Lecture 4: Principles of Virus Replication Flashcards
What is a one step growth experiment?
A single or one cycle of virus growth is observed, called as one step
growth experiment
• Def: It is the time of a complete process of infection by a given virus in a permissive host cell until the release of new viral progeny
What is true about about methods of replication of members of each virus order/ family?
Members of each virus order/family may have unique method of
replication
What are the steps to studying the viral growth curve?
- ) Virus infects cell culture
- ) Incubate viral infected cells in CO2 incubators
- ) Examine the viral infected cells daily under inverted microscope
- ) Harvest cell lysates at various time intervals (15 minutes, 30 minutes, 1 hour- 24 hours)
- ) Serial dilution of collected cell lysates.
- ) Do plaque assay and calculate the number of plaques per each time point.
- ) Analyze the data and draw the viral growth curve.
What are the 2 main phases of the viral growth curve?
Eclipse phase
Exponential growth phase (logarithmic)
What occurs during the eclipse phase of the viral growth curve?
Eclipse phase: the time from the entry of the virus
into the host cell until the assembly of new progeny
viruses (1- 20 hrs)
What occurs during the Exponential growth/ logarithmic phase of the viral growth curve?
Exponential growth phase: no of produced viruses
increase exponentially until reaching a plateau thus
no more virus particles will produce
What is the difference between the bacterial growth curve and the viral one step growth curve?
Bacterial growth has a mild lag phase which then gives rise to an exponential growth phase which then becomes stationary for a certain period of time before death.
Viral growth has a period wher the virus must first be attached and penetrated. into the cell and then has a eclipse phase which is longer than the bacterial lag phase. After this point the virus begins to mature and will either start its exponential growth phase ( in the case of cell associated viruses) or will remain in that maturation phase before increasing in number during the release phase ( this is truein cases of cell free virus)
What kind of parasites are viruses? What do they require for survival?
Viruses are obligate intracellular parasites. They require a living cells for multiplication
How to viruses synthesize viral protiens? How does this effect the cell?
Viruses hijack the cellular machineries and direct them for the synthesis of viral proteins instead of cellular proteins
resulting in cell damage.
What are the typical steps / stages of viral replication?
1- Attachment/adsorption
2- Penetration
3-Uncoating
4- Synthesis of viral proteins= translation
5- Synthesis of viral nucleic acids
6- Assembly and maturation of Virions
7- Release of the progeny virus
How does a virus attach to its target cell?
- Ligands on the viral surface called virus attachment proteins
- Receptors on the plasma membrane of the target cell
- Members of same virus family may use different receptors
What can be determinant of trophism?
Viral receptors
What is an example of viral receptors determining trophism?
–Viral receptors Can be the primary determinant of tropism
- -Cell receptors: CD4+
Tissue tropism - e.g. measles (skin cells) vs. mumps (salivary
gland) Species tropism - e.g. BHV-1 (Infects cattle but not human)
What is trophism?
• Tropism: Affinity of certain viruses to certain tissues • the way the virus responds to external stimuli in order to attach
to and infect cells
What do attachment protiens on the cell surface bind to? How do viruses utilize this binding?
Attachment proteins on the cell surface binds to the host cell
receptors (proteins, glycoproteins, carbohydrates, and lipids)
Viruses use these receptors for the attachment and entry to the host cells
What are the factors of affecting the efficacy of virus attachment to cell?
- Density of receptors on the host cells
- Density of the ligands on the surface of the virus
- Ratio of the virus/host cells
- Temp, and pH
- Presence of specific ions such as Ca
• Some other viruses require the presence of co/receptors
What are neurotropic viruses? What is an example?
Nervous tissue (brain, SC, etc)
Examples: Rabies virus, AEV
What are Pneumotropic viruses? What is an example?
Respiratory tissues (lung, trachea, bronchi, etc)
Examples: Influenza, Rhinovirus, SARSCoV-2,
MERS-CoV, RSV
What are Viscerotropic viruses? What is an example?
GIT tissues
Examples: Rotavirus, NDV, Enterovirus,
What are Dermotropic viruses? What is an example?
