viruses: biological background produced by greg mitchell, kenan fellow, durham school of the arts

Viruses: Biological Viruses: Biological background background

Produced by Greg Mitchell,

Kenan Fellow,

Durham School of the Arts

What do the following diseases What do the following diseases have in common?have in common?

The common coldEbolaAIDS

They are all caused by viruses!!!

Now take 1 minute to Now take 1 minute to write down at least 3 write down at least 3

things you know about things you know about virusesviruses

Here is the story of one virus…Here is the story of one virus…In 1976 in Zaire (now Congo) several hundred people died horrific deaths in 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?What is Ebola?

http://science.education.nih.gov/home2.nsf/2cacf5795f2bdd8885256ea900678748/ee23d70d186c504c85256ea2006cd6da/$FILE/PHIL_1833low.jpg

•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

What does Ebola virus do?What does Ebola virus do?Fever

Headache

Sore throat

Diarrhea

Coughing blood

Vomiting blood

Bleeding from eyes

Shock and death

Progression of sym

ptoms

What does Ebola virus do?What does Ebola virus do?Scary things about this

disease:

1. Natural reservoir is unknown

2. No standard treatment—only “supportive therapy”

3. Few prevention measures exist

4. 50-90% mortality rate

Health care workers must be VERY cautious around patients with Ebola

http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola.htm

What is a virus?What is a virus?Non-cellular particle made of:

Protein

http://en.wikipedia.org/wiki/Protein

RNA or DNA

http://de.wikipedia.org/wiki/DNA

And sometimes Lipids

http://www.answers.com/topic/lipid-bilayer

How big is a virus?How big is a virus?

Viruses are very small

Usually 20-400 nanometers in size and cannot be seen with a light microscope

Approx. 1000 viruses would fit across the width of a hair

c) 2000-2003 Mitchell N Charity

What do viruses do?What do viruses do?Viruses

invade cellsThen they

multiply

That’s about it!http://www.history.navy.mil/photos/images/d00001/d02370c.htm

Kind of like this!

How do they know what to do?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 ownViruses 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

Aka they “gotta” live inside cells

In a nutshell…In a nutshell…What viruses do is invade

cells and reproduceThey just happen to kill

cells in the process!!

Now take 1 minute and Now take 1 minute and sketchsketch what you think a what you think a

virus looks like….virus looks like….

Over the next few minutes we will see what a few different viruses look like…

All viruses contain:

Structure of a VirusStructure of a Virus

2) Nucleic acid which stores the information—this can be either DNA or RNA

1) A shell of protein called a capsid

http://www.bioinvision.dk/plantvir2.html

Some viruses contain an additional outer layer outside the capsid called the envelope

Structure of a VirusStructure of a Virus

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

www.ae.gatech.edu/ ~ptsiotra/airplanes.html

Stealth Bomber

Structure of an Enveloped VirusStructure 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?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?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

Typical adenoviruses cause:

•Sore throat

•Pneumonia

•Diarrhea

•Pink eye.

What is special about Adenovirus?What is special about Adenovirus?

Adenoviruses are among the most common viruses and usually cause mild infections of the respiratory or digestive systems.

What do viruses look like?What do viruses look like?

This is the virus that causes herpes

Capsid Envelope


Herpes is a DNA virus.

What’s the deal with Herpes virus?What’s the deal with Herpes virus?

http://www.etsu.edu/cpah/dental/dcte760/apul.htm

Herpes viruses can cause fever blisters, sexually transmitted genital infections, mononucleosis and even chickenpox.


This is Rotavirus

Rotavirus is interesting because it actually has two capsids instead of just one!

The smaller capsid fits inside the larger capsid


More about RotavirusMore about Rotavirus

•Rotaviruses are RNA viruses.

•Rotaviruses are usually transmitted through the fecal-oral route.

•Usually cause infections of the digestive system – diarrhea.


•Influenza Virus – the cause of the flu!

•This is an RNA virus


Influenza virusInfluenza virus

•Influenza virus has an envelope

•Influenza killed over 20 million people in 1918-1919.


•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?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 Virus life cycles: What do viruses do all day?do viruses do all day?


Kenan Fellow,


Some Virus ReviewSome Virus Review What are viruses made of?

