Viruses- Chapter 18
Objectives
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Know the structure of a virus
Know some scientists that studied viruses
Are viruses living or non-living
Know how viruses reproduce
Know some examples of viruses and the
disease they cause
Viruses
I. Virus - a tiny, parasitic
particle that consists of
nucleic acids (DNA or
RNA) surrounded by a
protein coat. They fall
somewhere between
living cells and nonliving things.
Head
DNA
Neck
Collar
Sheath
Tail Fiber
Base
Plate
Viruses: Structure
NOT CELLS!!
Viruses contain
1. Nucleic Acids (DNA/
RNA)
2. A Protein Coat
(CAPSID)
Some have structures
designed to attach to
host cells (tail fibers;
envelope proteins,
spikes, etc)
All viruses are made of 2 things:
1. DNA/ RNA
2. Protein Coat (capsid)
Comparing the size of a virus, a
bacterium, and an animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Why non-living:
1. Viruses cannot reproduce on their own
(Must have a living host-cell)
2. Viruses cannot metabolize (obtain and
use energy)
3. Viruses are not made of cells
4. Viruses do not grow/ develop
5. Viruses do not respond to stimulus
6. But Viruses DO have genetic material
Scientists
1st discovered in 1892 (Iwanowski)
• Tobacco Mosaic virus
• Originally thought to be a bacterial disease
• Disease could be caused even after sap was
filtered through extremely small filters
1796 - Edward Jenner - 1st vaccination
• Injected cowpox extract into people to immunize
them against smallpox..1st vaccination
1885 - Louis Pasteur
• developed rabies vaccination
1936 - Dr. William Stanley
Smallpox virus
• isolated tobacco mosaic viruses for the 1st
time; obtained them as crystals
• Living organisms do not crystallize, so
concluded were not living
Sizes of Viruses
Extremely small; largest virus is 1/10 size of
smallest bacteria. Range in size from 20 200 nm (nanometer); 1 nanometer = 1
millionth of a meter
Shapes of Viruses
1. Helical - rod-like appearance.
Ex: Tobacco mosaic virus
2. Isometrical - spherical or polyhedral
shaped; some appear to have spikes for
cellular recognition and attachment
Ex: Polio or HIV
Shapes (cont.)
3. Geometric - usually a geometric pattern;
Ex: bacteriophage
4. Filovirus - filamentous strands
Ex: Ebola
Characteristics of Viruses
• non-cellular; do not respire, grow/
develop, respond to stimuli
• Parasitic on living cells - nucleic material in
viruses give information for production of
new viruses
• viruses are spread from infected cells by
direct contact or indirectly through air,
water, food, fecal material
Simple Lytic
and Lysogenic
Cycle to Draw
in notes:
Viral Reproduction:
requires host cell
A. Lytic Cycle - the viral replication process
that rapidly kills host cell
1. A virus invades a host cell and takes control
2. Begins to replicate immediately producing
many new viruses
3. Host cell lyses (breaks apart), releasing the
newly made viruses
4. New viruses may then enter other cells and
repeat the cycle.
Lytic Cycle
B. Lysogenic Cycle - viral replication in
which a virus does not immediately
kill a host cell
1. virus attaches to host cell and injects
nucleic acid; viral DNA attaches to host
cell's chromosome
2. The nucleic acid becomes part of the
host chromosome (prophage/provirus)
3. Host cell divides, replicating viral DNA
along with its own DNA
4. The viral DNA is activated and enters
into the lytic cycle
Lysogenic Cycle
Lytic vs Lysogenic
Lytic vs lysogenic
• http://highered.mcgrawhill.com/sites/0072556781/student_view0/
chapter17/animation_quiz_2.html
Fig. 19-6
See Review Sheet
Phage
DNA
Daughter cell
with prophage
The phage injects its DNA.
Cell divisions
produce
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial
chromosome,
initiating a lytic cycle.
