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Viruses

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Chapter 18 Viruses 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. – PowerPoint PPT presentation

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Title: Viruses


1
Chapter 18
  • Viruses

2
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.

3
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.

4
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.

5
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)

6
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.

7
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

8
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

9
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.

10
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.

11
Viroids
  • Viroids are composed of a single circular strand
    of RNA with no protein coat.
  • They cause diseases in plants.

12
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.
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