– Nucleic acid, protein capsid, and sometimes an envelope.

Give two examples of viruses.– 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?How do viruses work?The basic steps of the Lytic CycleThe 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


2. The virus injects its nucleic acid (DNA or RNA) into the host cell

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


4. Many new viruses assemble inside the cell



5. New viruses are released from the cell by one of two means:

a) Non-enveloped viruses usually cause the host cell to burst

http://www.aids-info.ch/e_te/aas-e-imm.htm(Lennert Nilsson, Karolinska Inst., Stockholm)© Boehringer Ingelheim International GmbH

The cell is beginning to burst because of all of the herpesviruses

How do viruses work?How do viruses work?The basic steps of the Lytic CycleThe basic steps of the Lytic Cycle5. New viruses are

released from the cell by one of two means:

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


6. All of the many newly released viruses are free to infect lots of other cells!!


7. The process then repeats itself over and over again, making us sick.

A little bit of review: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: 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 The Lysogenic Cycle: A Major Variation on the virus life cycle 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 CycleThe Lysogenic Virus CycleJust 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 CycleThe Lysogenic Virus CycleThen 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.

The Lysogenic Virus CycleThe Lysogenic Virus CycleThen 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 CycleThe Lysogenic Virus CycleThen when conditions

are right, the provirus will activate the lytic cycle.

•Temperature

•Stress level

•Immune system weakness

The viral DNA makes viral proteins

Virus destroys host DNA

Virus replicates

New viruses burst cell and spread

The Lysogenic Virus CycleThe Lysogenic Virus Cycle

Because 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

The Lysogenic Virus CycleThe Lysogenic Virus CycleExample = herpes

“cold sores” that keep infecting the mouth

Herpes 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

Review QuestionReview QuestionWhat 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 How do viruses cause illness and what can illness and what can be done about it?be done about it?


Kenan Fellow,


Think back to the life Think back to the life cycle of a virus…cycle of a virus…

Why do you think the Why do you think the viral life cycle would viral life cycle would

make us “sick”?make us “sick”?

Why do viruses 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

3. By making it easier for other pathogens—such as bacteria—to infect us

4. By promoting cancer in our bodies

Why do viruses make us sick?Why do viruses make us sick?

http://www.niehs.nih.gov/kids/colillness.htm

With the person next to you, With the person next to you, take 1 minute to devise a take 1 minute to devise a

way to stop a viral illness.way to stop a viral illness.

Is There Anything We Can Do to Is There Anything We Can Do to Combat Viral Infections?Combat Viral Infections?

1. Washing hands prevents the spread of viruses from person to person

2. Antibodies –your bodies defense against foreign invaders

http://www.cryst.bbk.ac.uk/PPS2/course/section10/mosaic.html

Is There Anything We Can Do Is There Anything We Can Do to Combat Viral Infections?to Combat Viral Infections?

3. Antiviral drugs—these drugs can prevent:

viral penetration nucleic acid synthesisviral assemblyviral release

Is There Anything We Can Do Is There Anything We Can Do to Combat Viral Infections?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 We can “help” our immune system to produce antibodies by using to produce antibodies by using

vaccinesvaccinesImportant vocabulary in understanding vaccines:

1) Immune response: your bodies 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 When you receive a vaccine you are injected with either:you are injected with either:

An “inactivated” or “killed” pathogenAn antigenic part of the pathogen (such as part

of the cell wall or flagellumA 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???

How is the most common How is the most common flu vaccine made?flu vaccine made?

How is the most common flu How is the most common flu vaccine made?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 How is the most common flu vaccine made?vaccine made?

The live virus strains are injected into chicken eggs

The virus replicates inside the eggsThe 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…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…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…

So in a nutshell…So in a nutshell…A vaccine exposes your immune system to a form of the disease Causing it to

“remember” that antigen

So that the next time you get the disease, your immune system can act quickly

Preventing you from getting sick

Important viral diseases with Important viral diseases with vaccinesvaccines

•Chickenpox

•Smallpox

•Measles

•Mumps

•Rubella

•Influenza

•Polio

•Hepatitis A

•Hepatitis BWe currently DO NOT have vaccines for HIV or the common cold

viruses: biological background produced by greg mitchell, kenan fellow, durham school of the arts

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