WHAT DO THE COMMON COLD, INFLUENZA, MEASLES, & POLIO HAVE IN COMMON?

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THEY ARE ALL VIRUSES!

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Influenza Virus Biology 20 Cold Virus HIV

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What is a virus?

 Non-living strand of genetic material within a protein coat  Smallest disease causing structures known  Does not grow, respire, or respond to stimuli but it does reproduce

Why are viruses so unique?

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 Neither living nor non-living! • Outside of a host, viruses are considered non-living. Inside a host, viruses can reproduce and therefore are living. As such they are not classified in any kingdom!

 They are host-specific! • E.g. Tobacco mosaic virus is harmful only to tobacco plants.

Why are viruses so unique?

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 Smallest beings in existence! They range from 5 300 nm (a nanometer is a one billionth of a meter).

• It would take 10,000 cold viruses to span the period at the end of this sentence.

 Can only be seen with electron microscopes.

Structure

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 Outer core is called the capsid, which is made of proteins  Inner core is genetic material, either RNA or DNA, but not both

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The capsid gives the virus its shape.

Phylogeny

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 No fossil evidence but many theories.

 One theory, now considered most likely, is that viruses came from parts of cells.

 Virus genetic material is very similar to cellular genes.

 These genes somehow developed the ability to exist outside the cell.

Viral Infection

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 In order to replicate, a virus must enter a host cell.

 The virus attaches to the host cell using receptors.  Different types of organisms have receptors for different types of viruses, explaining why viruses cannot be transmitted between species.

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Viral Infection

 Once the virus attaches, the genetic material of the virus enters the cell’s cytoplasm  The virus now uses the host cell to replicate by either the lytic cycle or the lysogenic cycle

Lytic Cycle

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 The host cell makes many copies of the viral RNA or DNA, which in turn help produce many more viruses.

 These new viruses cause the cell to burst, releasing new viruses that are free to infect other cells.

 • Viruses that replicate this way often cause active infections, meaning symptoms of the virus start to occur within 1-4 days after exposure. E.g. Common cold and influenza

Lysogenic Cycle

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 In this situation, the viral DNA enters the nucleus and integrates with the chromosome of the host cell.

 Once integrated, the host cell will have the viral genes permanently.  • The viral genes may remain dormant for months or years before becoming activated. E.g. Herpes simplex I  Activation results in the lytic cycle.

Retroviruses

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 Viruses are classified by their genetic material.

 Viruses that have RNA, rather than DNA, are called retroviruses. • E.g. human immunodeficiency virus (HIV)  Retroviruses have a complicated replication cycle.

Viruses & Disease

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 Two defenses the body has against viruses are interferon and antibodies.

 Viruses are often name for the diseases they cause.

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Viruses & Disease

 Vaccination is the process of preventing viral disease by deliberately exposing the body to a weakened or killed pathogen, such that the body will develop an immunity to it.

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Bacteria Versus Viruses

Bacteria

 Prokaryotic cells  Carry out all life functions  Contain both DNA and RNA

Viruses

   Not cells, just simple molecular structures Only capable of reproduction and can do this only inside a specific host Contain either DNA or RNA

Prions

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 A protein that can cause infection or disease is called a prion.

 Prions normally exist in cells and are shaped like a coil. Mutated prions are shaped like a piece of paper folded many times.

Prion Disease

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 Mutated prions are associated with many diseases known as transmissible spongiform encephalopathies.  E.g. mad cow disease in cattle, Creutzfeldt-Jakob disease in humans, & chronic wasting disease in deer and elk