Lytic cycle
Lysogenic cycle
The bacterium reproduces,
copying the prophage and
transmitting it to daughter cells.
The cell lyses, releasing phages.
Lytic cycle
is induced
or
New phage DNA and proteins
are synthesized and
assembled into phages.
Lysogenic cycle
is entered
Prophage
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
***Retrovirus – virus contains RNA and
enzyme called reverse transcriptase
1. virus attaches to target cell and inject
RNA and reverse transcriptase
2. the reverse transcriptase “reads” the
RNA and produces DNA
3. the new viral DNA begins the lytic cycle
4. when the viruses emerge from the
infected cell, they often surround
themselves with the cell’s plasma
membrane material – making it harder
for the body’s defenses to recognize it.
Ex: HIV
Reverse Transcription video
RNA  DNA  RNA  Protein
Viral RNA is converted into host DNA
(reverse transcription), which is then
integrated into the host genome. Then when
normal transcription occurs, the viral
“message” is expressed, synthesizing a
viral protein.
Types of viruses
Bacteriophage – infects bacteria
Papovavirus – (DNA) causes warts
Adenovirus - (DNA) respiratory infections
Herpesvirus - (DNA) causes herpes,
chicken pox, shingles
5. Parvovirus – (DNA) infects dogs
6. Rhinovirus – (RNA) common cold
7. HIV – Human immunodeficiency virus
– retrovirus (RNA) – causes AIDS
(Auto Immune Deficiency Syndrome)
1.
2.
3.
4.
T4 bacteriophage infecting an E. coli cell
0.5 m
Viruses are pathogenic
• Treatment: Really no treatment for viral
diseases… Interferon therapy (IF) has
some beneficial effects
• PREVENTION IS CRUCIAL: Vaccination;
sterilization, proper hygiene, avoidance of
contagious individuals
Ways to prevent viral infection
• Sterilization – with disinfectants
(cleaners)
– Disinfectants are used to sterilize hospitals
• Avoid contact
• Vaccinations – build up antibodies for
future infection
Herd Immunity
Human Immunodeficiency Virus
• Retrovirus
– Information is stored in
RNA (mutates easily);
hijacks host-cell DNA
• Attacks helper T cells
in the immune system
and lowers the immune
system
• Causes Auto Immune
Deficiency Syndrome
Defenses Against Viral Infection
1st line of defense – skin, mucous
membranes, tears
– Most viruses enter body through nose or
mouth
2nd line of defense (once inside body)
1. White blood cells called phagocytes engulf
and destroy invading virus
2. Immune response – body detects foreign
proteins called “antigens.” The body then
produces antibodies to attack invader
Vaccine – a solution of weakened or killed
pathogens (disease-causing organism)
that causes the body to produce
antibodies without causing the disease.
The immune system “remembers” the
pathogen in case it encounters it again.
3rd Interferon (IF) – a protein that interferes
with viral replication – it is produced by
cells under viral attack which triggers
healthy cells to produce protective
enzymes. IF works faster than antibody
production.
Viroids & Prions
Viroids – single stranded RNA molecules that have NO
surrounding capsids. Infect many plants, including
potatoes, tomatoes, apples, and citrus fruits.
Prions – protein infectious particles. Bovine Spongiform
Encephalopathy (mad cow disease) and CreutzfeldtJakob disease, a similar disease in humans, may be
caused by prions.
Flu Attack! How a Virus Invades
your Body
• http://www.youtube.com/watch?v=Rpj0em
EGShQ
Lytic & Lysogenic Cycle and
Virus vs Cells Diagrams
• Get a textbook and a sheet of paper
• Turn to page 481:On ONE side of the
paper, draw the diagram in figure 19-10
comparing the lytic and lysogenic cycles.
Be sure to include all text/ info
• Turn to page 483: On the OTHER side of
the paper, draw Figure 19-11 comparing
viruses and cells. Be sure to include all
text/ info