The Virus
What they are and how they work
What is a virus
Viruses are nonliving particles of
DNA, RNA,
proteins, and in
some cases, lipids.
 *Virus is the Latin
word for poison

Characteristics….
•Must invade a living host cell to
reproduce.
•Viruses are parasitic.
•Viruses are smaller than bacteria.
•Viruses cannot move, grow or
use energy.
I am sick with a virus can I have
some antibiotics?
*NO!!!!


Antibiotic means against life . . . Viruses
aren’t living!!!!
We can vaccinate beforehand, but
once you are sick with a virus, Dr.’s
can only give you stuff for symptoms.
Viral Structure

Basic Structure:
 DNA or RNA core
 Protein coat =
capsid – This
determines the
shape of the virus
Other structures
Tobacco Mosaic Virus
Polyhedral Virus (Many sides)
(Icosahedral)
Causes foot-and-mouth disease
Retroviruses
“retro-” means backwards – these viruses work
backwards. Instead of copying RNA from the DNA,
they copy DNA from their RNA.
Retroviruses use RNA
– Make a copy of DNA from their RNA using
enzyme reverse transcriptase.
– DNA is inserted into host DNA
– Can remain dormant for a long
time
Ex: HIV causing AIDS
Retrovirus = Hard to fight
Bacteriophage
Bacteriophage is a virus which attacks
bacteria!
* “phage” = Greek = eating
Lytic Cycle
Lytic Infection:



Virus insert its
DNA or RNA,
Our cells
replicate the viral
DNA, makes
copies of the
virus, and the
virus causes the
cell to burst
*Lysis = burst
Think – high jacking the cell!!
Lysogenic Cycle

Lysogenic Infection: The virus integrates its own DNA
into the DNA of the host cell, and the viral genetic
information replicates along with the cell’s DNA
It lies dormant in your body and
can be undetected for several
years.
(You may be infected by a virus
and not even know it!)
The virus can become harmful
and lyse at anytime.
Ex: Herpes simplex, HIV,
chickenpox virus causing shingles
A Lysogenic Infection
The prophage may leave the
host’s DNA and enter the
lytic cycle.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
Lytic vs. Lysogenic
Lytic cycle
Lysogenic cycle
•Viral DNA takes
over host cell’s
DNA.
•Host cell is
destroyed
(Lysis = to burst)
Both are viral
replication
cycles
•Viral DNA
becomes
part of the host
cell’s DNA
Host cell is not
destroyed –
virus replicates
with host cell.
*Are they alive?

Living things:






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Are made up of cells NO
NO/YES
Reproduce
Based on a universal genetic code YES
Grow and develop NO
Obtain and use materials and energy NO
Respond to their environment NO
Maintain stable internal environment NO
Change over time YES - MUTATE
Show organization NO