Skin
Example: Poxvirus, LSDV, HPV, Cattle papillomavirus
What is a pantropic virus? What is an example?
Many tissues and organs
Example: SARS-CoV-2, NDV, YFV
What are picornaviruses?
Picornaviruses : create a pore either at plasma or endosomal membrane to inject their genomic RNA
What is another way a virus such as influenza A attaches to its host cell?
Via cell surface receptors.
What are some things being done to prevent viral receptor use by pathogens?
- removal of viral receptors
- Application of monoclonal antibody therapy for cellular receptors ( competition for placement)
What are the mechanisms of viral entry (penetration) of a cell?
- Binding of the virus to the host cell receptors
- Invagination of the membrane
- Formulation of an endocytic vesicle
- Release of the Virion into the cytoplasm
How do enveloped viruses penetrate into cells?
Receptor mediated fusion with host cell plasma membrane
What is Membrane fusion? What are examples of viruses and their level of uncoating?
Surface fusion has direct and partial uncoating. Partial uncoating provides the virus within the viral core. This is called nuclear import.
Direct uncoating is complete uncoating. This is when the viral nucleocaspid is released directly into cytoplasm.
Examples:
Ex: Paramyxoviruses: direct uncoating
Ex: HIV: partial uncoating
What is receptor mediated endocytosis (fusion in the endosome)?
Virus creates pit in cell wall and then pushes through into the cytoplasm but is coated in the vesicle. The viral cell within the vesicle undergoes acidification and then is free in the cytoplasm where it is uncoated, leaving the remaining nucleocapsid to begin transcription.
- Fusion occurs directly in the endosome.
Example: Influenza virus
What is receptor mediated endocytosis (lysis of the endosome)?
Same procedure as fusion with endosome but this is for a non enveloped virus. After virus comes through endosome into a coated pit, it is within a vesicle that undergoes acidification. Then the partially uncoated virus travels to nuclear membrane for nuclear import.
Example: Adenovirus
What is the general criteria of DNA virus replication strategies?
General criteria of the DNA virus replication strategies
Most DNA viruses replicate in the nucleus and use the host cell’s DNA-dependent DNA polymerase
Exceptions:
Poxvirus- replicates in the cytoplasm and has its own DNA polymerase.
polymerase.
What is the general criteria of RNA virus replication strategies?
General criteria of the RNA virus replication strategies
Most RNA viruses replicate in the cytoplasm.
Exceptions: Retro and Influenza viruses RNA viruses have their own RNA dependent RNA polymerase
Negative sense ? prepackaged with the genome in a capsid
Positive sense ? Directly translated
The Retrovirus (positive sense)- Reverse transcriptase.
Also prepackaged with the genome in a capsid
Where do most DNA viruses typically replicate?
Most DNA viruses replicate in the nucleus and use the host cell’s DNA- dependent DNA polymerase.
Where do most RNA viruses typically replicate?
Most RNA viruses replicate in the cytoplasm.
How many classes are viruses divided into? What is their divisions based on?
Viruses are divided into 6/7 classes based on their genome structure and the pathways they use to produce their mRNAs
How are viral envelopes acquistioned?
- Viral glycoproteins migrates from the ribosomes via ER towered the cell membranes and grouped into cylindrical spikes
- The NC located underneath the plasma or nuclear membranes
- The matrix or the inner protein coat become incorporated above the NC which is underneath the lipid membranes to immobilize the glycoprotein spikes
How does budding release the virus from the cell? What kind of viruses are released in this way?
- Most enveloped viruses are released from the cell membrane of the infected cells by budding
- Virus buds to the extracellular fluid to start the second replication cycle in a new cell
What is the site of replication for an adenovirus? What type of genome does it have?
Site of replication: Nucleus
Type of genome: DNA
Family: Adenovirdae
Virus: Adenovirus
What is the site of replication for the Poliovirus ? What type of genome does it have?
Type of genome: RNA
Family: Picornaviridae
Virus: Poliovirus
Site of replication: Cytoplasm
What is the site of replication for the rabies virus ? What type of genome does it have?
Type of genome: RNA
Family: Rhabdoviridae
Virus: Rabies virus
Site of replication: Cytoplasm
