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19–2 Viruses
What Is a Virus?
What Is a Virus?
Virus comes from the Latin word for “poison.”
Viruses are particles of nucleic acid, protein, and
in some cases, lipids.
Viruses can reproduce only by infecting living cells.
Viruses differ widely in terms of size and structure.
All viruses enter living cells and use the infected
cell to produce more viruses.
One of the first viruses to be discovered and
named was the tobacco mosaic virus.
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19–2 Viruses
T4 Bacteriophage
Head
Tail
sheath
DNA
What Is a Virus?
Tobacco Mosaic
Virus
RNA
Influenza Virus
RNA
Capsid
Tail
fiber
Membrane
envelope
Capsid
proteins
Surface
proteins
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19–2 Viruses
What Is a Virus?
Most viruses are so small, they can be seen only
with the aid of a powerful electron microscope.
A typical virus is composed of a core of DNA or
RNA surrounded by a protein coat.
A virus can have a few simple genes, or more than
a hundred genes.
A capsid is the virus’s protein coat, which includes
proteins that enable a virus to enter a host cell.
Capsid proteins bind to receptors on the cell surface
and “trick” the cell into allowing it inside.
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19–2 Viruses
What Is a Virus?
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.
Most viruses are highly specific to the cells they infect
because they must bind precisely to proteins on the
cell surface and then use the host’s genetic system.
Viruses that infect bacteria are called
bacteriophages.
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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
Lysogenic Infection
Lytic Infection
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19–2 Viruses
Viral Infection
Lytic Infection
In a lytic infection, a virus enters a cell, makes copies
of itself, and causes the cell to burst.
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.
Unlike lytic viral infections, lysogenic viruses do not
lyse the host cell right away.
Instead, a lysogenic virus remains inactive for a
period of time.
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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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19–2 Viruses
Viral Infection
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Viral Infection
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Viral Infection
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Viral Infection
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19–2 Viruses
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.
Retroviruses may remain dormant for varying lengths
of time before becoming active, directing the
production of new viruses, and causing the death of
the host cell.
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19–2 Viruses
Retroviruses
Retroviruses get their name from the fact that their
genetic information is copied backward—from RNA to
DNA.
The viruses that cause AIDS and some types of
cancer are retroviruses.
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19–2 Viruses
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.
Therefore, viruses can be considered parasites:
they depend entirely on another living organism for
existence, harming that organism in the process.
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Viruses and Living Cells
Are Viruses Alive?
Viruses have many of the characteristics of living
things.
After infecting living cells, viruses can reproduce,
regulate gene expression, and even evolve.
Viruses are at the borderline of living and nonliving
things.
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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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