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WHAT DO THE COMMON COLD, INFLUENZA,

MEASLES, & POLIO HAVE IN COMMON?

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Biology 20

Influenza Virus Cold Virus HIV

THEY ARE ALL VIRUSES!

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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

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Why are viruses so unique? 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.

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Why are viruses so unique? 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.

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Structure 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.

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Phylogeny 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.

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Viral Infection 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

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Lytic Cycle 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

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Lysogenic Cycle 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.

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Retroviruses 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.

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Viruses & Disease 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

Prokaryotic cells

Carry out all life functions

Contain both DNA and RNA

Not cells, just simple molecular structures

Only capable of reproduction and can do this only inside a specific host

Contain either DNA or RNA

Bacteria Viruses

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Prions 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.

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Prion Disease 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