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Questions and Answers
What is the term used for the mechanism by which some non-envelope viruses enter a host cell?
What is the term used for the mechanism by which some non-envelope viruses enter a host cell?
What occurs during the uncoating of the viral genome?
What occurs during the uncoating of the viral genome?
Which phase of viral replication begins with genome replication?
Which phase of viral replication begins with genome replication?
What is a defining characteristic of positive-sense RNA viruses in comparison to negative-sense RNA viruses?
What is a defining characteristic of positive-sense RNA viruses in comparison to negative-sense RNA viruses?
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What role do Viral Attachment Proteins (VAPs) play in viral infection?
What role do Viral Attachment Proteins (VAPs) play in viral infection?
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What is the process by which envelope viruses enter a host cell?
What is the process by which envelope viruses enter a host cell?
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What is the consequence of differences between positive-sense and negative-sense RNA viruses for the composition of the virus particle?
What is the consequence of differences between positive-sense and negative-sense RNA viruses for the composition of the virus particle?
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Defective interfering particles are best described as:
Defective interfering particles are best described as:
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What characterizes a positive-sense RNA virus?
What characterizes a positive-sense RNA virus?
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Which statement about negative-sense RNA viruses is true?
Which statement about negative-sense RNA viruses is true?
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What is the primary function of reverse transcriptase in retroviruses?
What is the primary function of reverse transcriptase in retroviruses?
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Which viruses are known to be positive-sense RNA viruses?
Which viruses are known to be positive-sense RNA viruses?
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What is the fate of the viral genome once inside the host cell?
What is the fate of the viral genome once inside the host cell?
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In which stage do early viral proteins get formed?
In which stage do early viral proteins get formed?
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What distinguishes the replication process of double-stranded RNA viruses?
What distinguishes the replication process of double-stranded RNA viruses?
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How does a negative-sense RNA virus replicate its genome?
How does a negative-sense RNA virus replicate its genome?
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What type of viral gene products are mainly involved in the replication of the genome?
What type of viral gene products are mainly involved in the replication of the genome?
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Which process allows complex enveloped viruses to interact with host cells?
Which process allows complex enveloped viruses to interact with host cells?
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During which phase is the transcription of late gene products initiated?
During which phase is the transcription of late gene products initiated?
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How do enveloped viruses primarily exit the infected cell?
How do enveloped viruses primarily exit the infected cell?
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What is the main characteristic of viruses that undergo lysis for their release?
What is the main characteristic of viruses that undergo lysis for their release?
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In what form does single-stranded DNA replicate during viral synthesis?
In what form does single-stranded DNA replicate during viral synthesis?
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Which of the following is a characteristic of late proteins in viral formation?
Which of the following is a characteristic of late proteins in viral formation?
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What role does the viral RNA polymerase play in the synthesis of (−) RNA viruses?
What role does the viral RNA polymerase play in the synthesis of (−) RNA viruses?
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What is the eclipse period in viral replication?
What is the eclipse period in viral replication?
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Where does the replication of most RNA viruses predominantly occur?
Where does the replication of most RNA viruses predominantly occur?
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Which type of virus does NOT require transfer to mRNA for protein synthesis?
Which type of virus does NOT require transfer to mRNA for protein synthesis?
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What enzyme do negative-sense RNA viruses rely on during replication?
What enzyme do negative-sense RNA viruses rely on during replication?
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What process allows a virus to merge with infected cells, forming multinucleated giant cells?
What process allows a virus to merge with infected cells, forming multinucleated giant cells?
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What is the main distinction between positive-sense and negative-sense RNA viruses?
What is the main distinction between positive-sense and negative-sense RNA viruses?
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Which of the following can result from mutations in essential viral genes?
Which of the following can result from mutations in essential viral genes?
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Which viruses contain RNA polymerases to synthesize mRNA?
Which viruses contain RNA polymerases to synthesize mRNA?
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What is the result of uncoating during the viral replication process?
