9 Viruses (1) Flashcards
Define “virus”
A subcellular agent that must use the metabolic machinery of a living host to produce NEW virus particles
Are viruses cellular or non-cellular?
non-cellular
Two basic components of a virus particle?
- Nucleic acid (genetic info)
2. Protein coat
What does it mean for a virus to be an “obligate intracellular parasite”?
Viruses must enter a cell before they can infect and reproduce
Viruses are much ______ than bacterial cells
smaller
Avg size of a virus?
20-300 nm
T or F: Viruses are easily seen using a light microscope.
F
Viruses can only be seen w/ electron microscopes
Would a filter of 0.2 microns be useful in filtering out viruses?
No. Viruses are usually smaller than 0.2 microns (200 nm).
[Viruses are usually 20-300 nm]
T or F: Viruses are very diverse in appearance, size, and shape.
T
Name the three structural features of a virus:
- Nucleic acid core
- Protein coat (aka “capsid”)
- +/- Envelope (surrounds the capsid)
Two types of viruses in terms of nucleic acid core.
- DNA viruses
- RNA viruses
[Note: A virus will NEVER have both DNA and RNA]
Can be either single OR double stranded.
The protein coat (capsid) of a virus is composed of _____ subunits
capsomere
Two types of viruses in terms of envelope.
- Naked virus
2. Enveloped virus
If a virus is enveloped, what is the envelope made up of?
Lipids
A(n) ____ virus is composed of a nucleic acid core and a capsid coat.
naked
A(n) ____ virus is composed of a nucleic acid core, a capsid coat, and a lipid envelope around the capsid.
enveloped
Glycoprotein spikes are attached to the _____ in naked viruses.
capsid
Glycoprotein spikes are attached to the _____ in enveloped viruses.
envelope
T or F: A virus can infect any type of living cell.
F
Viruses have a defined HOST RANGE (i.e. they can only infect one type of cell or host)
What is host range (i.e. range of hosts a virus can infect) determined by? (2)
- Presence of receptors on host cell
2. Presence of suitable metabolic machinery in host cell
T or F: Viruses are restricted to the types of cells they can infect.
T
T or F: Viruses are inert when outside of host cell.
T
5 basic steps of a virus’ life cycle?
- Adsorption
- Penetration/uncoating
- Gene expression
- Assembly
- Release
What facilitates a virus’ ability to adsorb onto a host cell?
Virus “spike” glycoproteins
The interaction b/w what two structures is an important determinant of the virus’ host range and tissue specificity?
The glycoprotein spike’s interaction w/ the host cell’s receptor
What is the pharmacological significance of the adsorption step of the life cycle of viruses?
This step could potentially be blocked by drugs/vaccinations
T or F: Human viruses use the SAME receptor type in the human body
F
Diff viruses use diff receptors
5 basic steps of a virus’ life cycle?
- Adsorption
- Penetration/uncoating
- Gene expression
- Assembly
- Release
After adsorbing, what process do naked viruses use to penetrate into cells?
receptor-mediated ENDOCYTOSIS
After adsorbing, what process do enveloped viruses use to penetrate into cells?
receptor-mediated FUSION
This virus type is surrounded by a vesicle once it gains entry into the host’s cell.
a. Naked virus
b. Enveloped virus
Naked virus
Membrane fusion occurs w/ this virus type as it gains entry into the host cell.
a. Naked virus
b. Enveloped virus
Enveloped virus
Which step in a virus’ life cycle corresponds to the event of viral nucleic acids being transcribed, translated, and replicated via the host cell’s enzymes?
Gene expression
What occurs during “assembly” - the fourth step in a virus’ life cycle?
New virus particles are produced and assembled (e.g. capsids, nucleic acids, etc.)
What occurs during the “release” phase of a virus’ life cycle?
Mature, infectious viruses are released from the host cell
T or F: The release of naked viruses does not kill the host cell.
F
The host cell is lysed due to the accumulation of naked viruses inside of it.
T or F: The release of enveloped viruses does not necessarily kill the host cell right away.
T
Where does the envelope of enveloped viruses come from?
The host cell’s membrane (as the capsid buds out)
5 basic steps of a virus’ life cycle?
- Adsorption
- Penetration/uncoating
- Gene expression
- Assembly
- Release
Naked viruses are released via…
host cell LYSIS
Enveloped viruses are released via…
BUDDING
Why don’t antibiotics work on viruses?
Viruses do not contain - at any point in its life cycle -
the same biological targets as bacteria
List the 4 types of viral infections:
- Lytic (acute) infection
- Persistent (chronic) infection
- Latent infection
- Oncogenic infection (“Host Cell Transformation”)
This type of viral infection is associated w/ viral nucleic acids being inserted into the host cell’s chromosome.
Latent infection
Viruses can be REACTIVATED in this viral infection type.
