Chapter 19: Viruses
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What is a virus?
A virus is a non-cellular, infectious agent that contains a protein coat and a nucleic acid core (DNA or RNA).
Why are viruses considered to on the fringe between living and non-living?
Viruses are non-cellular and do not have a metabolism. They must sabotage a host cell and hijack the cell's bio-synthetic machinery to produce copies.
Why do our immune systems have a difficulty fighting off some viruses?
Viruses mutate frequently (once with each viral generation). It's difficult to keep up with these mutations.
Are viruses generic or are the specific?
- Viruses are Specific.
- They can only infect and multiply in specific hosts cells.
Obligate intracellular parasite
Parasite is obligated to multiply inside of a host cell by taking over the cells machinery.
The specific range of hosts a virus can infect
- Specific cells within a host that a virus will target.
- Rabies targets neurons, hepatitis targets liver cells, etc.
What are the 5 basic steps of the viral multiplication cycle?
- 1. Attachment
- 2. Penetration
- 3. Replication and synthesis
- 4. Assembly
- 5. Release
Step one: Attachment
Virus targets specific host cell that it can chemically recognize, and attaches to specific groups on the cells surface.
Step 2: Penetration
Entire virus or it's genetic material is injected into the host cell's cytoplasm.
Step 3: Replication and Synthesis
- Referred to as "molecular piracy."
- DNA or RNA directs the host cell to make copies of the viral components.
Step 4: Assembly
Newly created viral protiens and nucleic acids are pieced together to form new viruses.
Step 5: Release
Host cell bursts to release newly formed viruses.
What is the lytic pathway?
Steps 1-4 of the viral multiplication cycle proceed rapidly and lysis of the host cell releases virus particles.
What is the Lysogenic Pathway of Viral multiplication?
The lysogenic pathway is extended by a latent period.
What are the main steps of the Lysogenic Pathway?
- 1. Viral DNA is intergrated into bacterical chromosome
- 2. Bacterium replicates all DNA including that from the virus.
- 3. After binary fission, each daughter cell has chromosome with viral DNA incorporated.
- 4. Later a molecular signal or stimulus can reactivate the viral DNA.
What happens to the viral DNA within a bacterial chromosome once it becomes "switched on"?
The viral DNA becomes active and proceeds to go through the Lytic cycle.
Give an examples of a latent virus:
Herpes simplex virus, type I and II.
What are the two basic shapes of a virus?
Icosahedron & Helical
What shape is the tobacco mosaic virus?
Most animal viruses have which shape:
_____ viruses have elements of icosahedral and helical structures.
Give an example.
- Example: bacteriophages
Viruses contain either _____ or _____.
DNA or RNA
Give the properties of an RNA virus:
- 1. Single stranded
- 2. Replicate and assemble within the cytosol on eukaryotic cells.
3. high mutation rate due to error-prone replication.
Why do RNA viruses have higher mutation rates than DNA viruses?
RNA viruses are single stranded, which allows for more freedom for mutation to occur. RNA isn't tied to another strand.
Name 3 examples of RNA viruses:
Influenza, measles, and HIV (AIDS)
HIV is a special class of RNA virus known as a ______.
Give the properties of a DNA virus:
- 1. Most are double stranded
- 2. Viral DNA replicates inside the nucleus of the eukaryotic host cell
What are some examples of DNA viruses?
Smallpox & Herpes
HIV is which type of virus:
A. DNA Virus
B. RNA Virus
B. RNA virus
When was HIV first reported within the United States?
First reported in 1981.
HIV is closely related to which virus found in chimpanzees?
(this multiple choice question has been scrambled)
What is the relation between HIV and smallpox?
People with mutated CCR5 gene receptors for smallpox appear to be more resistant to HIV.
What receptor does HIV target on the host cell?
Which type of cell does HIV target?
T-Helper Cells (CD4+ cells)
What do T-helper cells do?
T-helper cells are the man power behind mounting a defense against invading viruses and bacteria.
What types of infections are AIDS patients more susceptible to?
What is an opportunistic infection?
An opportunistic infection is an infection that arises when the immune system is weak.
Kaposi Sarcoma, taxoplasmosis, and yeast infections are examples of ______.
Examples of opportunistic infections
How do HIV tests work?
HIV tests search for the presence of HIV antibodies. If a person has contracted HIV, these antibodies will be present in the blood.
Why is HIV so difficult to control?
