Chapter 3 Flashcards
What challenges does a virus face in order to replicate?
- Must find the correct host
- Must be able to get into the host (counter barriers at cell surface)
- Must be able to find correct cell
- Must be able to enter cell
- Must be able to take over host cell machinery
To replicate genome, produce proteins, assemble - Must be able to evade the immune response
- Must successfully leave the cell and enter neighboring cells
- Not kill the host too quickly
what are the 6 steps to virus replication?
- Attachment
- Entry
- Replication of genome and protein production
- Virus regulation of cellular activities
- Assembly
- Release
what happens during attachment?
- Virus moieties interact with the cell surface
-
during attachment Virus surface proteins ________ bind to cell surface molecules called _______, not always a protein
glycoproteins, receptors
Ensure the virus gets into the correct host cell is called?
cell tropism
Influenza virus cell tropism appears to be controlled by the linkage between ____-____ and ________on the cell surface
sialic acid and ___________
_____ are a ‘mixing vessel’ for influenza as they have both α 2,3 and α 2,6 linked sialic acids
pigs
influenza virus has a surface protein that detaches the virus from inappropriately bound non-cellular sugars __________
(neuraminidase)
cell receptors for viruses includes both ______ and ________
proteins and sugars
Multi-stage binding process ensures fidelity of cellular________
tropism
the herpes simplex virus is _______ cell tropism
promiscuous
epstein bar virus is _____ cell tropism
restricted
describe entry with enveloped virus
fusion with the surface membrane or endosomal membrane
describe entry with non enveloped virus
particle internalized by the cell, crosses membrane by lysis or permeabilization
describe entry into a vacuole (3 steps)
- clathrin mediated endocytosis
- calveolae mediated endocytosis
- macropinocytosis
calveolae
describe macropinocytosis
- virus coated with phosphatidylserine stimulates cell to make lamelipodia
- large vessicle clalled macropinosomes form
- carry virus into the cell and fuse with an endosome
when the capsid enters the cytosol acidification can cause …
- change in envelope structure and capsid is released into cytosol
- change in conformation of capsid proteins altered virion enters cytosol or genome eneters cytosol
function of amantadine
- Binds to the inside of the M2 ion channel, blocking its ability to pump protons
- Resistance developed with changes in amino acids lining the M2 channel (drug can’t bind)
replication has 3 phases
- eclipse phase
- Right after virus entry there is a period of time where no infectious particles can be detected in the infected cell - log phase
- phase infectious particles are being released, cell taken over by virus - cell death
- In many cases the culmination of a virus infection
what are some sites for virus replication
- Most DNA viruses and some RNA viruses replicate in the nucleus
- Most RNA viruses replicate in the cytoplasm
- Large DNA viruses set up “second nucleus” in the cytoplasm and replicate there
how does a virus enter the nucleus
- viral proteins have nuclear localization signals
- Small size to enter through the nuclear pore intact
- Disassemble into subunits
- Enter the nucleus during cell division when nuclear envelope is dissolved
*cell tropism
all of the above
*which virus has a promiscuous cell tropism?
herpes simplex virus
*how does an enveloped virus enter a cell?
fusion with the cell membrane
*How does influenza virus use acidification of the endosome to enable entry?
changes HA molecule to allow fusion of viral envelope with the endosomal membrane
synthesis of viral proteins must occur in the ________
cytoplasm
Viruses that replicate their genomes in the nucleus must export viral mRNAs to the cytoplasm for ___________
translation
In general,________proteins are made prior to structural proteins
non structural
DNA synthesis is catalyzed by ______-_______
dna polymerase
DNA is made in a __________ direction
5’ to 3’
issues faced by viruses include
- Manipulation of the host cell machinery to replicate the genome
- Replication of unique genomic material
- Appropriate use of cellular resources for high levels of replication
what is the first strategy to obtain machinery to replicate the genome?
- induce the cell to enter S phase
- Makes a protein, large T antigen, that interacts with host Rb, blocks its function and drives the cell into S phase
what is the second strategy to obtain machinery to replicate the genome?