Latent infection
This viral infection is associated w/ low-level, continuous release of viruses over a longer period of time
Persistent (chronic) infection
This viral infection is associated w/ rapid virus replication and host cell death upon release.
Lytic (acute) infection
Disease symptoms are usually less severe in this viral infection type, despite virus particles constantly being present in the infected person.
Persistent (chronic) infection
Why is it so easy for viruses to be spread?
Bc the presence of infectious virions are not necessarily associated w/ the presence of disease symptoms.
That is, just because a person is infected doesn’t mean that he/she is experiencing negative effects, and so they may think that they’re healthy and non-infectious, but the contrary is true.
This viral infection type is associated w/ viruses that can cause their host cell to grow in an unctrled way.
Oncogenic infection
What’re two virus-related mechanisms of oncogenesis?
- Viral nucleic acids inserted into host’s chromosome disrupts the host cell’s ability to ctrl its growth
- Viral proteins produced during virus’ life cycle destroy/inhibit host cell factors that’re needed to ctrl host cell growth
What is a potential problem with using anti-viral agents?
The virus’ life cycle is closely linked to the host cell’s processes > difficult to target the virus without damaging the host cell
T or F: There’re relatively few anti-viral agents compared to anti-bacterial agents.
T
T or F: Due to commonalities in viral life cycles amongst all viruses, there’re more broad-spectrum anti-virals than there are broad-spectrum anti-bacterials.
F
There’re only a few agents that’re truly “broad-spectrum” anti-viral
How do current anti-virals work?
By interrupting some part of the viral life cycle.
Do current anti-virals kill pre-existing viruses?
No, they only block the production of new viruses in the host cell
What are the most ideal steps in the virus life cycle to block via anti-viral agents? (2)
- Adsorption
2. Penetration
Selzentry is an anti-viral agent for HIV treatment. How does it work?
It binds on the receptor that HIV usually binds to > prevents HIV from binding
Docosanol (Abreva TM) is used in the treatment of herpes. How does it work?
It’s a fatty alcohol that becomes incorporated into the host cell’s membrane > prevents herpes virus from fusing w/ the host cell’s membrane
What is the purpose of anti-virals that block the uncoating step (step 2 of viral life cycle) of viruses?
The purpose is to prevent the breakdown of the capsid by binding to it, and thus viral nucleic acids cannot be released into the host cell
Why are anti-virals that block the uncoating of viral nucleic acids highly virus-specific?
Because diff viruses have diff capsid structures
What type of anti-viral drugs are present in the greatest variety w/ reasonable selective action?
Drugs that block viral gene expression and replication (step 3 of the viral life cycle)
Name two types of drugs that block viral gene expression and replication:
- Viral polymerase inhibitors
2. Nucleic acid analogues
What is foscarnet, and what does it do?
It is a viral polymerase inhibitor, and it binds to the viral DNA polymerase of cytomegalovirus and HSV > no new viral DNA is produced
What is acyclovir?
It’s a guanine analogue that’s used in HSV treatment
What is azidothymidine?
It’s a thymine analogue that’s used in HIV treatment
Why is acyclovir given in a pro-drug form?
So that it doesn’t have activity in non-infected cells > reduces toxicity
Name two types of anti-viral drugs that block viral assembly (step 4) or viral release (step 5 of viral life cycle).
- Protease inhibitors
2. Neuraminidase inhibitors
What’re the proteases that’re produced by some viruses needed for?
For the synthesis and ASSEMBLY of viral structural proteins
What is neuraminidase?
It’s an enzyme produced by the influenza virus that’s needed for its RELEASE
Oseltamivir and zanamivir are examples of what type of antiviral?
Neuraminidase inhibitors (blocks the release of influenza viruses from host cells)
What’s interferon? What does interferon do?
A NATURALLY-PRODUCED anti-viral chemical. It’s released by virus-infected cells and signals to neighbouring cells to make various anti-viral factors.
MOST anti-virals act at this step of the viral life cycle.
Genome replication
5 types of anti-viral agents?
- Drugs that block viral adsorption and penetration (steps 1 and 2)
- Drugs that block un-coating of viral nucleic acids (step 2)
- Drugs that block viral gene expression and replication (step 3)
- Drugs that block viral assembly and/or release (steps 4 and 5)
- Interferon
Name 5 problems w/ anti-viral agents.
- Many are toxic in varying degrees
- Latent viruses (i.e. those that aren’t actively replicating constantly) are not susceptible
- Some anti-virals are less effective if given late in the infection
- Resistance can develop quickly
- New agents are difficult to develop
Why are latent viruses a problem when it comes to anti-viral therapies?
Most anti-viral therapies specifically target viral replication, so most drugs currently available are useless against latent viruses
Why is developing new anti-virals a challenge?
Few animal models for human viruses
[Remember: Viruses have a defined host range, so human viruses affect ONLY humans, thus can’t test anti-virals in other organisms]
