Test 1 - Virus Replication (7) Flashcards
A cell in which a virus is able to replicate, i.e. the cell machinery supports replication of the virus, is known as a _________.
Permissive cell
Cells in which a factor or factors necessary to viral reproduction is not present or one detrimental to viral reproduction is present, eg. Absence of appropriate receptors, are known as ________.
Non-Permissive cells
MOI (Multiplicity of Infection) refers to what?
The number of virions that are added per cell during infection
When determining the one-step virus growth curve, the virus titer is examined by measuring _____.
PFU/ml of intracellular virions and extracellular virions
Describe the one-step virus growth curve. What are the different phases?
- Adsoprtion: viral attachment and entry into cells. Titer of free virus may decline.
- Eclipse period: the time interval between uncoating and appearance, intracellulary of first progeny irions. No infectious virus can be determined. Usually ranges from 2-12 hours.
- Latent period: Time before the appearance of new infectious virus in the medium. i.g. from uncoating to just prior to release of first extracellular virions. No extracellular virions detected.
- Bust size: the number of virions released = height of curve.
What are the steps of virus replication?
- Attachment (adsorption)
- Penetration (injection)
- Uncoating
- Synthesis of viral components (nucleic acids and proteins)
- Assembly and maturation/packaging
- Release (lysis)
______ are structures on the surface of a cell (or inside a cell) that selectively receive and bind a specific substance, and mediates its entry or action into the cell.
What may they be made of?
Receptors
may be proteins, carbohydrates, glycoproteins, lipids, lipoproteins, or even complexes
What mediates the interaction between the virus and host cell?
Receptors
TRUE/FALSE.
Binding to a cellular receptor is sufficient for infection.
FALSE.
An additional cell surface molecule, or co-receptor, may be required for entry.
TRUE/FALSE.
If a cell is lacking the receptor, it will evade infection.
TRUE.
Why do host cells have receptors for viruses?
Host cells certainly do not maintain receptors specifically for viruses, these cellular receptors carry out normal functions for cell, such as uptake of proteins. Viruses have evolved to use this receptors.
Some viruses can even use more than one host cell receptor.
What are the different ways a virus can penetrate the host cell?
- Endocytosis
- Surface Fusion
- Pore-mediated Penetration
- Antibody-mediated penetration
Describe the process of endocytosis.
What are the different types?
The process in which a substance gains entry into a cell without passing through the cell membrane. The process involves invagination and pinching off of small regions of the cell membrane, resulting in the nonspecific internalization of molecules.
Types: Phagocytosis, pinocytosis, Receptor-mediated
What kinds of viruses undergo receptor-mediated endocytosis?
Nake viruses
Most enveloped viruses
Describe the process of clathrin-mediated endocytosis of virus by the host.
- The virion attachment to the host receptor induces the binding of an adaptor protein to the receptor cytoplasmic tail.
- Adaptor proteins bind to another protein known as clathrin.
- The increase in local concentration of adaptor proteins on the inside face of the cell membrane allows clathrin to multimerize to form characteristic invaginations or Clathrin-Coated Pit (CCP).
- Membrane scission proteins Dynamin pinch off the Clathrin-Coated Pit from the host membrane, thereby releasing the Clathrin-Coated Vesicle (CCV).
- The clathrin basket is subsequently released from the vesicle.
- The vesicle then delivers its viral content to early endosomes (membrane bound
compartment in host cell cytoplasm).
- pH in endosome changes to acidic, viral genome is released in host cell.
Describe the difference in capsid release between enveloped viruses and non-enveloped viruses.
Enveloped viruses
Fusion of virus membrane with host endosomal membrane release Viral Genome.
Non-enveloped viruses
Most: Lysis occurs when a viral capsid induces rupture of endosomal membrane, releasing Viral Capsid or genome.
Some: induce local permeabilization of host endosomal membrane to allow virus capsid penetration into to the cytoplasm
What are some other types of receptor-mediated endocytosis?
Caveolin-mediated endocytosis of virus by host (specialized lipid rafts)
Clathrin and caveolin-independent endocytosis of viruses by host
What are the ONLY virus types to undergo membrane/surface fusion?
Enveloped viruses
What may result due to viral membrane fusion? Why?
Antibody-Dependent Cell Mediated Cytotoxicity
Viral glycoproteins are retained on the cell surface, and since these are antigenic, the cell can become a target of the immune system of the host.
