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Questions and Answers
What is the defining characteristic of viruses regarding their replication?
What is the defining characteristic of viruses regarding their replication?
Which type of microscope uses electrons reflected from the surface of the sample for imaging?
Which type of microscope uses electrons reflected from the surface of the sample for imaging?
What is the genome of a virion made of?
What is the genome of a virion made of?
How is the viral envelope lipid component acquired?
How is the viral envelope lipid component acquired?
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What is the function of the viral capsid?
What is the function of the viral capsid?
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What is the tegument or matrix in a virus?
What is the tegument or matrix in a virus?
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What are the building blocks of a viral capsid?
What are the building blocks of a viral capsid?
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What distinguishes an RNA-dependent RNA polymerase?
What distinguishes an RNA-dependent RNA polymerase?
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What distinguishes a permissive cell from a susceptible cell in the context of viral infection?
What distinguishes a permissive cell from a susceptible cell in the context of viral infection?
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What is a characteristic of a restrictive viral infection?
What is a characteristic of a restrictive viral infection?
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During which phase of the viral infectious cycle can no infectious virus particles be recovered from the cell?
During which phase of the viral infectious cycle can no infectious virus particles be recovered from the cell?
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What is the primary determinant of a virus's host range and tissue tropism?
What is the primary determinant of a virus's host range and tissue tropism?
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What is the role of anti-receptors in viral infection?
What is the role of anti-receptors in viral infection?
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If a cell is susceptible but not permissive to a virus, what type of infection occurs?
If a cell is susceptible but not permissive to a virus, what type of infection occurs?
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In enveloped viruses, what type of molecules typically function as anti-receptors?
In enveloped viruses, what type of molecules typically function as anti-receptors?
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What cellular components typically function as receptors and co-receptors for viruses?
What cellular components typically function as receptors and co-receptors for viruses?
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What is the term for any observable alteration in a host cell caused by a viral infection?
What is the term for any observable alteration in a host cell caused by a viral infection?
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Which of the following is NOT typically considered a cytopathic effect of viral infection?
Which of the following is NOT typically considered a cytopathic effect of viral infection?
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Viruses are obligate intracellular parasites. What does this mean?
Viruses are obligate intracellular parasites. What does this mean?
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Which of the following is commonly used for the growth of some viruses for vaccine production?
Which of the following is commonly used for the growth of some viruses for vaccine production?
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What is the primary purpose of adding antibiotics to tissue culture media?
What is the primary purpose of adding antibiotics to tissue culture media?
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Primary cell cultures are derived by dissociating tissues. Which of the following is used for this process?
Primary cell cultures are derived by dissociating tissues. Which of the following is used for this process?
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Why do primary cell cultures typically have a limited life span?
Why do primary cell cultures typically have a limited life span?
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What process is used to transition primary cell cultures into secondary cell cultures?
What process is used to transition primary cell cultures into secondary cell cultures?
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What is the primary mechanism by which enveloped viruses deliver their nucleocapsid or genome into the host cell cytoplasm?
What is the primary mechanism by which enveloped viruses deliver their nucleocapsid or genome into the host cell cytoplasm?
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Which of the following best describes the 'uncoating' process in viral replication?
Which of the following best describes the 'uncoating' process in viral replication?
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What is the primary role of the host cell in the macromolecular synthesis phase of viral replication?
What is the primary role of the host cell in the macromolecular synthesis phase of viral replication?
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What is a characteristic feature of viral exit via 'budding'?
What is a characteristic feature of viral exit via 'budding'?
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During the assembly phase of the viral replication cycle, how do viruses form their capsid?
During the assembly phase of the viral replication cycle, how do viruses form their capsid?
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What is the key distinction between pore-mediated penetration and receptor-mediated endocytosis in viral entry?
What is the key distinction between pore-mediated penetration and receptor-mediated endocytosis in viral entry?
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What happens to a virion during the exit from the host cell by cytolysis?
What happens to a virion during the exit from the host cell by cytolysis?
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In the viral replication cycle, what is packaged during the assembly phase of some viruses?
In the viral replication cycle, what is packaged during the assembly phase of some viruses?
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What is a major challenge in developing antiviral drugs?
What is a major challenge in developing antiviral drugs?
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Which of the following is a characteristic of continuous cell lines?
Which of the following is a characteristic of continuous cell lines?
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Which method for viral infection diagnosis is described as labor-intensive and poorly sensitive?
Which method for viral infection diagnosis is described as labor-intensive and poorly sensitive?
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What is the primary target of most antiviral drugs?
What is the primary target of most antiviral drugs?
