Viruses: Biological background Produced by Greg Mitchell, Kenan Fellow, Durham School of the Arts

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 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

What does Ebola virus do? Fever Headache Sore throat Diarrhea Coughing blood Vomiting blood Bleeding from eyes Shock and death Progression of symptoms

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 % mortality rate Health care workers must be VERY cautious around patients with Ebola

What is a virus? Non-cellular particle made of: Protein RNA or DNA And sometimes Lipids

How big is a virus? Viruses are very small Usually nanometers in size and cannot be seen with a light microscope Approx viruses would fit across the width of a hair c) Mitchell N Charity

What do viruses do? Viruses invade cells Then they multiply That’s about it! Kind of like this!

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 Aka they “gotta” 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…

All viruses contain: Structure 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

Some viruses contain an additional outer layer outside the capsid called the envelope Structure 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 ~ptsiotra/airplanes.html Stealth Bomber

Structure of an Enveloped Virus Nucleic Acid Capsid Envelope (this virus is in “stealth mode”)

What do viruses look like? These are adenoviruses They do not have envelopes They are DNA viruses

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,

Typical adenoviruses cause: Sore throat Pneumonia Diarrhea Pink eye. 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? This is the virus that causes herpes Capsid Envelope © Copyright Linda M Stannard, Herpes is a DNA virus.

What’s the deal with Herpes virus? Herpes viruses can cause fever blisters, sexually transmitted genital infections, mononucleosis and even chickenpox.

What do viruses look like? This is Rotavirus Rotavirus is interesting because it actually has two capsids instead of just one! The smaller capsid fits inside the larger capsid © Copyright Linda M Stannard,

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,

Influenza virus Influenza virus has an envelope Influenza killed over 20 million people in

What do viruses look like? HIV is a retrovirus HIV has an envelope HIV is an RNA virus

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? Produced by Greg Mitchell, Kenan Fellow, Durham School of the Arts

Some 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? The basic steps of the Lytic Cycle 1. The virus must attach to the host cell

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

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 by one of two means: a) Non- enveloped viruses usually cause the host cell to burst (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? The basic steps of the Lytic Cycle 5. New viruses are released from the cell by one of two means: b) Enveloped viruses usually leave the infected cell by budding (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.

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. 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 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 Cycle Example = 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 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? Produced by Greg Mitchell, Kenan Fellow, Durham School of the Arts

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

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?

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 bodies defense against foreign invaders

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

We can “help” our immune system to produce antibodies by using vaccines Important 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 you are injected with either: An “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???

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”

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…

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 vaccines Chickenpox Smallpox Measles Mumps Rubella Influenza Polio Hepatitis A Hepatitis B We currently DO NOT have vaccines for HIV or the common cold