HIV has a long latency period of 8-10 years! An infected person might be unaware that they are infected and this increases the chance of them infecting others.
What allows HIV to overcome the immune system?
Random mutations or a failed immune response.
In HIV, viral ______ fits on the cell surface marker ____.
Explain the steps of HIV infection
- 1. Viral glycoprotien fits on cell surface marker CD4+
- 2. CCR5 receptor activates endocytosis of virus
- 3. HIV particle enters the cell
- 4. HIV sheds it's protective coat
- 5. Viral RNA and reverse transcriptase are release in cytoplasm.
- 6. Reverse transcriptase maked double-stranded DNA complimentary to viral RNA.
- 7. RNA incorporated into host DNA (as provirus)
- 8. Replicated viruses budded off host cell by exocytosis.
When RNA is incorporated into the host cell's DNA, it is called a ____.
Reverse transcriptase is much more accurate than DNA polymerases, which results in fewer mutations.
True or False?
False. Reverse transcriptase is much less accurate than DNA polymerase and results in a high mutation rate.
Most approved AIDS treatment drugs do what?
Act as inhibitors to either the reverse transcriptase or protease in infected cells.
If HIV is eleminated from the blood, it is eliminated from the body.
True or False?
False! Some treatments have removed HIV from the blood, but HIV may still hide dormant elsewhere until activation.
Influenza is a (DNA or RNA) virus.
Influenza is a highly mutagenic RNA virus.
Which type of Flu virus is the most serious? (Type A or Type B)
Type A is the most serious
How do different strains of the subtyes vary from one another?
These flu subtypes vary in their antigenic protein spikes.
How often can flu viruses mutate?
Once per viral generation. They undergo antigenic shifts often.
Why is it difficult to make effective flu vaccines?
It is difficult because this virus has a high mutation rate and there are numerous strains at any given time. This is due to the frequent antigenic shifts.
In what part of the world do most new strains of flu originate?
Most new flu strains come from the Far East (Asia).
Why do most new flu strains come from the Far East?
Most strains originate here due to the current living conditions.
What is an Emerging Virus?
An emerging virus is a virus that begins in one species and is transferred to another species, causing a super virus.
How are flu viruses from ducks and birds passed to humans?
The Pig is the common link between fowl and human flues. The pig is a melting pot where these flu viruses can mutate together to form a deadly super virus.
Why are emerging viruses a threat?
Emerging viruses are a large threat due to worldwide travel.
- A single-stranded RNA virus
- Associated with rodents (deer mice)
Define Ebola Virus:
- a filamentous virus causing hemorrhagic fever, may have a 50-90% mortality.
- Natural host unknown.
What is the natural host of the ebola virus?
The natural host is unknown
- Severe Acute Respiratory Syndrome;
- pneumonia-like symptoms;
- caused by coronavirus
- low mutation rate could make a vaccine possible.
A species of coronavirus causes what disease?
Can viruses cause cancer?
What types of cancers can be caused by a virus?
Hepatitis B, Liver cancer, Human Papilomavirus (HPV), cervical cancer
How can viruses cause cancer?
Viruses can act as a stimulus for congenital oncogenes to become active.
What percentage of human cancers may be caused by a virus?
Possibly 15% of all human cancers
What is another way that viruses can cause cancer?
Some viruses carry and integrate oncogenes into the host cell's genome.
Prions and Viroids are _____ particles.
What does TSE stand for?
Transmissible spongiform encephalopathies
What is a TSE?
A transmissible disease that affects brain tissue and causes the tissue to become hole-like (spongy).
Name some examples of TSE's
- -Scrapie in sheep
- -Bovine Spongiform encephalopathy (BSE) aka "Mad Cow Disease"
- -Chronic wasting diseases in deer and elk
- -Kuru; Creutzfeldt-Jacob disease (CJD) and it's varient (vCJD) in humans
How are TSEs commonly spread?
- Commonly spread by:
- -tissue transplants
- -contaminated food
- -injecting infected brain tissue into recipient animal's brain
What is a Prion?
Proteinaceous infectious particles consisting of a misfolded tertiary form of a protein.
What is strange about prions?
The misfolding of prions causes a chain reaction of misfolding of normal proteins-- leading to disease.
What causes TSE's?
Prions may cause TSEs
What is a Viroid?
Viroid means virus-like, but a Viroid is NOT a virus.
What is the difference between a viroid and a virus?
- A viroid is an infectious RNA without a protein coat.
- Viroids only infect plants.
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