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What is a notable difference between antiviral and antibacterial drugs?
What is a notable difference between antiviral and antibacterial drugs?
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What is a common feature of viruses that contributes to antiviral drug resistance?
What is a common feature of viruses that contributes to antiviral drug resistance?
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Which diagnostic method uses the detection of viral nucleic acids?
Which diagnostic method uses the detection of viral nucleic acids?
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What is the main purpose of using serology to diagnose a viral infection?
What is the main purpose of using serology to diagnose a viral infection?
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What is the primary mechanism of action of docosanol in the treatment of herpes simplex virus?
What is the primary mechanism of action of docosanol in the treatment of herpes simplex virus?
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Which drugs are effective against influenza A by blocking the uncoating process?
Which drugs are effective against influenza A by blocking the uncoating process?
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Nucleoside analogs contribute to viral genome termination through what mechanism?
Nucleoside analogs contribute to viral genome termination through what mechanism?
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What role do HIV protease inhibitors play in viral assembly?
What role do HIV protease inhibitors play in viral assembly?
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Which of the following is a function of zanamivir and oseltamivir?
Which of the following is a function of zanamivir and oseltamivir?
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Which statement accurately reflects the role of interferons in viral infections?
Which statement accurately reflects the role of interferons in viral infections?
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The antiviral immune response consists of how many phases?
The antiviral immune response consists of how many phases?
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What is a characteristic of nucleoside analogs used in antiviral therapies?
What is a characteristic of nucleoside analogs used in antiviral therapies?
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Study Notes
General Characteristics of Viruses
- Viruses are obligate intracellular parasites
- They lack the ability to generate energy, or synthesize proteins and replicate independently of the host cell
- They are small, less than 300 nanometers in size
Viral Structure
- A virion (viral particle) consists of:
- Nucleic acid (either DNA or RNA, but not both). The genome.
- A protein shell, the capsid.
- An envelope (in some viruses).
- Enzymes or other proteins essential for replication.
Viral Structure (capsid)
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The capsid is a rigid protein structure
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Individual proteins form successively larger units: protomers, capsomers and finally the complete capsid
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Some capsids are:
- symmetric (helical or polyhedral), and are built stepwise
- nonsymmetric (complex). These structures are associated with bacteriophages.
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An envelope is an external coating to the capsid composed of lipids and glycoproteins
- The lipid portion of the envelope is derived from the host cell membrane. Specifically the infected cell.
- Viral proteins are inserted into the lipid layer
- The tegument/ matrix is situated between the inner face of the envelope and the nucleocapsid. It is composed of viral proteins
Viral Replication
- The replication cycle depends on the host cell; the host cell functions as a factory
- The cell provides substrates, energy and machinery needed for viral protein synthesis and genome replication
- Viral infections occur with susceptible and permissive cells
- Susceptible cells have specific receptors for a given virus, not necessarily permissive
- Permissive cells have the capacity to replicate the virus, not necessarily susceptible
- Infections can be:
- Productive (both susceptible and permissive)
- Abortive (susceptible but non-permissive)
- Restrictive (susceptible but semi-permissive - inefficient replication)
- Latent (susceptible and permissive, but the viral genome is silent in the host cell- with possible reactivation)
Diagram of the Infectious Cycle
- The major steps in viral replication are the same for all viruses
- Attachment
- Penetration
- Uncoating
- Replication
- Assembly
- Release
Steps in Viral Replication
- Recognition of a target cell
- Attachment
- Penetration (non-enveloped, enveloped)
- Uncoating
- Macromolecular synthesis (Early mRNA, replication of genome, Later mRNA, and protein synthesis),
- Assembly
- Release (Budding, lysis)
Viral Replication (Steps 1-2)
- Initial contact is a random collision
- Efficient attack requires anti-receptor on the virion and its corresponding receptor (and co-receptor) on the host cell membrane
- Receptor binding determines host range and tissue tropism (the specific cells that the virus can infect)
- Anti-receptors are capsid proteins in naked viruses, glycoproteins in enveloped
HIV Entry in the Host Cell
- HIV attaches to either CD4 or CCR5 (or CXCR4 ) on the host cell
Viral Replication (Step 3: Penetration)
- Non-enveloped viruses may penetrate the plasma membrane directly
- Enveloped viruses fuse their membranes to cell membranes, delivering the nucleocapsid directly into the cytoplasm or through receptor-mediated endocytosis, and fusion in the endosome.
Viral Replication (Step 4: Uncoating)
- Uncoating is a stepwise process to release the genome from the capsid.
- The specifics of uncoating vary per virus and are not fully understood.