- replication machinery is encoded by the virus
- RNA viruses encode for their own RNA dependent RNA polymerase
describe viruses with dsDNA genomes
- Often large viruses with complex virions
- Produce many virus-specific enzymes and proteins for replication
- Replication in nucleus
- rolling circle replication
describe viruses with ssDNA genomes
- Very small genomes (1759 – 11,000 bp) ex: Parvoviridae
- Does not have the ability to turn on cellular DNA synthesis
- Replication in the nucleus
Hepadnaviridae
Hepatitis B virus is the only virus with this genome type that infects humans
DNA viruses that use RNA intermediates include
- hep B
- Genome is a partially dsDNA genome
- Reverse transcriptase and primers are covalently attached to the genomic DNA
Why doesn’t Hepadnaviridae use the host cell’s DNA polymerase?
- It would have to infect only actively dividing cells, induce cell cycle etc.
(Instead, it uses a reverse transcriptase that has DNA polymerase activity too) - Its reverse transcriptase is a low fidelity polymerase
(It will introduce errors that may produce mutants with an evolutionary advantage)
describe RNA syntheis
- Uses ribonucleic acids instead of deoxyribonucleic acids
- Synthesis occurs in a 5’ to 3’ direction
- RNA polymerase does not require a primer
what are the types of RNA genomes?
- Single-stranded RNA genomes (positive sense, negative sense, ambisense)
- double stranded RNA genomes
- segmented genomes
- RNA genome with DNA intermediate
Problems with RNA genomes:
- RNA less stable than DNA (especially ssRNA)
Gets degraded by nucleases in the cell - Cell does not have an RNA dependent RNA polymerase
- RNA polymerases are notoriously low fidelity enzymes
(+) ssRNA genomes are infectious (T F)
true
poliovirus replication makes more (+) then (-) (T F)
true
poliovirus replication is associated with the _____ _____
smooth ER
describe viruses with (-) ssRNA genomes
- Genome is not infectious, cannot be translated directly, therefore virus must package RNA polymerase in particle
- Once in the cell, both mRNA and genomic copies are made
Viruses with (+) ssRNA genomes that use a DNA intermediate are integrated into hist DNA as _________
provirus
HIV can ________- cells
transform
what are the 2 possible mechanisms for HIV to transform cells:
- Integration of genome affects adjacent cell cycle controlling genes disrupting (or enhancing) their function
- Expression of viral oncogenes
describe ambisense genomes
- Replicate similarly to (-) ssRNA viruses in that they package an RNA dep RNA polymerase in their particles
- Rather than have a genome that can be directly translated
*Small DNA viruses replicate their genomes:
in the nucleus
*How does a virus get its genome into the nucleus?
all of the above
- Small enough to transport whole virion through the nuclear pore
- Bind to nuclear pore and inject genome through and into nucleus
- Genome alone enters nucleus through pore
- Enter nucleus during mitosis and nuclear envelope degradation
*Which of the following describes the temporal regulation of HSV-1 replication?
all of the above
viral regulation of cellular activities involves what strategies:
- Drive cell into S phase in order to access resources for replication
- Disrupt cellular metabolism to alter activity of transcription factors
- Transfer of caps from cellular to viral mRNAs
Influenza virus doesn’t make it’s own 5’ cap (T F)
T
segmented (-)ssRNA influenza virus occurs in the _______
nucleus
what are the two hypothesis for segmented genome localization
- There is active sorting of the genome segments to ensure that a full set is incorporated into a forming virus particle
- RNAs are encapsidated at random, and only those with a full set of segments are infectious
simple virus assembly includes:
- VP1-4 (capsomers) assemble into pentamers
- Fifth protein, vpg, associates with RNA genome
- Genome is either
1. Inserted into preformed capsid
2. Condenses with the proteins during capsid formation
comple virus assembly includes:
- Icosahedral capsid but larger genome and more complex structure
- Uses scaffolding proteins
- Proteins required for assembly but not included in the infectious particle
virus release includes:
- cell lysis
2. budding
cell lysis involves:
- Viroporin: (Adenoviridae, Picornaviridae) encode proteins late in infection that disrupt the cell membrane
- Lytic phospholipids: (Phycodnaviridae: infects algae) virus may induce the synthesis of lytic phospholipids
- Lysozymes: (prokaryotic viruses) encode lysozymes that digest the cell wall, causing lysis by osmotic shock
* non enveloped some enveloped
*What polymerase do ssDNA viruses use to copy their genomes?
Host-encoded DNA dependent RNA polymerase
Which virus genome would be considered infectious?
+ ssRNA
*What phospholipid bilayer is used by viruses for their envelope?
all of the above
- Plasma membrane
- Nuclear membrane
- ER membrane
- Golgi membrane
- All of the above
