Module 9 - Viruses Flashcards
What are viruses?
Small particles that can replicate inside living host cells
How are viruses obligate intracellular parasites?
They rely on taking over the host cell to function
True or false: viruses can infect all forms of life
True
How many viruses exist on the planet?
Millions
How many viruses have been extensively characterized?
5000
How long have viruses plagues humans?
Since before we knew about them
How old is the history of virology?
Nearly as old as the history of life itself
What is virus Latin for?
Poison
When was yellow fever virus discovered?
1901
What did Walter Reed do?
Showed yellow fever was causes by a virus transmitted by mosquitos in 1901
Who discovered yellow fever virus?
Walter Reed
What did Stanley (from Columbia) do?
Crystallized TMV
When was TMV crystallized?
1935
Who crystallized TMB?
Stanley (from Columbia)
How was TMV isolated?
In a filtered, bacteria-free fluid
What is the typical size of a virus?
10-100 nm
What is the typical genome size of a virus?
A few thousand to 200,000 nucleotides
How many genetic molecules do viruses typically have?
One
How can viruses have more than one genetic molecule?
By having a segmented genome
What is the structure of the genetic material in viruses?
Linear or circular, single or double stranded DNA or RNA
What is the host for poliovirus?
Humans
What is the structure of poliovirus?
Non-enveloped, icosahedral
What is the size of poliovirus?
30 nm
What is the genome size of poliovirus?
7.7 kbp
What is the genetic material of poliovirus?
ssRNA
What is the host for TMV?
Tobacco and related plants
What is the structure of TMV?
Non-enveloped, helical
What is the size of TMV?
300x18 nm
What is the genome size of TMV?
6.4 kbp
What is the genetic material of TMV?
ssRNA
What does TMV stand for?
Tobacco mosaic virus
What is the host of T4?
E. coli
What is the structure of T4?
Non-enveloped
What is the size of T4?
200x90 nm
What is the genome size of T4?
170 kbp
What is the genetic material of T4?
dsDNA
What is the host for variola virus?
Humans
What is the structure of variola virus?
Enveloped, complex
What is the size of variola virus?
300x250 nm
What is the genome size of variola virus?
186 kbp
What is the genetic material of variola virus?
dsDNA
What is the most for mimivirus?
Amoeba
What is the structure of mimivirus?
Enveloped, complex
What is the size of mimivirus?
400 nm
What is the genome size of mimivirus?
1200 kbp
What is the genetic material of mimivirus?
dsDNA
What surrounds the viral genome?
A capsid
What is a capsid?
Viral proteins that surround the viral genome
What are capsids composed of?
Capsomeres
What is a capsomere?
A subunit of a capsid
What is a capsomere composed of?
One or more polypeptides
What are the common shapes of a capsid?
Helical or icosahedral
What type of genome do helical viruses have?
ssRNA
What are some examples of helical viruses?
Ebola, TMV
What are some examples of icosahedral viruses?
Rhinovirus, adenovirus, poliovirus
What does rhinovirus do?
Causes the common cold
True or false: capsid shapes are always simple
False: they can have irregular or complex shapes
What is the typical structure of bacterial viruses?
Icosahedral head and a helical tail
What do the tail fibers do in bacteriophages?
Help binding to host cells
What is the nucleocapsid?
The capsid and genome of the virus
What is an enveloped virus?
A virus where a plasma membrane surrounds the nucleocapsid
What is a naked virus?
A virus where there is no plasma membrane surrounding the nucleocapsid
What is another name for a naked virus?
A non-enveloped virus
What are some examples of enveloped viruses?
Influenza and HIV
What are spikes?
Glycoproteins inserted into the lipid membrane of enveloped viruses
What are some examples of spike proteins?
Hemagglutinin and neuraminidase
Which virus has hemagglutinin and neuraminidase?
Influenza
What does hemagglutinin and neuraminidase determine?
Subtype of influenza
What are the 4 steps for viral replication?
- Adhere
- Penetrate and uncoat
- Synthesis
- Assembly and exit
What happens when a virus sticks to the host cell?
It releases its genome into the cell
What happens once the viral genome is inside the host cell?
The host cell expresses the genes to make proteins, replicate genome, and put itself together
What is the most important part of the viral replication cycle?
Entry into the host cell
What does the entry mechanisms depend on?
The host cell
How are animal cell viruses different from bacterial, plant, and fungal viruses?
Animal viruses do not have to contend with cell wall, while bacterial, plant, and fungal viruses do
What does entry of viruses into plant cells depend on?
Some damage on the plant tissue
Why do plant viruses rely on plant tissue damage?
It opens a spot on the cell wall
How can plant tissue be damaged?