Viral Replication (Step 5: Macromolecular Synthesis)
- The replication phase of the genome (by cellular or viral enzymes). There are many strategies
- Transcription to mRNA and virus protein synthesis (structural and non-structural). Viral proteins are synthesized
Viral Replicative Strategies (Baltimore system)
- Viruses have different genome configurations (ss (+) RNA or ss (-) RNA, Double Stranded RNA, etc etc) which affect replication. The diagram shows a classification of viruses by the replication mechanisms.
Viral Replication (Step 6: Assembly)
- New viral particles are assembled from genome copies and viral proteins
- Capsomers interact to create the full capsid. (helical, icosahedral)
- Some viruses assemble the capsid around their genome. Other virus first assemble an empty capsid.
Viral Replication (Step 7: Exit/Release)
- Cytolysis, the virions accumulate, forming clusters and are released by the bursting of the host cell - Budding— Enveloped virions escape through cellular membranes (plasma membranes, nuclear, or endoplasmic reticulum membranes). Their envelopes are derived from host membranes.
Release of a Virus by Budding
- The diagram illustrates the process of budding.
Viral Cytopathgenesis
- Different mechanisms are used by viruses
- Examples of mechanisms of cytopathic effects, e.g, inhibition of cellular proteins, inclusion bodies, syncytia formation, and effects on the cell structure, disruption of cytoskeleton.
Viral Infection Causes Cytopathic Effect (CPE)
- Any changes in a host cell due to viral infection is CPE
- CPE may include cell rounding, swelling, shrinking, detachment, cytoplasmic/nuclear inclusions, or syncytium formation.
Viral Cultures
- Viruses are obligate intracellular parasites reliant on host cells
- Viral cultures can use:
- Embryonated eggs (still used for vaccines)
- Experimental animals (rarely used in clinical labs)
- Tissue culture cells (primary, continuous)
Tissue Culture Cells
- Primary cells — Isolated from specific organs, grown as monolayers and suspension, limited lifespan
- Secondary cells— Derived from primary cultures, more homogeneous
- Continuous cell lines— Derived from transformed cells or tumors, grow indefinitely (immortal).
- Normal cells form a monolayer, stop dividing
- Cancer cells do not exhibit density-dependent inhibition
Diagnosis of Viral Infection
- Culture-based systems (slow, labor-intensive, poorly sensitive)
- Electron microscopy (expensive, not sensitive enough)
- Molecular methods - detection of viral nucleic acid by PCR.
- Serology— detection of virus-specific antibodies
Antiviral Drugs
- Viruses rely on host machinery for replication, making it hard to develop effective drugs
- Antivirals primarily target viral enzymes or structures critical to replication.
- Some stimulants of host innate immune responses. (i.e., interferon)
- Antiviral drugs usually have a narrow spectrum of activity (specific for certain types of viruses), have resistance issues (like bacterial resistance) due to high mutation rates in viruses and long term treatment
Antiviral Drugs
- Examples include inhibitors of:
- Entry/fusion
- Uncoating
- Viral protein/nuclei acid synthesis
- Viral release
- Viral assembly
- Examples like nucleoside analogs, protease inhibitors, neuraminidase inhibitors, and others
Antiviral Drugs: Examples
- Docosanol— inhibits herpes simplex virus entry into cells; used topically. - Amantadine/ rimantadine— Inhibits influenza A virus uncoating
Host Defenses Against Viral Infection
- Initial response is innate immunity (nonspecific), follow it by adaptive immunity. Innate immune mechanisms (nonspecific), and adaptive immune mechanisms (highly specific). - Type I IFNs.
Interferons (IFNs)
- A group of signaling proteins produced by the body's cells in response to viruses. Important modulators of innate immunity
- They have an antiviral role.
- Type I IFNs — generated by infected cells in response to viruses.
- IFN-A — generated by lymphocytes, and other cells
- IFN-B — generated by fibroblasts and other cells
- IFN-γ produced by NK cells and CD8+ cells
Type I Interferons
- Viruses induce type I interferons
- They activate an "antiviral state"
- Interferons bind to receptors on the cells
- Activates protein kinase R and RNAse L. This inhibits viral protein synthesis, destroying viral particles, by degrading the viral mRNA
Type I Interferons
- Induce protection of cells for a short time
- Stimulate immune responses to control viral infections
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Description
Test your knowledge on key concepts of virology, including virus replication, structure, and infection cycles. This quiz covers essential topics such as the viral capsid, genome composition, and the distinctions between susceptible and permissive cells. Perfect for students studying microbiology or virology.