What is the result of uncoating during the viral replication process?
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What do defective interfering particles (DIPs) result from?
What do defective interfering particles (DIPs) result from?
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How does the ribosome interact with a positive-sense RNA virus?
How does the ribosome interact with a positive-sense RNA virus?
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What is a potential consequence of viral mutations that alter antigenicity?
What is a potential consequence of viral mutations that alter antigenicity?
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In the context of viral replication, what can pressure selection include?
In the context of viral replication, what can pressure selection include?
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Viral vectors can be modified for what purposes?
Viral vectors can be modified for what purposes?
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What is one factor that influences the selection of viral mutants?
What is one factor that influences the selection of viral mutants?
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How do mutations in viruses typically manifest in terms of variants?
How do mutations in viruses typically manifest in terms of variants?
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What defines viropexis?
What defines viropexis?
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Which step of the virus replication cycle involves the recognition and binding to specific receptors on the host cell?
Which step of the virus replication cycle involves the recognition and binding to specific receptors on the host cell?
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What is the primary mechanism by which non-enveloped viruses are internalized into host cells?
What is the primary mechanism by which non-enveloped viruses are internalized into host cells?
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During the uncoating process of a virus, what occurs?
During the uncoating process of a virus, what occurs?
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What role do glycoproteins or oligosaccharides play in the viral replication cycle?
What role do glycoproteins or oligosaccharides play in the viral replication cycle?
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Which of the following statements about the entry process of enveloped viruses is correct?
Which of the following statements about the entry process of enveloped viruses is correct?
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What happens to the viral infectivity during uncoating?
What happens to the viral infectivity during uncoating?
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Which step directly follows penetration in the viral replication cycle?
Which step directly follows penetration in the viral replication cycle?
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Study Notes
Viral Replication
- Viral replication is a process of separating into phases
-
Early phase infection:
- Recognition of the appropriate target cell
- Attachment to the cell
- Penetrating the plasma membrane
- Release (uncoating) of the genome into the cytoplasm
- Delivery of the genome to the nucleus (if needed)
-
Late phase infection:
- Begins with genome replication and viral macromolecular synthesis
- Proceeds through viral assembly and release
- The steps in viral replication include:
- Recognition of the target cell
- Attachment
- Penetration
- Uncoating
- Macromolecular synthesis (early and late mRNA, replication of the genome, and protein synthesis and modification)
- Assembly of the virus
- Budding (for enveloped viruses)
- Release of the virus
How a Virus Enters its Host Cell
- Internalization of the virus into a cell requires
- Interactions between VAPs and cellular receptors
- Structure of the virion
- Cell type
- Envelope viruses: Fuse their membranes with cellular membranes, releasing the genome/nucleocapsid directly into the cytoplasm
- Non-envelope viruses: Enter cells through receptor-mediated endocytosis or viropexis
Viropexis
- Defined as the process by which some viruses (e.g., picornaviruses and Papovaviruses) enter a cell and are internalized by phagocytosis
- Facilitated by exposed hydrophobic structures on the capsid proteins
Uncoating
- A physical separation of the viral nucleic acid from the outer structural components of the virion
- Facilitated by host cell enzymes in endosomes or lysosomes
- The infectivity of the parental virus is lost
Gene Expression and Genome Replication
- Happens after viral genome uncoating
- Most RNA viruses complete replication in the cytoplasm, except for retroviruses, influenza viruses, and hepatitis D virus
- Viral DNA replicates in the nucleus, except for poxviruses and herpes viruses
