Download Viruses - Madeira City Schools

Viruses: Section 10.17—10.21
Bacteria: Section 8.3, section 16.1– 16.7, and pg. 55
Immune Response: Chapter 24
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)
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 couldn’t find any
Conclusions he made…
1. The Tobacco Mosaic
disease is
contagious
2. He assumed it was
caused by a very
small bacteria
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.
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”.
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.
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 protein…also 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.
Ebola virus
Rabies virus
These are
bacteriophages. This
is the “head, tail”
structure. They attack
bacteria.
Marburg virus
AdenovirusAfrican swine fever
Foot and Mouth
disease virus
Polio virus
Tobacco Mosaic
Virus
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.
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 GerstmannStrassler Syndrome in humans
a. neurological disease
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)
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
measles…humans only)
4. Why? host cells have receptor proteins on their
membranes that give off signals. Viruses only
identify certain ones.
III. Virus Replication – there are two ways in which viruses
can reproduce.
A. The Lytic Cycle
1. discovered in the 1940’s
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.
A
D
B
E
C
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 cell’s DNA
e. The viral DNA becomes a part of the host cell’s DNA
f. The host cell replicates by mitosis (any new cell has
viral DNA in it)
g. Viral DNA pinches off of the host cell’s DNA and forms
a circle.
h. This initiates the lytic cycle, therefore killing the cell.
A, B
G
C
H
D, E
H
F