Chapter 13

What does it mean when viruses are “obligatory intracellular parasites”?

They must live inside a host cell and use the cell’s stuff to make more viruses (multiplying)

Do viruses have both RNA and DNA?

No, each virus has only one type: RNA or DNA.

What does “host range” mean for viruses?

Each virus can only infect certain cell types in one species

How do we see viruses since they’re so tiny?

With an electron microscope

How big are viruses and are viruses bigger or smaller than bacteria?

• Viruses mostly range from 20 - 1000 nm

• Smaller than bacteria 

How big are red blood cells compared to a virus? 

A red blood cell is 10,000 nm wide (bigger)

What is a virion?

A complete virus, it has genetic stuff inside (DNA or RNA) and a protein coat outside 

Do viruses have DNA and RNA at the same time? How do DNA/RNA look?

• No, they either have DNA or RNA not both

• It can be single or double stranded, and straight or round

What is usually the main genetic material in viruses? 

DNA 

What protects the virus’s genetic stuff (DNA/RNA) and what is that made of?

• Capsid protects (protein coat)

• Capsid makes of tiny proteins called capsomeres

What is an envelope in a virus?

A soft outer cover around the capsid that can have fats (lipids), sugars (carbs), and proteins 

What extra things stick out from some viral envelopes? 

Spikes that help the virus grab onto host cells 

What does it mean if a virus in non-enveloped?

It means the virus has no outer cover

What protects the virus’s genetic stuff in a non-enveloped virus?

The capsid which is a protein coat

What do non-enveloped virus capsids protect against?

They block nuclease enzymes, which normally destroy DNA or RNA

What are viral spikes and where are they located?

Little protein “sticks” that poke out from the virus’s envelope (outer surface) used for attachment and identification

How does the immune system react to spikes?

It makes antibodies against them but some viruses can escape them due to mutations 

Why is influenza difficult to control in terms of viral spikes?

Influenza virus constantly changes its spikes, helping it evade immunity

What are complex viruses?

Viruses with extra parts that are more “complicated”, like tails or special walls

What is so special about bacteriophages? What contains the genetic material?

• Bacteriophages have capsids with additional structures (tail, sheath)

• The capsid head contains the nucleic acid (DNA/RNA)

Can Influenza A cross species barriers?

• Yes, it can cross species barriers.

• Before 1998, H1N1 was only in pigs 

• In 1998, H3N2 spread from humans to pigs 

What causes Influenza A subtypes?

• Different proteins on the virus surface (NA and HA)

• H5 and H7 mostly in birds 

• Bird viruses usually don’t infect humans 

How can avian flu be transmitted to humans?

Directly from birds/contaminated environments or through pigs as an intermediate host

What happens when pigs are infected with both human and avian flu?

Virus genes mix, forming a new strain (antigenic shift)

How do scientists grow viruses in living animals?

They put the virus into the animal and watch for sickness 

What can scientists do if the animal gets sick?

They may study its tissues to learn about the virus

What is another way viruses can be grown besides animals? Why?

In embryonate eggs (chicken eggs) so it can be used to make vaccines like a flu shot 

How are viruses grown in cell cultures?

Cells are taken from infected animal tissue and grown in culture media (a solution that mimics the human body)

What are immortal cell lines and what is their history?

Immortal cell lines (HeLa) were isolated from Henrietta Lacks in 1952 without her permission. Her cells were commercialized, but her family lacks basic healthcare ($200-$10,000 per vial)

What is the main goal of viral multiplication?

To replicate and process viral nucleic acid to make new viruses

What are the basic steps of viral multiplication?

1. Attachment - virus sticks to the host cell

2. Entry - virus gets inside the cell

3. Replication - virus copes its nucleic acid 

4. Assembly - virus parts are put together 

5. Release - new viruses leave the cell 

What happens in the lytic cycle of a virus?

1. Virus injects DNA into host cell

2. Viral DNA takes over and makes new phage DNA + proteins

3. New viruses are assembled

4. Host cell bursts (lysis), releasing many new viruses

What happens in the lysogenic cycle of a virus?

1. Virus injects DNA into host cell

2. Viral DNA integrates into host DNA (becomes a prophage)

3. Host cell reproduces normally, copying viral DNA each time 

4. Prophage may later switch to the lytic cycle

Why are lysogenic cells immune to reinfection by the same phage?

