Download Viruses: Biological background

Viruses
What do the following diseases
have in common?
 The
common cold
 Ebola
 AIDS
They are all caused by viruses!!!
Now take 1 minute to
write down at least 3
things you know about
viruses
Here is the story of one virus…
In 1976 in Zaire (now Congo) several hundred
people died horrific deaths in a matter of days from
an unknown illness. Symptoms in infected people
began 2-21 days after infection and often resulted in
massive internal hemorrhaging (bleeding from
internal organs). More than half of those who got
sick from this disease died. We now know that this
disease is…
EBOLA
What is Ebola?
•This is the virus that
causes Ebola
Hemorrhagic Fever
•Ebola is a filamentous
virus known as a filovirus
•Ebola is transmitted from
person-to-person by body
fluids
http://science.education.nih.gov/home2.nsf/2cacf5795f2bdd8885256ea900678748/ee23d70d186c504c85256ea2006cd6da/$FILE/PHIL_1833low.jpg
What does Ebola virus do?
Fever
Headache
Sore throat
Diarrhea
Coughing blood
Vomiting blood
Bleeding from
eyes
Shock and death
What does Ebola virus do?
Scary things about this
disease:
1. Natural reservoir is
unknown
2. No standard
treatment—only
“supportive therapy”
Health care workers must be
VERY cautious around patients
with Ebola
3. Few prevention
measures exist
4. 50-90% mortality rate
http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola.htm
What is a virus?
Non-cellular particle
Protein
http://en.wikipedia.org/wiki/Protein
RNA or DNA
http://de.wikipedia.org/wiki/DNA
made of:
And
sometimes
Lipids
http://www.answers.com/topic/lipid-bilayer
How big is a virus?

Viruses are
very small
 Usually 20400
nanometers in
size and
cannot be seen
with a light
microscope
About 1000 viruses
would fit across the width
of a human hair!!
Is this larger or smaller
than a bacterium????
(smaller)
What do viruses do?
Viruses
Kind of like this!
invade cells
Then they
multiply
http://www.history.navy.mil/photos/images/d00001/d02370c.htm
How do they know what to do?
 Viruses
contain information (DNA
or RNA) that enables them to:
1) Take over the
host cell
2) Make more of
themselves and
3) Spread to
other cells
INFORMATION
Viruses can’t do this on their own
 Viruses
are dependent on the help of YOUR
host cells to reproduce!!
 Therefore viruses are known as:
Obligate Intracellular Parasites
So they must live
inside cells!
In a nutshell…
What
viruses do is invade
cells and reproduce
They just happen to kill
cells in the process!!
Now take 1 minute and
sketch what you think a
virus looks like….
Over the next few minutes we will see what a
few different viruses look like…
Structure of a Virus
 All
viruses contain:
1) A shell of
protein called a
capsid
2) Nucleic acid
which stores the
information—this
can be either DNA
or RNA
http://www.bioinvision.dk/plantvir2.html
Structure of a Virus
 Some
viruses contain an
additional outer layer
outside the capsid called
the envelope
Stealth Bomber
www.ae.gatech.edu/ ~ptsiotra/airplanes.html
The envelope allows the virus
to attack its hosts in “stealth
mode” because their envelope
(outer covering) is usually
made from the host cell
membrane
Structure of an Enveloped Virus
Nucleic Acid
Capsid
Envelope
(this virus is in
“stealth mode”)
http://biodidac.bio.uottawa.ca/thumbnails/filedet.htm?File_name=Viru008b&File_type=gif
What do viruses look like?
•These are
adenoviruses
•They do not
have envelopes
•They are DNA
viruses
http://www.virology.net/Big_Virology/BVDNAadeno.html
What is special about Adenovirus?
Adenoviruses use
special proteins that
stick out from the
capsid (called penton
fibers) to attach to the
host cell.
© Copyright Linda M Stannard, 1995. http://web.uct.ac.za/depts/mmi/stannard/emimages.html
What is special about Adenovirus?
Adenoviruses are among the most common viruses and usually cause
mild infections of the respiratory or digestive systems.
Typical
adenoviruses cause:
•Sore throat
•Pneumonia
•Diarrhea
•Pink eye
What do viruses look like?
This is the
virus that
causes herpes
Herpes is a DNA
virus.
Capsid
Envelope
© Copyright Linda M Stannard, 1995. http://web.uct.ac.za/depts/mmi/stannard/emimages.html
What’s the deal with Herpes virus?
Herpes viruses can
cause fever blisters,
sexually transmitted
genital infections,
mononucleosis and
even chickenpox.
http://www.etsu.edu/cpah/dental/dcte760/apul.htm
What do viruses look like?
This is Rotavirus
The smaller
capsid fits inside
the larger capsid
Rotavirus is interesting because it actually has
two capsids instead of just one!
© Copyright Linda M Stannard, 1995. http://web.uct.ac.za/depts/mmi/stannard/emimages.html
More about Rotavirus
•Rotaviruses are RNA viruses.
•Rotaviruses are usually transmitted through the fecal-oral
route.
•Usually cause infections of the digestive system – diarrhea.
What do viruses look like?
•Influenza
Virus – the
cause of
the flu!
•This is an
RNA virus
© Copyright Linda M Stannard, 1995. http://web.uct.ac.za/depts/mmi/stannard/emimages.html
Influenza virus
•Influenza
virus has an
envelope
•Influenza
killed over 20
million people
in 1918-1919.
What do viruses look like?
•HIV is a
retrovirus
•HIV has an
envelope
•HIV is an
RNA virus
http://research.amnh.org/exhibitions/epidemic/microbes.html
What’s the deal with retroviruses?
Retroviruses come packaged with
a VERY SPECIAL enzyme called
reverse transcriptase
Reverse transcriptase allows
them to make DNA copies of
the RNA that they carry with
them
That DNA can then integrate
into the host genome
Virus life cycles: What
do viruses do all day?
Some Virus Review