Through insects feeding on plants, wind damage, hail/rail damage, fire damage, or human-induced damage
True or false: bacterial viruses enter the host cell
False: they do not enter the host cell after attachment
What happens when bacterial viruses attach to the host cell?
There is a conformational change in the tail protein subunits, which moves DNA from capsid head into bacterial cytoplasm
What happens when animal viruses attach to the host cell?
Some form of virion capsid or viral genome enters the cytoplasm
How do non-enveloped animal viruses enter the cell?
The entire viral particle enters through endocytosis
How do enveloped viruses such as HIV enter the cell?
They undergo a membrane fusion event at the cell surface
How do enveloped viruses such as influenza enter the cell?
They first enter through endocytosis, and then the viral capsid undergoes a specific set of disassembly to release the genome into the cytoplasm
What is the consequence of viruses only being able to replicate within living host cells (in terms of evolution)?
Their origin must be connected with the host cell
What three hypotheses have been proposed for the origin of viruses?
Coevolution, regressive, and progressive
What is another name for the coevolution hypothesis?
The virus first hypothesis
What does the coevolution hypothesis state?
Viruses co-evolved with current cellular hosts, or existed before cells
What are the strengths of the coevolution hypothesis?
It can explain the origin of many RNA viruses
What are the weaknesses of the coevolution hypothesis?
It has little support outside of RNA viruses
What does the regressive hypothesis state?
Viruses were cells that lost their metabolic and replicative features over time, thus depending on the host cell
What are the strengths of the regressive hypothesis?
There is support from nucleocytoplasmic large DNA viruses
What are the weaknesses of the regressive hypothesis?
It does not explain the origin of RNA viruses
What does the progressive hypothesis state?
Existing genetic elements gradually gained the ability to move between cells
What are the strengths of the progressive hypothesis?
There is evidence in the similarity of replication of retroviruses and replication of transposons and retrotransposons
Based on the progressive hypothesis, how did retroviruses evolve?
From eukaryal retrotransposons
How do transposons move within a genome?
By converting DNA to RNA, and then converting back into DNA
How is RNA converted into DNA for transposons?
Through the enzyme reverse transcriptase
What is done with the DNA copy produced by reverse transcriptase in transposons?
It can be integrated into a different location on the chromosome
How are retroviruses similar to retrotransposons?
They also undergo RNA to DNA conversion, and they have a similar genetic organization
True or false: there is evidence for all three hypothesis of viral origin
True: each of them has different evidence
True or false: one hypothesis of viral origin completely explains the origin of viruses
False: each of them have a partial description
What traits of viruses makes them difficult to work with?
They are small and can only replicate within appropriate host cells
What is needed to cultivate viruses?
Host cells must be inoculated with virus, and progeny viruses must be harvested
How are bacteriophages cultivated?
A culture of actively growing bacteria are inoculated with a small sample of phage
How can a researcher tell that a bacteriophage is replicating?
The growth media changes from turbid to clear
What happens after incubation of bacteriophages?
The medium is centrifuged, and the resulting supernatant is filtered
What is done to isolate a viral clone?
Molten nutrient agar is added to the cells and poured on a plate
What is the purpose of the molten nutrient agar?
It inhibits free movement of virus
What does a virus produce when plated?
A visible plaque of dead cells
What does each discrete plaque represent?
A clone (the progeny of a single virus)
What is done to cultivate animal viruses?
A small amount of virus is added to host cells growing in a flask
What do the viruses in the animal cell flask do?
New viral particles are released from infected cells into the medium, and infect other cells
What does the replication of many animal viruses lead to?
CPEs
What is an example of a virus that produces CPEs?
Poliovirus
What does CPE stand for?
Cytopathic effects
What are CPEs?
Visible changes in cellular morphology
What are CPEs often associated with?
Cell damage or death
What are CPEs commonly used as a marker for?
Extent of viral replication
What cell line was used to develop the tools to study animal viruses?
HeLa cells
What is done for simple and relatively crude purifications?
A viral suspension can be centrifuged and filtered through a very small pore filter
What does differential centrifugation do?
Separates particles by mass
What does low speed centrifugation lead to?
Pelleting of large objects, including cells
What does high speed centrifugation lead to?
Pelleting of small objects, such as viruses
What does gradient centrifugation do?
Separates particles by density (concentration)
What is the setup for gradient centrifugation?
The sample is layered on top of a density gradient made up of sucrose concentrations (ranging from 20-70%)
How does gradient centrifugation work?
During centrifugation, particles move down until they encounter a sucrose concentration equivalent to particle density
What is the product of gradient centrifugation?
Bands at different locations in the tube
What is the product of differential centrifugation?
Pellets and supernatant