Viruses
Definition = Latin for POISON
• NON – Living because
–Do not grow
–Do not reproduce on their own
–Do not metabolize
• Living Because
–Evolve/Adapt
–Mutate and recombine
Discovery of the Virus
• Adolph Meyer (1833) a German
Scientist studied the Tobacco
Mosaic Virus.
• Thought it was caused by a very
small bacterium because it could
not be viewed through the
microscope.
Infection by tobacco mosaic virus
(TMV)
VIRUSES: GENES IN PACKAGES
• Viruses sit on the fence between
life and nonlife
• VIRION = a virus particle
• Our body sees viruses as non
living antigens and makes
antibodies to fight them.
• Viruses are parasites of cells.
• A virus takes over its cellular
metabolism (think cellular
factory) and makes hundreds, if
not thousands of copies.
• Viruses are specific for the kinds
of cells they infect. For example:
–Influenza attacks only cells of
the lungs.
–Hepatitis C attacks only liver
cells.
–HIV attacks T4 white blood
cells.
Examples of Different Types of
Human Viruses
Influenza (Flu)
HIV
Herpes Virus
Measles
Ebola Virus
Polio
2 MAIN PARTS OF A VIRUS:
1. Protein coat (capsid): Protects
the nucleic acid core
2. Nucleic acid core – can be DNA
or RNA
• An envelope is found in some
viruses (usually those that infect
animal cells). It is an additional
protective coating.
Membranous
envelope
RNA
Protein
coat
Protein
spike
EXAMPLE OF A
BACTERIOPHAGE
(attacks bacteria)
PROTEIN
COAT
(CAPSID)
VIRAL DNA
SHEATH
TAIL FIBERS
Compare the size of a Eukaryotic
cell, Bacterial Cell and a Virus
• Viruses come in many shapes
and sizes
• Measured in nanometers (nm)
• There are 109 nm in a meter
(1 nm = 10-9 m)
• Cannot be seen with a light
microscope
HOW ARE VIRUSES
CLASSIFIED?
A. TYPE OF NUCLEIC ACID
(DNA or RNA)
B. SHAPE
B. HOST
C. FUNCTION
Bacteriophages
• Bacteriophage is a virus that
attacks bacteria
• Injects its DNA and takes over the
bacteria cell.
• Phage: means to eat or destroy
0.5 m
Head
Tail
Tail fiber
DNA of virus
Bacterial
cell
HOW DO VIRUSES
MULTIPLY?
• Viruses perform replication in
other cells called hosts.
• The two replication cycles that
viruses go through are the LYTIC
and the LYSOGENIC cycles.
• The two cycles differ because in
the lytic cycle a virus immediately
replicates after entering the cell
and in the lysogenic cycle the
viral DNA replicates itself in a way
that doesn’t kill the host cell
immediately.
LYTIC CYCLE
LYSOGENIC CYCLE
TREATMENT OF VIRAL
DISEASES
• Viral diseases cannot be
cured by antibiotics.
• Generally with viral
infections you are limited to
relieving symptoms while
your immune system battles
the virus.
Prevention of Viral Infections –
VACINES
• When you inject a person with a
harmless (weakened or dead)
form of a virus
• This stimulates the immune
system to produce cells and
proteins that will recognize and
destroy that type of virus, if it
enters the body again
• This helped to end epidemics
of smallpox, polio and
measles
HUMAN USES FOR VIRUSES
• GENETIC ENGINEERING
–Viruses can help correct
genetic defects by carrying
desirable genes from one cell
to another
AGRICULTURE
• Help control pests that destroy
human food crops
• They eliminate harmful insects
without creating pollution