What are viruses made of?
–

Give two examples of viruses.
–

Nucleic acid, protein capsid, and sometimes an
envelope.
Ebola virus, rotavirus, adenovirus, herpes virus,
influenza virus, etc.
What is unique about a retrovirus?
–
It carries RNA as its nucleic acid, which it converts into
DNA by using the enzyme reverse transcriptase.
How do viruses work?
The basic steps of the Lytic Cycle
1. The virus must attach to the host cell
http://www.cat.cc.md.us/courses/bio141/lecguide/unit2/viruses/u2fig5a.html
How do viruses work?
The basic steps of the Lytic Cycle
2. The virus
injects its
nucleic acid
(DNA or
RNA) into
the host cell
How do viruses work?
The basic steps of the Lytic Cycle
3. The host cell
unwittingly makes
viral proteins from
the viral nucleic
acid which:
– Destroy the host
DNA
– Copy the viral
nucleic acid
– Make more viral
capsid proteins
How do viruses work?
The basic steps of the Lytic Cycle
4. Many new
viruses
assemble
inside the cell
How do viruses work?
The basic steps of the Lytic Cycle
5. New viruses
are released
from the cell in
two ways:
a) Nonenveloped
viruses usually
cause the host
cell to burst
The cell is beginning to
burst because of all of
the emerging viruses
http://www.aids-info.ch/e_te/aas-e-imm.htm
(Lennert Nilsson, Karolinska Inst., Stockholm)
© Boehringer Ingelheim International GmbH
How do viruses work?
The basic steps of the Lytic Cycle
OR…
b) Enveloped
viruses usually
leave the
infected cell by
budding
http://www.aids-info.ch/e_te/aas-e-imm.htm
(H.R. Gelderblom, Robert-Koch-Institut, Berlin)
Virus budding
off the cell
membrane
How do viruses work?
The basic steps of the Lytic Cycle
6. All of the
many newly
released viruses
are free to
infect lots of
other cells!!
How do viruses work?
The basic steps of the Lytic Cycle
7. The process then repeats itself over and over
again, making us sick.
A little bit of review:
 What
does a virus do once it
attaches to the host cell?
– It injects its nucleic acid into the host cell.
 What
does the viral nucleic acid
allow the virus to do?
– Destroy the host DNA, replicate the viral
nucleic acid, and make more capsids
Two Types of Virus Life Cycles:
1.Lytic Cycle—faster, simpler
cycle (ex: the flu)
• A virus with this cycle makes
you sick right away
Vs.
The Lysogenic Cycle: A Major
Variation on the virus life cycle
2. Lysogenic Cycle—slower, more complex
cycle (ex: herpes)
• This type of virus can “hide out” in
the DNA of your cells until it is
ready to attack
• This type of virus can keep
reoccurring…
The Lysogenic Virus Cycle

Just like in the Lytic Cycle, the lysogenic
virus:
– Attaches to the host cell
– Injects its nucleic acid into the host cell
BUT THEN IT DOES SOMETHING
DIFFERENT!!!!
The Lysogenic Virus Cycle

Then the viral DNA
integrates into the
DNA of the host
cell…it becomes part
of the host
chromosome!!!!
When its DNA is
inside the host
DNA it is called a
provirus.(prophage)
The Lysogenic Virus Cycle

