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Biology
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19–2 Viruses (p )
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What Is a Virus?
What Is a Virus?
Viruses are particles of nucleic acid, protein, and in some cases, lipids.
Viruses can reproduce only by infecting living cells.
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What Is a Virus?
T4 Bacteriophage
Tobacco Mosaic Virus
Influenza Virus
RNA
DNA
Head
RNA
Capsid
Tail sheath
Tail fiber
Membrane envelope
Surface proteins
Capsid proteins
Viruses come in a wide variety of sizes and shapes. A typical virus is composed of a core of either DNA or RNA, which is surrounded by a protein coat, or capsid.
Photo Credits: l. ©M.Wurtz/Biozentrum, University of Basel/Science Photo Library/Photo Researchers, Inc. m. ©Dr. O. Bradfute/Peter Arnold, Inc. r. ©National Institute for Biological Standards and
Control, England/Photo Researchers, Inc.
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What Is a Virus?
A typical virus is composed of a core of DNA or RNA surrounded by a protein coat.
A capsid is the virus’s protein coat.
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Viral Infection
Viral Infection
1. The capsid proteins of a typical virus bind to receptors on the surface of a cell and “trick” the cell into allowing it inside.
2. Once inside, the viral genes are expressed.
3. The cell transcribes and translates the viral genetic information into viral capsid proteins.
4. Sometimes that genetic program causes the host cell to make copies of the virus, and in the process the host cell is destroyed.
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Viral Infection
Viral Infection
Once the virus is inside the host cell, two different processes may occur.
Some viruses replicate immediately, killing the host cell.
Others replicate, but do not kill the host cell immediately.
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Viral Infection
Viral Infection
Because viruses must bind precisely to proteins on the cell surface and then use a host's genetic system, most viruses are highly specific to the cells they infect. 
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Viral Infection
Bacteriophage injects DNA into bacterium
Bacteriophage DNA forms a circle
Lytic Infection
Lysogenic Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Lytic Infection
In a lytic infection, a virus enters a cell, makes copies of itself, and causes the cell to burst.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Lysogenic Infection 
Other viruses cause lysogenic infections in which a host cell makes copies of the virus indefinitely.
In a lysogenic infection, a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates along with the host cell's DNA.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Viral Infection
Bacteriophage may infect cells in two ways: lytic infection and lysogenic infection.
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Retroviruses
Retroviruses
Retroviruses contain RNA as their genetic information.
When retroviruses infect cells, they make a DNA copy of their RNA.
This DNA is inserted into the DNA of the host cell.
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Retroviruses
Retroviruses
Retroviruses get their name from the fact that their genetic information is copied backward—that is, from RNA to DNA instead of from DNA to RNA. (The prefix retro- means “backward.”)
Retroviruses are responsible for some types of cancer in animals, including humans. The virus that causes acquired immune deficiency syndrome (AIDS) is a retrovirus.
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Are viruses alive?
Are viruses alive?
If we require that living things be made up of cells and be able to live independently, then viruses are not alive.
Viruses are at the borderline of living and non living things.
Viruses have many of the characteristics of living things:
After infecting living cells, viruses reproduce, regulate gene expression and even evolve.
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19–2
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19–2
Viruses that contain RNA as their genetic information are known as
prions.
oncoviruses.
retroviruses.
bacteriophage.
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19–2
The first type of virus to be studied was the
bacteriophage.
tobacco mosaic virus.
influenza virus.
AIDS virus.
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19–2
Which of the following statements about viruses is true?
Viruses appear similar to bacteria when studied with a light microscope.
Viruses display the essential characteristics of living things.
Viruses can reproduce independently if they contain DNA.
Viruses cannot reproduce unless they infect a living cell.
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19–2
A virus integrates its DNA into the DNA of the host cell but remains inactive for a while in
a lytic infection.
a lysogenic infection.
neither a lytic nor a lysogenic infection.
retroviral infection.
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19–2
Retroviruses are considered unique because
they have RNA in their capsid and not DNA.
they have DNA in their capsid and not RNA.
after infection of a host cell, their RNA makes DNA.
after infection of a host cell, their DNA makes RNA.
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