosaic virus on bottom

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Viral multiplication cycles

• Attachment
• Virus binds to a membrane protein (tail fibers)
• Penetration (entry)
• Replication and synthesis
• virus can insert its genetic material into its
  host, literally taking over the host's functions
• Assembly
• An infected cell produces more viral protein and
  genetic material instead of the cells usual
  products.
• Release

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Two viral reproductive cycles

• Lytic cycle virulent viruses (lytic infection)
  infected bacterium bursts (lyses) releasing its
  progeny. The cell makes mRNA from the DNA of the
  virus these turn off the production of
  molecules important to the cell
• Lysogenic clycle temperate viruses. infected
  bacterium does not lyse, but imbeds itself into
  the hosts DNA and is replicated along with the
  host cells DNA for many rounds of cell division
  before lysing.
• The embedded viral DNA is called a prophage.

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Lytic cycle

• Lytic cycle - virulent, cycle occurs rapidly.
  Lysis means that the cell wall, and/or plasma
  membrane are damaged.

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lysogenic phase

• Some viruses may remain dormant inside host cells
  for long periods, causing no obvious change in
  their host cells

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Lytic phase

• When a dormant virus is stimulated, it enters the
  lytic phase new viruses are formed,
  self-assemble, and burst out of the host cell,
  killing the cell and going on to infect other
  cells

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How to prevent a virus infection

• Contain the spread.
• Without a susceptible host the virus will cease
  to exist Smallpox was eradicated this way
• vaccination a preparation of weakened virus or
  viral protein

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Viruses and cancer

• Oncogenic viruses disrupt cell growth and division

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Retroviruses

• Contain RNA as their genetic material. Their
  genetic information is copied backward- from RNA
  to DNA. Aids is a retrovirus. AIDS stands for
  acquired immune deficiency syndrome
• animation
• Advanced animation

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Prions and viroids

• Prions particles that contain no DNA or RNA only
  proteins protein infectious particles.
• Creutzfeldt-Jakob disease
• Mad cow disease
• Viroids single strand of RNA that has no capsid.
  Important in plants