Viruses

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Viruses Bacteria Immune Response
Viruses Section 10.1710.21 Bacteria Section
8.3, section 16.1 16.7, and pg. 55 Immune
Response Chapter 24
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VIRUSES
General Information 1. Virus comes from the
Latin word for poison. 2. It took
approximately 52 years to discover viruses. 3.
The following people were involved in the
discovery by researching a disease that was
attacking their tobacco crop (later called
the Tobacco Mosaic Virus. Adolf Mayer
(German) Dmitri Ivanovsky (Russian) Martinus
Beijerinck (Dutch) W.M. Stanley (American)
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Adolf Mayer -- 1883
What he did 1. Transferred sap of a sick
tobacco plant to a healthy one. 2. The healthy
plant got sick. 3. He looked for bacteria in
the sap but couldnt find any Conclusions he
made 1. The Tobacco Mosaic disease is
contagious 2. He assumed it was
caused by a very small bacteria
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Dmitri Ivanovsky - 1892
What he did 1. Ground up diseased leaves to
collect sap from plant. 2. Filtered the sap
through a bacteria trapping filter. 3. Applied
the filtered sap to healthy plant (should have no
bacteria in it) 4. The healthy plant caught the
disease. Conclusions he made 1. A very small
bacteria leaked through the filter. 2. The
bacteria in the infected plant secretes toxins
and it is these that is causing the
infection.
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Martinus Beijerink - 1897

• What he did
• 1. He took the sap from infected plants, diluted
  it and applied it to healthy plants.
• 2. Then he repeated the procedure from the newly
  infected
• plants, repeated the above procedure, so on
  and on.
• What he concluded
• 1. The newly infected plants were as infected as
  the original. If it were a toxin, it
  would have been diluted thousands of times,
• and it would have showed signs of weakening
  infection.
• 2. He hypothesized that the disease was caused
  by something smaller and simpler than
  bacteria.
• 3. He called this agent a virus.

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W. M. Stanley - 1935
What he did 1. Tried to grow the small
bacteria on agar. 2. Tried to kill it using
alcohol (bacteria would have died) 3. Isolated
and crystallized the tobacco mosaic virus What
he concluded 1. Because viruses can be
crystallized, they are not cells (and
therefore not bacteria), but something
much simpler. 2. Now that viruses were
isolated, they could study the
structure and behavior.
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II. Structure of a Virus A. Basic parts 1.
Inner core that is either DNA or RNA (not
both) 2. Inner core is surrounded by a CAPSID
a. made of proteinalso called a
protein coat b. can be complex (head,
tail) c. can be simple (rod) d.
arrangement of proteins in the capsid
determines the shape 3. Some, not all, have an
ENVELOPE that surrounds the capsid.
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Marburg virus
Rabies virus
Ebola virus
These are bacteriophages. This is the head,
tail structure. They attack bacteria.
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Adenovirus-African swine fever
Polio virus
Tobacco Mosaic Virus
Foot and Mouth disease virus
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B. VIROID 1. A small circular piece of RNA
with no capsid. 2. The RNA pairs with itself to
make it circular. 3. Contain only enough
information to allow an RNA polymerase to
copy them but not enough to make a protein
message. 4. Their presense causes side
effects. a. may interfere with the plants
production of ribosomes b. shown
to cause tumors on potatoes 5.
Little is known about how they spread.
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C. PRION 1. A self replicating protein 2.
Causes Scrapie (degenerates nervous system in
sheep and goats) 3. Causes Mad Cow
Disease 4. Causes Creutzfeldt-Jakob disease and
Gerstmann- Strassler Syndrome in humans
a. neurological disease
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D. RETROVIRUS 1. Virus with RNA and an
enzyme called Reverse
Transcriptase 2. Enters cells and enzyme
copies viral RNA into DNA. 3. DNA then
becomes a provirus (virus DNA
integrated with host cells DNA)
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E. Specificity 1. Viruses are specific in the
cells they attack. 2. Plant viruses only attack
plants and animal viruses only attack
animals. 3. May only attack one species (polio
and measleshumans only) 4. Why? host
cells have receptor proteins on their
membranes that give off signals. Viruses only
identify certain ones.
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III. Virus Replication there are two ways in
which viruses can reproduce.
A. The Lytic Cycle 1. discovered in the 1940s
2. This type of replication causes an Active
Infection the infection occurs
immediately. 3. The viruses that replicate this
way are Virulent viruses they kill
the host cell they invade. 4. Steps
a. Virus attaches to the cell membrane
(specific) b. Virus injects DNA into
the cell c. Virus DNA takes over the
cell, telling it to replicate the viral
DNA and make new capsids. d. Capsids and
DNA assemble into new viruses e. An
enzyme from the virus Lyses (breaks) the cell
membrane, releasing new viruses.
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D
A
B
E
C
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B. Lysogenic Cycle 1. discovered in
1953 2. This type of replication causes a
Latent Infection the infection DOES NOT
occur immediately. 3. Examples of this type of
virus Herpes virus, HIV, 4. Steps a.
Virus attaches to the cell membrane b. Virus
injects its DNA into the cell c. Viral DNA
forms a circle inside the host cell d. The
viral DNA attaches to the host cells DNA e.
The viral DNA becomes a part of the host cells
DNA f. The host cell replicates by mitosis
(any new cell has viral DNA in it)
g. Viral DNA pinches off of the host cells DNA
and forms a circle. h. This
initiates the lytic cycle, therefore killing the
cell.
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A, B
G
C
H
D, E
H
F
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