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Viruses (18.2)
State Standard
SB3D. Compare & contrast viruses with living
organisms.
Viruses—An Exception
 A virus is a nucleic acid (core)
surrounded by a protein coat (capsid).
Why aren’t they considered to be
living?
 Viruses are not cells, nor are they made of cells.
 Viruses cannot reproduce without a host cell.
 Viruses lack the machinery needed to carry out the
functions of life.
 Because viruses are nonliving, they usually are
not placed in the biological classification system.
More virus traits…
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the core can be made
of DNA or RNA (but
never both!)
very small.
NOT living.
Can reproduce by
hijacking living cells.
The capsid helps the
virus bind to the host
cell.
T4 Bacteriophage
- A virus that attacks E. coli bacterial cells
E. Coli and the
Bacteriophage
What it looks like in real life
The HIV Virus
The Chicken Pox Virus
The Influenza Virus
The Small Pox Virus
The Ebola Virus
The Polio Virus
There are two types of viral
infection cycles:
• Lytic (Active)
• Lysogenic (Dormant)
Lytic (active) Infection
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Virus enters the host cell.
Virus makes copies of itself.
The new copies cause the cell to burst.
The
Lytic
Cycle
Lysogenic (dormant) Infection
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A virus embeds its DNA into the DNA of
the host cell.
The virus’s DNA is replicated (copied)
along with the host cell’s DNA.
The cell does NOT burst/lyse
immediately.
Certain conditions can cause the dormant
virus to switch to an active lytic cycle.
The
Lysogenic
Cycle
Cycle of Lytic and Lysogenic
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Viruses often cause observable symptoms
Antibiotics are useless against viruses, because
viruses are not alive
Ex. AIDS, Smallpox, Polio, Measles, the
Common Cold
Vaccines
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Since viruses can’t be cured, our only
defense against them is prevention.
We accomplish this through the use of
vaccines.
Vaccines contain a weakened or dead form
of the virus that is injected into the body.
Retrovirus
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Changes DNA
into RNA.
Mutates quickly
And example of a
Retrovirus is HIV