Positive-sense and Negative-sense RNA Viruses
- Positive-sense RNA virus: The viral RNA acts as mRNA, directly translated into protein by the ribosome
- Negative-sense RNA virus: Requires an RNA polymerase to convert the viral RNA into mRNA for protein translation
Double-stranded RNA Viruses
- Use viral RNA polymerases to synthesize mRNA
Retroviruses
- Positive-strand RNA genome
- No means for replication in the cytoplasm
- Carry two copies of the genome, tRNA molecules, and RNA-dependent DNA polymerase (reverse transcriptase)
- Reverse transcriptase translates the RNA into DNA, allowing the viral DNA to be integrated into the host cell DNA
Viral Components Synthesis
- The viral genome directs the synthesis of viral mRNA and protein
- The viral genome creates identical copies of itself
- Transcription translates the viral genetic information into mRNA
- Translation converts mRNA into protein
Transcription and Translation Stages
- Early proteins: Polymerases, regulatory proteins, viral antigens
- Late proteins: Structural capsid proteins, enzymes for viral release
Viral Macromolecular Synthesis Steps
- Double-stranded DNA (dsDNA) uses host machinery to produce mRNA (except for poxviruses)
- Single-stranded DNA (ssDNA) converts to dsDNA and replicates like dsDNA
- Positive-sense RNA (mRNA-like) is translated into protein by ribosomes
- Negative-sense RNA requires RNA polymerase to make mRNA
Differences between Early and Late Viral Gene Products
- Early gene products are often DNA-binding proteins and enzymes (including viral polymerases), typically present in smaller quantities
- Late gene products are structural proteins and other proteins essential for viral packaging and are not required before genome replication
Replication of a Complex Enveloped DNA Virus
- Transcription and translation occur in three phases: immediate early, early, and late
Assembly and Maturation
- Irreversible combination of viral nucleic acid and protein coat to form new-infectious virions
- Capsid synthesis is directed by late genes
- Enveloped virus assembly needs plasma membrane interaction
Release of Viral Particles
- Lysis: Cytolytic viruses cause cell death to release viruses
- Budding: Cytopathic viruses do not kill infected cells; viruses bud off from the cell using plasma membrane
- Non-enveloped viruses are released via exocytosis
Re-initiation of Replication (Spread)
- Viral release to the extracellular medium
- Through cell-to-cell fusion
- Into multinucleated giant cells (syncytia) cells
- Becoming huge virus factories
- Vertically into daughter cells
Defining the Concept of Viral Mutation
- Spontaneous and frequent in viral genomes
- Resulting in new strains of viruses with differing properties (compared to the parental form)
- Mutations in essential genes can:
- Inactivate the virus (lethal mutations)
- Cause antiviral drug resistance
- Alter viral antigenicity or pathogenicity
Mutation and Selection
- Viral mutants are selected based on their ability to use host cell machinery and withstand conditions
- Cellular properties act as pressure selections
- Includes cell growth rate and tissue-specific protein requirements (e.g., enzymes, glycoproteins, transcription factors)
- Viruses that cannot adapt or escape the host's defense mechanisms are eliminated
Defective Interfering Particles (DIPs)
- Also known as defective interfering viruses or mutants
- Result from unexpected virus generation
- Loss of a critical portion of the genome either from defective replication or non-homologous recombination
Viruses as Vectors for Therapy
- Modification of viral genetic composition is used for:
- Gene replacement therapies
- Vaccines to stimulate immunity
- Replacing harmful genes with desired ones
- Using viral vectors (attenuated/defective) in clinical trials (e.g., retroviruses or adenoviruses).
Consequences of Viral Properties
- Viruses lack life (non-living)
- Viruses must be infectious to survive
- Viruses utilize host cellular processes to generate components (mRNA, proteins, identical genome copies)
- Viruses must encode required functions not present in the cell
- Viral components self-assemble
Steps in viral replication
- Recognition of the target cell
- Attachment
- Penetration
- Uncoating
- Macromolecular synthesis (early mRNA, replication of the genome, late mRNA, posttranslational modifications of protein)
- Assembly of the virus
- Budding of enveloped viruses (if present)
- Viral release
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Description
Test your knowledge on the mechanisms of viral entry and replication. This quiz covers essential concepts related to non-envelope viruses, uncoating processes, and the differences between positive-sense and negative-sense RNA viruses. Assess your understanding of viral attachment proteins and defective interfering particles.