What facilitates membrane fusion?
Fusion protein
TRUE/FALSE.
Release of the capsid into the cytoplasm is always pH dependent.
FALSE.
Viruses may have pH independent fusion proteins that can directly fuse with the host cell and release nucleic acid into the cytoplasm. This occurs at a neutral pH.
Describe pH dependent fusion and uncoating.
What is this similar to?
Low pH in endosome promotes fusion of envelope with endosomal membrane; lysis of nucleocapsid by lysosomal proteases, and release of viral genome.
This is the same as receptor mediated endocytosis.
Describe pore-mediated penetration.
What kind of viruses undergo this type of penetration?
Some non-enveloped viruses inject their genome into the host cytoplasm through creation of a pore in the host membrane.
FIP (feline infectious peritonitis) virus uses antibody mediated attachment and penetration. Describe how this virus performs this.
- FIP enters the host macrophage by attachment of its spike proteins to the CD13 receptor.
- At the same time, Abs against FIP spike protein cannot clear the virus from the hosts. Rather, they bind to spikes and facilitate entry of the virus into the host through the Ab (IgG)-Fcy receptor. This enhances entry into the cell
During which step of replication can the virion no longer be detected?
Virus uncoating
TRUE/FALSE.
The virus remains infectious after uncoating.
FALSE.
It loses its infectivity of virions.
What are some ways that a virion can undergo uncoating?
Endocytosis
Uncoating within the endosome (enveloped viruses) - same as receptor mediated endocytosis
Uncoating at the nuclear membrane (endosome docks onto nuclear membrane)
For each of the groups, know the steps to generate mRNA.
Always remember,
DNA/RNA + —> (-) —> mRNA
When dealing with viral DNA, what enzymes are involved in:
- Genome replicaton
- Production of viral mRNA
- DNA-dependent RNA polymerase
- DNA dependent DNA polymerase
When dealing with viral RNA, what enzymes are involved in
- Genome replication
- Production of viral mRNA
- RNA-dependent RNA polymerase (the same for both!)
What kind of viral genome can be used directly as mRNA?
ssRNA (+)
mRNA may also be produced from the ssRNA (-) intermediate during genome replication.
What needs to be generated for translation viral protein production?
mRNA (+)
What modifications occur for viral mRNA to conform to the requirements of host cell translation system?
Processing of Primary RNA Transcript/Pre-RNA
- Capping: Addition of 7-methylguanosine to the 5’ end of RNA. Adds to stability of mRNA, binds mRNA to ribosomes and marks mRNA as “self)
- Addition of 3’ poly-adenylated tails: thse interact with polyA-binding protein (important for translation). This can be done by host/viral enzymes.
- Splicing: removal of introns
What are the two types of splicing?
- Constituitive: every intron is removed and all exons are spliced in.
- Alternative: all introns are spliced out, only select exons are spliced in. This results in mRNAs having different coding information derived from a single gene.
What are the types of viral mRNA?
Monocistronic (encodes one polypeptide)
Polycistronic (encodes several polypeptides) - may chopped to monocistronic mRNAs and then translated OR, directly translated into a polyprotein and cleaved by proteases.
What are some important viral proteins?
- Enzymes
- Structural proteins
- Viral nonstructural proteins
- Regulatory proteins
- Inhibitors
Components of a virus (nucleic acids/proteins) are packaged to form ______.
Where can this process take place?
mature virions
Nucleus, cytoplasma, plasma/cell membrane (most enveloped viruses)
Compare the release of naked virions vs enveloped virions.
Naked (non-enveloped) –> Lysis
Enveloped –> budding through the plasma membrane (attachment of viral glycoroteins) or exocytosis from golgi apparatus/ER
What viruses undergo release through exocytosis?
Flaviviruses, Arteriviruses, Coronaviruses, Bunyviruses
What are the different ways a virus can spread?
- Extracellular spread: thrugh release into the environment and travel to a new host.
- Intercellular spread: do not contact the enviroment. Results in Rapid Virus
Dissemination, evasion of Immune system, and Persistent Infections. Viruses can either utilize existing cell-cell contacts or exploit basic cell adhesion biology to deliberately establish contact between infected and uninfected target cells for the purpose of efficient spreading. Seen in HIV, Herpesvirus, Measles, etc. See types in image.
- Nuclear spread: viral genome is integrated into host genome and passed down to progeny (e.g. retrovirus