Then every time the
host cell reproduces, it
copies all of its
DNA…including the
provirus!!!
•So each new host
cell will contain the
provirus!!!
The Lysogenic Virus Cycle

Then when conditions
are right, the provirus
will activate the lytic
cycle.
The viral
DNA
makes
viral
proteins
Virus
destroys
host
DNA
•Temperature
•Stress level
•Immune system
weakness
Virus
replicates
New
viruses
burst cell
and spread
The Lysogenic Virus Cycle
lysogenic viruses can “lurk”
in host cell DNA, they can be difficult
for the body to eradicate
 As a result, they can stay inside cells as
proviruses and can keep causing
infections
 Because
The Lysogenic Virus Cycle
 Example
= herpes
“cold sores” that
keep infecting the
mouth
may “go
away” temporarily, but
as long as the provirus
lurks in the DNA of
your mouth cells, they
can enter the lytic cycle
to make you miserable
Herpes
Review Question
 What
makes a lysogenic virus
different from a lytic virus?
– It can hang out in the host cell DNA
as a provirus for many generations of
host cell replication!
– Every time the host cell reproduces,
the “hidden” provirus is also
reproduced!
How do viruses cause
illness and what can
be done about it?
Think back to the life
cycle of a virus…
Why do you think the
viral life cycle would
make us “sick”?
Why do viruses make us sick?
1. By destroying our
cells when newly
assembled viruses are
ready to spread to
other cells
2. By causing our
immune system to
respond in a way that
gives us symptoms
Why do viruses make us sick?
3. By making it
easier for other
pathogens—such
as bacteria—to
infect us
4. By promoting
cancer in our
bodies
http://www.niehs.nih.gov/kids/colillness.htm
With the person next to you,
take 1 minute to devise a
way to stop a viral illness.
Is There Anything We Can Do to
Combat Viral Infections?
1. Washing hands prevents
the spread of viruses from
person to person
2. Antibodies –your
body’s defense against
foreign invaders
http://www.cryst.bbk.ac.uk/PPS2/course/section10/mosaic.html
Is There Anything We Can Do
to Combat Viral Infections?
3. Antiviral drugs—these
drugs can prevent:
 viral penetration
 nucleic acid synthesis
 viral assembly
 viral release
Is There Anything We Can Do
to Combat Viral Infections?
4. Interferons –
natural proteins
made by the body:
 usually produced in
response to one
infection
 make cells resistant
to infection by
other viruses
http://www.gpnotebook.co.uk/cache/-1643773914.htm
We can “help” our immune system
to produce antibodies by using
vaccines
Important vocabulary in understanding vaccines:
1) Immune response: your body’s defenses that
attack a disease-causing agent (pathogen)
2) Antigen: a substance (such as a virus or
bacterium) that triggers an immune response
When you receive a vaccine
you are injected with either:
“inactivated” or “killed” pathogen
 An antigenic part of the pathogen (such as part
of the cell wall or flagellum)
 A living but weakened version of the pathogen
– This is the most effective type of vaccine,
but why might it not be advisable for
patients with weakened immune systems???
 An
How is the most common
flu vaccine made?
How is the most common flu
vaccine made?
 In
February, the scientists at the World
Health Organization predict which
versions of the flu virus are going to
affect the Northern Hemisphere
 They usually choose three strains of the
virus
 In the best years, their prediction is only
75% effective
How is the most common flu
vaccine made?
 The
live virus strains are injected into
chicken eggs
 The virus replicates inside the eggs
 The virus is then extracted and purified
from the eggs and killed with a chemical
such as formaldehyde
 Then the dead virus can be injected into
people as the “flu shot”
http://www.bbc.co.uk/gcsebitesize
Once you receive the vaccination…
Your immune system responds in two ways:
1) Some of the cells of your immune system
produce antibodies that bind to the
disease-causing organism…eventually
leading to its death.
2) Memory cells are
formed that
“remember” what the
antigen looked like
These memory cells are key…
 Because
the next time you get the
disease, these memory cells recognize
the antigen and produce antibodies
VERY QUICKLY
 The quicker your immune system
responds, the less sick you get…
http://www.bbc.co.uk/gcsebitesize
So in a nutshell…
A vaccine exposes your
immune system to a form
of the disease
Preventing you
from getting sick

Causing it to
“remember” that
antigen
So that the next time
you get the disease,
your immune system
can act quickly
Important viral diseases with
vaccines
•Chickenpox
•Influenza
•Smallpox
•Polio
•Measles
•Hepatitis A
•Mumps
•Hepatitis B
•Rubella
We currently DO NOT have
vaccines for HIV or the common
cold