end show slide 1 of 34 biology mr. karns viruses
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Biology Mr. KarnsMr. Karns
VirusesViruses
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19–2 Viruses
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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?
Viruses differ widely in terms of size and structure.
All viruses enter living cells and use the infected cell to produce more viruses.
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What Is a Virus?
What is the structure of a virus?
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What Is a Virus? Sketch these 3 shapes
Head
Tail sheath
DNA
T4 Bacteriophage Tobacco Mosaic Virus
Influenza Virus
RNA
Membrane envelope
Tail fiber
RNA
Capsid proteins
Capsid
Surface proteins
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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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What Is a Virus?
Capsid proteins bind to receptors on the cell surface and “trick” the cell into allowing it inside.
Once inside, viral genes are expressed and the cell transcribes and translates them into viral capsid proteins.
The host cell may makes copies of the virus, and be destroyed.
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What Is a Virus?
Most viruses are highly specific to the cells they infect.
Viruses that infect bacteria are called bacteriophages.
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Viral Infection
How do viruses cause infection?
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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
Bacteriophage injects DNA into bacterium
Bacteriophage DNA forms a circle
Lytic Infection Lysogenic Infection
Cell lysis or budding Cell lysis or budding takes placetakes place
DNA takes over the cellDNA takes over the cell
Uses the cell machinery to buildUses the cell machinery to build new virus partsnew virus parts
Prophase incorporates inProphase incorporates inHost DNAHost DNA
Viral DNA is copied along Viral DNA is copied along with bacterial DNAwith bacterial DNA
Can stay Lysogenic or Can stay Lysogenic or can be triggered to go Lyticcan be triggered to go Lytic
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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
First, the bacteriophage injects DNA into a bacterium.
The bacteriophage DNA forms a circle.
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Viral Infection
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Viral Infection
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Viral 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
A lysogenic infection begins the same way as a lytic infection.
The bacteriophage injects DNA into a bacterium.
The bacteriophage DNA forms a circle.
The viral DNA embedded in the host's DNA is called a prophage.
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Viral Infection
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Viral Infection
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Viral Infection
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Viral 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
A retrovirus’ genetic information is copied backward—from RNA to DNA. This requires a reverse transcriptase enzyme, which allows for reverse transcription.
The virus that causes AIDS is a retrovirus.
Retroviruses have a very high mutation rate, almost once every generation! This makes them dangerous for us.
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Viruses and Living Cells
Viruses and Living Cells
Viruses must infect a living cell in order to grow and reproduce.
They take advantage of the host’s respiration, nutrition, and all other functions of living things.
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Viruses and Living Cells
Viruses have many of the characteristics of living things.
After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.
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Viruses and Living Cells
Because viruses are dependent on living things, it seems likely that viruses developed after living cells.
The first viruses may have evolved from genetic material of living cells.
Viruses have continued to evolve over billions of years.
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Viruses that contain RNA as their genetic information are known as
a. prions.
b. oncoviruses.
c. retroviruses.
d. bacteriophage.
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The first type of virus to be studied was the
a. bacteriophage.
b. tobacco mosaic virus.
c. influenza virus.
d. AIDS virus.
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Which of the following statements about viruses is true?
a. Viruses appear similar to bacteria when studied with a light microscope.
b. Viruses display the essential characteristics of living things.
c. Viruses can reproduce independently if they contain DNA.
d. Viruses cannot reproduce unless they infect a living cell.
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A virus integrates its DNA into the DNA of the host cell but remains inactive for a while in
a. a lytic infection.
b. a lysogenic infection.
c. neither a lytic nor a lysogenic infection.
d. retroviral infection.
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Retroviruses are considered unique because
a. they have RNA in their capsid and not DNA.
b. they have DNA in their capsid and not RNA.
c. after infection of a host cell, their RNA makes DNA.
d. after infection of a host cell, their DNA makes RNA.
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