Because they already carry the prophage DNA inside their chromosome

What is phage conversion?

When a lysogenic phage makes the host cell show new properties

What is specialized transduction?

A process where bacterial genes are transferred to another microorganism because bacterial DNA is packaged along with phage DNA.

What happens during the attachment stage of animal virus multiplication?

Animal viruses attach to the plasma membrane of the host cell

How do animal viruses enter a host cell?

By receptor-mediated endocytosis or by fusion with the host cell membrane 

What is unceasing in animal virus multiplication?

Viral capsid is removed, and the viral nucleic acid is released inside the host cell (don’t by viral or host cell enzymes)

What happens during maturation of animal viruses?

Protein capsid assembles and envelope proteins are added to the plasma membrane

How are enveloped and non-enveloped viruses released?

Enveloped viruses bud off the host cell (doesn’t always kill it), while non-enveloped viruses exit by breaking the plasma membrane

What do oncogenes do in cells?

They cause malignant transformation, turning normal cells into tumor cells 

How can cancers be virus related?

About 10% of cancers are virus induced, caused by viruses or other agents activating oncogenes

What is a latent viral infection?

A virus that stays in a hosts cells without causing damage until activated by a stimulus. 

Give an example of a latent viral infection.

Varicellovirus (chickenpox) - it can stay dormant in nerve cells and later cause shingles

What are persistent viral infection? Name an example.

Infections that develop slowly over a long period of time and usually end up being fatal. (ex. AIDS)

When and where was AIDS first recognized in the United States?

1981, in young homosexual men with Pneumocystis pneumonia, Kaposi’s sarcoma, and immune system loss. 

When was HIV discovered and where did it originate?

HIV was discovered in 1983, it crossed into humans from chimpanzees in West/Central Africa (around 1908, bushmeat)

What are some early known cases of HIV/AIDS before 1981?

A Norwegian sailor who died in 1974, and a 1969 patient sample in Missouri confirmed HIV infection.

What type of virus is HIV?

HIV is a retrovirus that uses reverse transcriptase enzyme

What covers HIV and what special protein spikes does it have?

HIV has a phospholipid bilayer envelope with gp120 glycoprotein spikes

What do HIV gp120 spikes attach to?

They combine with CD4+ receptors on T helper cells, dendritic cells, and macrophages 

What happens during the fusion step of HIV entry?

The viral envelope merges with the host cell membrane 

What happens during the entry step of HIV?

The viral envelope stays behind, and HIV enters the host cell, releasing RNA.

What happens during latent HIV infection?

Viral DNA integrates into the hosts cells DNA as a provirus and stays inactive until later. 

What happens during active HIV infection?

The provirus is activated, viral RNA and proteins are made, and new viruses bud from the T cell. 

What happens in phase 1 of HIV infection?

Asymptomatic or swollen lymph nodes. Strong immune response; CTLs suppress virus.

What happens in phase 2 of HIV infection? 

CD4+ T cells steadily decline; few cells release virus; mild symptoms (fever, persistent infections, oral leukoplakia)

What happens in phase 3 of HIV infection?

AIDS develops when CD4+ T cell count drops below 200 cells; serious indicator conditions appear. Challenging to make a vaccine 

How does age affect survival with HIV?

Older adults and young children have weaker immune systems, making them more susceptible.

Why are some people exposed to HIV but not infected?

They may have a CCR5 mutation, which prevents HIV from entering cells.

What factors help long-term HIV survivors?

They have a low viral load and strong CTL (cytotoxic T lymphocyte) response. 

What are prions?

Infectious agents made of misfolded proteins, not nucleic acids

How do prions cause disease?

By converting normal host glycoprotein into the infectious misfolded form.

What causes Mad Cow Disease?

Infected prions that misfold proteins in the brain. They can’t be destroyed by heating or cooking infected beef.

What is the source of Mad Cow disease?

Prions that jumped species from infected sheep with scrapie?

What cause Creutzfeldt-Jakob disease and how common is it?

CJD is caused by prion infection. It is an extremely rare terminal brain disease.

What are the forms and early symptoms of Creutzfedlt-Jakob Disease?

Forms: Sporadic, hereditary, or acquired

Early symptoms: memory loss, loss of coordination, behavioral changes.