Viruses

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Chapter 18

• Viruses

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What is a virus?

• A virus is a disease-causing, non-living particle
  made of a nucleic acid on the inside and a
  protective protein coat on the outside.
• A virus can only replicate itself when it finds a
  host cell to live in.
• A special virus called a bacteriophage infects
  bacteria.

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What does a virus look like?

• Viruses are very very small, too small to be seen
  with a light microscope.
• A virus has RNA or DNA inside, and a coat made of
  protein outside to protect it, called a capsid.

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Attaching to a host

• Before a virus can replicate, it must enter a
  host cell, like a plant, animal, or bacterium.
• Viruses fit onto a host cell like a key fitting
  into a lock.

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Virus shapes

• Viruses can be many different shapes, including
• Polyhedral (warts)
• Helical (tobacco mosaic)
• Circular with protein studs (the flu)
• Polyhedral head and cylindrical tail with legs
  (bacteriophages)

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Virus Replication

• Viruses start to reproduce when they get inside a
  host, this is what makes the host sick.
• There are two ways a virus can reproduce, the
  Lytic Cycle or the Lysogenic Cycle.

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Lytic Cycle

• In the Lytic cycle, a virus uses the host cells
  energy to reproduce.
• It takes about 30 minutes to make 200 new virus
  particles.
• Once reproduction is complete, the host cell
  bursts open and dies, releasing the virus
  particles to infect more cells.
• Example colds and the flu, measles

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Lysogenic Cycle

• In the lysogenic cycle, the virus does not
  destroy the host cell all at once.
• Instead, it sticks its DNA into the hosts DNA,
  creating a provirus.
• When the host cell reproduces, it reproduces the
  virus as well.
• This virus can live in the host for years or a
  lifetime.
• Examples HIV, shingles

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Retroviruses

• A retrovirus is a special virus that has only RNA
  inside of it.
• When it infects a host, it uses that RNA to make
  DNA.
• This is backwards from the normal way, and is
  called reverse transcription.
• HIV, which causes AIDS, is a retrovirus.

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Prions

• A prion is a small ring of protein that can cause
  diseases even though it doesnt have nucleic
  acids.
• Mad cow disease is caused by a prion.

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Viroids

• Viroids are composed of a single circular strand
  of RNA with no protein coat.
• They cause diseases in plants.

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Plant Viruses

• The very first virus discovered by humans was the
  tobacco mosaic virus that infects plants.
• There are more than 400 plant viruses that cause
  over 1000 different diseases.
• They can cause stunted growth and crop loss.